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Drawing from a lifetime of teaching botany, Dr. Nels Lersten presents the study of the structures and processes involved in the reproduction of. PDF | On Jun 20, , Anil Kumar Thakur and others published Plant Anatomy and Embryology. Plant Anatomy and Embryology. Book · June with 2, Reads. Publisher: 1st. Publisher: In book: Meiosis - Molecular Mechanisms and Cytogenetic Diversity. Cite this . Embryology of Flowering Plants Applied to Cytogenetic Studies on Meiosis.


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Embryology, Accessible book, Angiosperms, Botany, Plants, Protected DAISY, Plant tissue culture, Congresses, Growth (Plants), Plant cell culture, Plant cells. These experiences convinced me of the need for an angiosperm embryology book that emphasizes economic plants. ix 1 Introduction Angiosperm embryology . Printer-friendly PDF Broadly defined, as indicated in the introduction of this book, embryology is the study of the structures A better understanding of plant embryology has also lead to biotechnological applications in plant improvement.

Neat labeled diagrams, tables and illustrations are included which will help in developing better understanding of the subject among the students. Summary in the end of each chapter is provided to give students a gist of the whole chapter. Exercise questions and Past University questions are included for each chapter for the benefit of students.

Hema Sane, a Senior Botanist is M. Sc, Ph. Phil in Indology and is an institution in Botany has diligently compiled this book. Her explanations will give students a liking for the subject and create interest in the subject.

Contents 1. Introduction to Plant Anatomy 1. Milestones in Plant Anatomy 2. Significance of Plant Anatomy 3. Importance of Anatomy with reference to other Branches 4. Types of Tissues 1. Tissue System - What it is? Pomology, Poland, Ser. Manning, J. Diversity of endothecial patterns in the angiosperms. McGregor, S. Insect pollination of cultivated crop plants. Milyaeva, E. Starch in developing anthers of Citrus sinensis: A cytochemical and electron microscope study.

Soviet Plt. Moss, G. Pacini, E. Bellani, and R. Pollen, tapetum and anther development in two cultivars of sweet cherry Prunus avium. Phytomorphology Schmid, R. Filament histology and anther dehiscence. Vautier, S. La vascularisation florale chez les Polygonacees. Candollea Whatley, J. Fine structure of the endothecium and developing xylem in Phaseolus vulgaris.

Woycicka, Z. Theme and Variations Chapter 2 dealt with aspects of the whole stamen apart from the production of pollen.

This chapter introduces pollen, outlines the stages of pollen development, and presents five examples to provide realism and illustrate events that typically occur as well as a few of the many variations.

Chapter 4 delves more deeply into the anther and stages of pollen development. In addition, a pollen grain has an advantage over an amphibious free-living gametophyte of a lower plant because it does not release the sperms into an external film of water to swim to their destination.

This means that even though pollination is a remarkably wasteful process for most flowering plants, when a pollen grain does land on the right stigma it will be nurtured and guided within the maternal tissue, with a good chance for successful fertilization.

Most pollen grains have a thick wall composed of two major layers. The usually conspicuous outer layer is called the exine, which in most grains is interrupted by one or three or more in some grains small circular-to-elongate thin areas called apertures.

The pollen tube will emerge through one of these at germination. A small number of species have a uniformly thin exine that lacks any special exit sites. Because there is only one pollen tube, one can ask why most pollen grains have more than one aperture. There is no definitive answer as yet, but Chapter 4 includes speculations. The exine is composed mostly of a tough material called sporopollenin, which defies natural and human efforts to erode or dissolve it.

This remarkably durable structure can persist for millions of years if buried under oxygenfree anaerobic conditions, and the standard methods used to prepare fossil or modern pollen for microscopic study all involve subjecting the grains to extremely harsh chemical treatment, which removes all but the exine.

Showers of pollen in pine forests, for example, can be gathered up in containers, where they do resemble flour. A few typical grains could fit on the period at the end of this sentence. Pollen is the common name. A microgametophyte is therefore a sperm-producing gametophyte. A pollen grain is a tiny, short-lived, non-photosynthetic haploid plant that consists of only two or three cells. Pollen grains of most economic plants, and of plants in general, live for only a few hours to a few days after leaving the anther Dafni and Firmage, , and successful ones never touch the ground.

A pollen grain is therefore an ephemeral speck that could well be regarded as a tenuous link in the life cycle. Pollen Development: It can be fairly said that the pollen exine and the similar sculptured outer layer of spores of other plant groups comprise the chief subject matter for the discipline of palynology the study of spores and pollen.

The inner pollen wall layer is the intine, and in contrast to the exine it is composed mostly of cellulose. The intine has its own complexities but it is at least structurally and architecturally simpler than the exine. Chapter 4 includes details about pollen wall structure and development. A consideration of pollen development must also include attention to the formation of the pollen sacs themselves, especially the tapetum, the important cell layer that completely lines the interior of each pollen sac, and which is involved with pollen development.

A diagram that is more specific Fig. The differences that Figure 3. The differences between pollen shed with two cells vs. The numbers 1—8 in Figure 3. Sporogenous cells proliferate by mitosis to a certain final number, and then each cell secretes an isolating callose sheath around itself. The microspore mother cells mmc undergo meiosis, and most monocots form a cell wall 3. Callose dissolves, releasing microspores into the fluid environment of the pollen sac.

Microspores enlarge, become vacuolate i. The vacuolate pollen now has a large vegetative cell and a small generative cell. The generative cell migrates within the vegetative cell, food reserves begin to accumulate, and the pollen may be shed at this two-celled stage.

In some groups the generative cell divides mitotically and produces two sperm cells becomes tri-celled before the vegetative cell completes its engorgement with food reserves. Descriptions of five actual examples follow. None of them illustrate all stages of pollen development; however, they complement each other. In addition, they provide glimpses of a few of the many variations in structure and processes that are known. A more detailed review of the processes that occur within anthers of angiosperms in general is that of Pacini The four pollen sacs develop more or less synchronously, but for ease of presentation each sac is shown diagrammatically at a different stage in Figure 3.

Sac 1 is smallest and shows the pre-meiosis situation: A detailed view at this stage is seen in Figure 3. General diagram of pollen development in dicots and monocots; stages 1—8 show sporogenous cell to pollen grain.

Further explanation in text.

Originally published in Laser and Lersten, Anatomy and cytology of microsporogenesis in cytoplasmic male sterile angiosperms, The Botanical Review, Vol. In most grasses every sporogenous cell is appressed to the tapetum and remains so during all stages of development until the mature pollen is released from the anther. A recent survey of pollen arrangement has verified this unusual, perhaps unique, physical relationship for most grasses Kirpes et al.

The enlarged single sporogenous cell at the right of pollen sac 1 in Figure 3. Just before meiosis begins, each mmc secretes callose between the cell membrane and the thin primary cell wall next single cell.

Callose is a gelatinous carbohydrate that isolates mmc's from each other and from surrounding diploid tissues during and after meiosis, when the transition to the haploid microgametophyte stage occurs. Most of the Chapter 3: Theme and Variations 25 Figure 3. Pollen development in sorghum. Pollen sacs 1—4 are at different developmental stages; individual cells clockwise around periphery show details.

From Christensen This uneven callose deposition helps to keep the mmc's appressed to the tapetum. The anther with its four internal pollen sacs enlarges during meiosis. After the first meiotic division a new cell wall forms, dividing the mmc into two cells, which is called a dyad next single cell in Fig.

After the second meiotic division two new cell walls partition 26 Flowering Plant Embryology Figure 3. Selected stages of sorghum pollen development from anther sections A—G, plastic-embedded; H, scanning electron micrograph.

Pollen sac with central sporogenous cells surrounded by tapetum. Enlarged view of mmc's with copious callose c in center of sac.

Plant embryology

Pollen sac with dyads and tetrads appressed to prominent tapetum. Individual mid-vacuolate microspore with single pore appressed to tapetum arrow. Late-vacuolate microspores with pore appressed to collapsing tapetum arrows.

Nearly mature pollen grain with central vegetative cell nucleus V , generative cell arrowhead , numerous starch granules, and diminishing vacuole; note pore appressed to collapsed and seemingly empty tapetum arrow.

Mature pollen grains filled with starch, with pore arrow appressed to collapsed tapetum longitudinal section shows artifact of seemingly separated grains; E shows true arrangement. Mature pollen grain with single pore. From Christensen and Horner Chapter 3: Theme and Variations each dyad cell into two new cells. The grass tetrad is unusual because it forms in one plane.

Imagine a grass mmc as a pie cut in half by the first cytokinesis; then each dyad cell bisected again by the second cytokinesis, yielding four microspore pie quarters. This tetrad pie remains appressed to the parietal tapetum, and in this way each microspore maintains physical contact with the tapetum, an arrangement shown in pollen sac 2 Fig.

Figure 3. Meiosis and successive cytokinesis occur within the callosic sheath, which persists around the microspore tetrad for some time after meiosis.

The exine layer of the pollen wall is now initiated around each microspore. Microspores enlarge as water is imbibed and stored in vacuoles Fig.

This enlargement occurs in concert with pollen sac and anther enlargement. The microspore tetrads are surrounded, except where they contact the tapetum, by a fluid of largely unknown composition.

Soon the callose that has encapsulated the microspore tetrad dissolves and contributes to the pollen sac fluid. Pollen sac 3 Fig. As microspore vacuoles continue to absorb water and swell Fig.

While a microspore is enlarging, this wall must retain flexibility to accommodate its expansion, except where the single aperture, found in all grasses, forms along the surface of contact with the tapetum. A microspore at about this stage is shown in Figure 3. The several small microspore vacuoles expand and eventually merge into one large vacuole, an expansion that occurs as food reserves are used for metabolic purposes and converted into components of the developing wall. At this late vacuolate stage the microspore undergoes mitosis and becomes a 27 pollen grain Fig.

The distinction between microspore and pollen is frequently ignored, and in many published studies all stages after meiosis are called pollen. But there are good reasons to call the one-celled direct product of meiosis a microspore, and to restrict the term pollen grain to the two-celled structure resulting from microspore mitosis.

During the transition from pollen sac 3 to 4 the microspore divides mitotically, producing a large vacuolate vegetative cell and a tiny, lens-shaped generative cell Fig.

The nucleus of the vegetative cell migrates toward the pore, followed by the generative cell, which can now move because the thin callose sheath that briefly attached it to the vegetative cell membrane has dissolved.

The tiny generative cell appears to float more or less freely within the vegetative cell, but in reality its movements are probably controlled by microtubules, tiny subcellular filaments that have been described from pollen of some plants. New food reserves in the form of starch appear in the vacuolated pollen grain, and the central vacuole gradually shrinks Fig.

As starch accumulates, the tapetum gradually empties itself and collapses in place, and the pollen wall is completed. There is evidence from other species that nutrients enter pollen from the tapetum, and it can be reasonably speculated that the intimate pore-to-tapetum connection in grasses provides an efficient nutrient entryway. While engorgement is proceeding, the generative cell divides to produce two sperm cells, a characteristic of grasses and several other families. Pollen sac 4 Fig.

During the last maturation events in pollen, wall thickenings in the form of bars appear in the cell wall of the endothecium, the subepidermis of the anther. As explained in Chapter 2, these bars help the anther to dehisce and release pollen. A detailed view of mature sorghum pollen, still with the single pore appressed to the now-collapsed tapetum, is 28 Flowering Plant Embryology shown in Figure 3.

Its pollen development is similar to that of other members of this family—for example, the tomato Sawhney and Bhadula, —as well as to many other dicots. Horner and Wagner illustrated the anther of sweet pepper with one of the four pollen sacs eliminated to show that there are three different types of crystals in the anther, each with its own characteristic distribution Fig. One crystal type druse is associated only with the stomium. Within the placentoidal pollen sac of sweet pepper, stages of pollen development were shown in cross-sectional view by Horner and Rogers Sporogenous cells form only two layers in the pollen sac Fig.

Here and in some other plants, it has been speculated without any evidence that the enlarged tapetal cells are larger because they are closer to vascular bundles in the connective and therefore more active in transfer of nutrients. Callose is secreted by each mmc early in prophase I of meiosis; unlike cereal grasses see sorghum example , callose in sweet pepper and in dicots in general is deposited quite uniformly Figure 3. Cutaway view of sweet pepper anther with one pollen sac removed to show crystal distribution and other anther features.

From Horner and Wagner The tetrad of microspores remains surrounded by callose, but when each microspore initiates its future pollen wall Fig. The inner tangential and radial walls of the tapetum also dissolve away at about this time, leaving the partially naked tapetal cells somewhat deformed, like partly deflated basketballs Fig.

The microspores released from callose become immersed in the fluid of the pollen sac, which contains several dissolved substances.

Breadcrumb

There is little information about this fluid, but polysaccharides have been reported by Gori in garlic, Allium sativa, and pectins in lily Aouali et al. Theme and Variations 29 Figure 3. Selected stages of sweet pepper pollen development from plastic-embedded cross sections of pollen sacs. Sporogenous cells in two layers S ; outer one abuts outer tapetum T , inner layer abuts larger inner tapetum and placentoid A ; note middle layers P at top.

Condensed chromosomes seen during second meiotic division; mmc's surrounded by callose are loosely connected in locule L.

Callose dissolving arrowheads at early microspore tetrad stage. Separated microspores after callose dissolution. Late vacuolated microspores with well-developed wall; note collapsing tapetum. Pollen in process of engorgement; note almost collapsed tapetum. Mature pollen ready to be shed; tapetum has almost disappeared and endothecium at top has characteristic wall thickenings.

From Horner and Rogers Microspores at the late vacuolate stage have a noticeably thicker wall and appear almost empty because of their large central vacuole Fig.

The tapetum in this figure shows signs of degeneration. Following microspore mitosis the pollen grain has a small generative cell within a large vegetative cell, food reserves begin to accumulate, and the parietal tapetal cells gradually collapse Fig. When the pollen grain is mature and fully engorged with food reserves, the tapetum has degenerated almost completely and wall thickenings on the endothecial cells indicate that the anther is ready to shed pollen Fig. But walnut anthers form during one year, reach a certain pre-meiotic stage, and remain dormant over winter, completing their development during the following growing season.

This behavior occurs in many temperate zone trees and perennial herbs, particularly among bulbous monocots. The developmental stages described here from Luza and Polito include certain features that differ from the previous examples. By the time sporogenous tissue and the layers external to it can be distinguished, the sporogenous cells form a multicellular column in each pollen sac Fig.

There are a large number of sporogenous cells; therefore, many of them do not contact the tapetum, unlike the cell configuration in pollen sacs of sorghum and sweet pepper. But the external anther layers are similar to those of the previous examples, consisting of tapetum, three middle wall layers, the endothecium subepidermis , and the epidermis. After the sporogenous cells complete their last mitotic division and become mmc's, each secretes a conspicuous callose deposit that becomes strikingly evident when viewed using a technique that causes the callose to fluoresce brightly Fig.

Meiosis without cytokinesis occurs while each mmc is encased in callose, and the resulting four haploid microspore nuclei spend a short period as a coenocyte. Simultaneous cytokinesis follows, which gradually pinches off the four microspores from each other by progressively invaginating constrictions, or furrows Fig.

The tetrad of separate microspores remains encased in callose for some time after meiosis Fig. As in the earlier examples of pollen development, callose dissolves and releases the four microspores of each tetrad into the pollen sac, where they gradually enlarge and become vacuolate as their food reserves are utilized to thicken the pollen wall.

Microspore mitosis occurs during the late vacuolated state and initiates the twocelled pollen grain, which then begins to replenish its food reserves as the wall approaches maturity Fig. The mature walnut pollen sac contains fully engorged pollen and a now completely collapsed tapetum. Thickenings appear in cells of the endothecium, indicating that the anther is ready to split open Fig.

The tapetum ruptures, releasing an oily substance called tryphine or pollenkitt, which enters the pollen sac and is deposited on and within the pollen exine. This tapetal coat makes pollen grains sticky, adhering them to each other as well as to pollinating insects and the stigma. Tryphine includes carotenoid and flavonoid pigments, as well as proteins that will act as molecular recognition signals when pollen Chapter 3: Selected stages of walnut pollen development from plastic-embedded pollen sac cross sections.

Sporogenous cells S in sac lined by tapetum T with middle layers between arrows and endothecium E toward exterior. Four sacs of one anther under fluorescence microscopy show mmc's black spots surrounded by white callose.

Microspores separated arrowhead but still retained as a tetrad within callose. Late vacuolate microspores above degenerating tapetum. Partly engorged pollen grains, one showing nucleus of vegetative and generative cells N , with exine Ex and intine I of pollen wall. Mature engorged pollen grains in sac; tapetum is gone and endothecium E has characteristic wall bars arrowheads. Reproduced from Luza and Polito, , Microsporogenesis and anther differentiation in Juglans regia L.: A developmental basis for heterodichogamy in walnut, Botanical Gazette Reprinted with permission from the University of Chicago.

Copyright by the University of Chicago. All rights reserved. The interaction of these proteins on the exine with recognition proteins produced by the stigma is one of the gatekeeping mechanisms by which the stigma can reject foreign pollen and accept and stimulate germination of compatible pollen. The example discussed and illustrated here is a combination of a study of rape, Brassica napus Grant et al.

A cross-sectional view of one pollen sac at the mmc stage shows that here, as in walnut, there are some mmc's deep within the central column that do not touch the tapetum. Also, it is evident that the very darkly staining parietal tapetum is quite conspicuous even at an early stage Fig.

The tapetum remains turgid and conspicuous through some of the later stages, for example at the early vacuolate microspore stage Fig. After meiosis the microspores start to fill up with food reserves even before they undergo mitosis to become pollen, another difference from previous examples. At the same time the tapetal cells begin accumulating dark lipid droplets Fig. A closer look at this excretion can be seen in a transmission electron microscope view of a radish anther Fig.

The microspores have a well-developed wall with many cavities, and the dark-stained excreted tapetal material can be resolved into two components, lipid droplets and larger vesicles with fibrous contents. Somewhat later in development the tapetal cell membranes rupture, sending tapetal cytoplasm into the pollen sac, where some of it flows onto the surface and into cavities in the exine of the pollen wall. The tapetum here, and in other plants where it ruptures and contributes much of its cytoplasm to the pollen surface, is especially intimately involved in pollen development.

The next example also emphasizes tapetal involvement. A B C Figure 3. Selected stages of pollen and tapetal development of oilseed rape from plastic-embedded cross sections of pollen sacs. Start of prophase of first meiotic division in microspore mother cells mmc ; note darkstaining tapetum T. Late microspores surrounded by enlarged tapetum with dense cytoplasm. Mature pollen and persistent tapetum filled with granular bodies that will be deposited as tryphine on the pollen wall.

From Grant et al. All members of this large dicot family examined to date exhibit an Chapter 3: Theme and Variations 33 A B Figure 3.

Before and after transmission electron micrograph views of tryphine release from tapetum of radish onto pollen wall of radish. Almost mature pollen grains above tapetum, which is still intact but with material-filled vacuoles V , elaioplasts E , and lipid bodies L ; these components will combine to form tryphine upon tapetal rupture.

Ruptured tapetal cells depositing tryphine on pollen wall surface and within exine spaces: Reproduced from Figures 13, 17 in Dickinson and Lewis. Reprinted with permission of The Royal Society. Invasive tapetal cells do not remain at the periphery of the pollen sac; instead, their cell walls dissolve at a certain stage and the tapetum collectively flows amoeba-like into the pollen sac and engulfs the developing microspores.

An invasive tapetum is common among monocots but uncommon in dicots except for the large family Asteraceae. A Pollen development in sunflower was described by Horner The early stages resemble those seen in the previous dicot examples, so the developmental sequence can be taken up after meiosis, starting with microspore tetrads encased in callose Fig.

Up to this stage the tapetum has remained peripheral, but somewhat later, when the callose begins to dissolve from around the microspores, the tapetal cells exhibit irregular swelling and begin to lose their wall Fig. Tapetal enlargement continues after the microspores are released by callose dissolution, and by the mid-vacuolate microspore stage the tapetal cells have elongated radially and are intruding deeply into the pollen sac, C B D E Figure 3.

Sunflower pollen development and invasive tapetum; cross sections of plasticembedded pollen sacs. Callose dissolving and releasing microspores as tapetum intrudes into sac. Mid-vacuolate microspores with spiny exine almost surrounded by invasive tapetum.

Highly vacuolated microspores engulfed by invasive tapetum. Mature pollen engorged with starch after invasive tapetum has disappeared; enlarged endothecium arrow abuts pollen sac. From Horner Theme and Variations some now thrusting between the microspores Fig. The late vacuolate microspores eventually become engulfed completely by the invasive tapetum Fig.

While the microspores are still engulfed they undergo mitosis to form the two-celled pollen grain; at the same time the tapetum begins to develop vacuoles and show other signs of degeneration. While the pollen grains accumulate food reserves and the central vacuole gradually disappears, the tapetum declines further. It finally disappears completely, probably by absorption into the pollen, by the time the mature pollen is ready to be shed Fig.

Laporte, and C. Pectin secretion and distribution in the anther during pollen development in Lilium. Christensen, J. Developmental Aspects of Microsporogenesis in Sorghum bicolor.

Dissertation, Iowa State University. Horner, Jr. Pollen pore development and its spatial orientation during microsporogenesis in the grass Sorghum bicolor. Dafni, A. Pollen viability and longevity: Practical, ecological and evolutionary implications. Dickinson, H. The formation of the tryphine coating the pollen grains of Raphanus, and its properties relating to the selfincompatibility system.

London, Ser. Gori, P. Accumulation of polysaccharides in the anther cavity of Allium sativa, clone Piemonte. Beversdorf, and R. A comparative light and electron microscopic study of microspore and tapetal development in male fertile and cytoplasmic male sterile oilseed rape Brassica napa.

A comparative light- and electron-microscopic study of microsporogenesis in male-fertile and cytoplasmic male-sterile sunflower Helianthus annuus. A comparative light and electron microscopic study of microsporogenesis in male-fertile and cytoplasmic male-sterile pepper Capsicum annuum. Kirpes, C. Clark, and N. Systematic significance of pollen arrangement in microsporangia of Poaceae and Cyperaceae: Review and observations on representative taxa.

Laser, K. Anatomy and cytology of microsporogenesis in cytoplasmic male sterile angiosperms. Invasive tapetum and tricelled pollen in Ambrosia trifida Asteraceae, tribe Heliantheae. Luza, J.

Comparative Embryology of Angiosperms Vol. 1/2

Microsporogenesis and anther differentiation in Juglans regia L.: A developmental basis for heterodichogamy in walnut. From anther and pollen ripening to pollen presentation.

Sawhney, V. Microsporogenesis in the normal and male-sterile stamenless-2 mutant of tomato Lycopersicon esculentum. Details of Stages Chapter 3 introduced the stages of pollen development, filled in some gaps in the general life cycle diagram of Chapter 1 Fig.

This chapter delves more deeply into anther, tapetum, and pollen development, and ends by considering the number of pollen grains produced by various plants. Turning to the opposite, inward facing part of a pollen sac, the tapetum here abuts on a few to several parenchyma layers in which are embedded one or more vascular bundles see Chapters 2 and 3 for examples ; this central core of the anther is called the connective.

Some investigators have proposed that tapetal cells next to the anther wall the outer tapetum have a different origin from tapetal cells facing the connective the inner tapetum , and it is true that in some families the inner tapetum is composed of two layers of cells that are often larger in size, as shown in Chapter 3, Figure 3.

Whether tapetal location makes any physiological difference is not known, although the inner tapetum is obviously closer to the vasculature and therefore possibly receives more nutrients, or receives them earlier. Davis , for example, recognized four types of anther wall formation.

But these are not mature static structures, and are therefore verifiable only by developmental study using statistically valid samples, which has seldom been done. A survey by Brunkener of early anther development that included several samples each of 60 genera from 38 angiosperm families, showed that individual anthers often deviate from their supposed types. Even Coulter and Chamberlain commented early that local physiological conditions within the young anther could affect patterns of cell division and influence the developmental pathway.

Mitotic divisions in the proliferating archesporium may or may not occur in synchrony; in lily, 36 Chapter 4: Details of Stages for example, mitoses are scattered irregularly throughout Walters, , In wheat, lily, and barley, successive cycles of mitoses occur, each cycle proceeding more slowly than the one before Bennett, In wheat, for example, the mitotic cycle that increased 12 cells to 25 cells took 25 hours, from 25 to 50 cells took 35 hours, and 55 hours were required to go from 50 to cells.

These successively slower mitotic cycles are not linked to any obvious cause, but they suggest that shifting from mitosis to meiosis requires a physiological transition period of as yet unknown nature. In walnut see Fig. All of the sporogenous cells of an anther are, however, connected to neighboring cells by numerous plasmodesmata. This is not simply a separation of the original 2N sets of chromosomes to return to the N number of the original egg and sperm, because during meiosis there occurs crossing over and exchange of parts of paired chromosomes, which is the basis for genetic variation.

There are several definitions of when meiosis begins Bennett, and no agreement on 37 what causes it to begin. Indirect evidence for such progressive accumulation comes from the earlier-mentioned observation that sporogenous cells of wheat anthers take a longer period of time to complete each successive cycle of mitosis. The progress of meiosis is marked by changes in appearance and behavior of the chromosomes, which provide convenient visible boundaries to define and identify stages, beginning with the onset of leptotene of prophase I.

These stages are well known and widely described; here it should be a sufficient refresher to merely illustrate some of them from barley, Hordeum vulgare Figs. Other events, less well known, will receive more emphasis.

Callose secretion coincides with, or may even precede, the onset of meiosis Fig. Callose is usually deposited uniformly around a mmc, although it differs in thickness among species. In cereal grasses, however, it is distributed quite unevenly, extremely thinly between mmc and tapetum but deposited copiously in the center of the pollen sac see sorghum example, Chapter 3.

Fresh unstained callose viewed microscopically appears as a rather translucent gelatin-like substance Fig. Callose is a common secretion from various types of cells in all plant organs. First meiotic division in barley microspore mother cells. Metaphase I. Telophase cell plate not yet formed.

From Ekberg and Eriksson Callose does not sever all cytoplasmic connections, however; some surviving plasmodesmata have been reported to even increase in diameter dur- Chapter 4: Second meiotic division in barley. Late interphase in the dyad vertical cell plate is inconspicuous. Metaphase II. Anaphase II. Telophase II horizontal second cell plates have not formed. Second cell plates define the tetrad of microspores.

Post-meiotic microspore. Such passages have been regarded by some investigators as necessary to maintain synchrony of meiosis. Walters spec- 40 Flowering Plant Embryology Figure 4. Cross section of walnut pollen sac viewed by fluorescence microscopy showing each mmc black spots embedded in callose white.

The still unknown stimulus that initiates meiosis does not spread throughout a pollen sac the same way in all species. In a survey of meiotic patterns in 42 species of dicots and monocots, Neumann found gradients from base to tip, tip to base, and middle to both ends.

Furthermore, many investigators have observed that different pollen sacs in the same anther are usually at somewhat different stages of meiosis. These many variations seem to defeat the formulation of a neat generalization. Although these decreases have not been detected in all of the few species examined, some RNA reduction is expected because removal of the sporophytic genetic coding machinery has been postulated to be necessary before repopulation with gametophytic RNA can occur in the microspores.

Mitochondria and plastids, however, do not decrease in number during meiosis, but they reportedly dedifferentiate to a simpler form that lacks the characteristic substructure of the mature organelles. According to Dickinson they can still synthesize DNA, and they often cluster closely around the nucleus, which suggests that these organelles have a functional relationship with the nucleus during meiosis.

The initiation of, and causal mechanism for, meiosis remains mysterious, but following the protracted prophase of the first meiotic division, all subsequent stages appear similar to those of mitosis.

It can occur in two fundamentally different ways. But contrary to expectation it occurs in only 40 families, of which 35 are monocots Davis, Successive cytokinesis is easy to visualize; Figures 4.

It is far more common for meiosis to produce all four microspore nuclei before any walls form, which creates a temporary 4-nucleate coenocyte cell with multiple nuclei. The two types of cytokinesis are therefore mostly Chapter 4: Simultaneous cytokinesis in Desmodium.

Paraffin sections in A—D. Coenocytic tetrads embedded in callose before furrowing begins. Furrowing in process. Furrowing complete except for cytoplasmic strands arrows.

Tetrads of separate microspores still embedded in callose. Parietal tapetal cells have separated because their cell walls are gone. Live microspore tetrad embedded in clear, unstained gelatinous callose. From Buss et al. Only four dicot and four monocot families have been reported to have species exhibiting both types of cytokinesis. Simultaneous cytokinesis is not as easy to visualize in three dimensions because the furrowing is not a simple two-dimensional quartering process.

Rather than a prose description, furrowing is illustrated, first from a species of Desmodium, a legume with pollen development identical to that of the common cultivated legumes.

Following meiosis, the coenocytic tetrad is still surrounded by callose Fig. Furrowing cleavage begins between each pair of nuclei Fig. Furrowing is eventually completed, but the four separated microspores are retained for some time within the original callose sheath Fig. A second illustration of simultaneous cytokinesis is a more intimate one that follows the organized framework of two sets of microtubules that control certain intracellular movements. After the second meiotic division, the first set of microtubules links all four microspore nuclei, positioning them precisely within the coenocyte.

A second set of microtubules later radiates from each nucleus, and cytokinesis proceeds along planes marked by the interaction of opposing arrays of these two sets of microtubules. The behavior of these microtubules in relation to the microspore nuclei and the planes of cleavage was shown beautifully by Brown and Lemmon in the honeysuckle shrub Lonicera japonica, Caprifoliaceae , and in Impatiens Balsaminaceae Fig.

It is generally agreed that simultaneous cytokinesis has evolved from successive cytokinesis Davis, , which certainly seems a logical progression. But why should the most common mode of cytokinesis occur by furrowing instead of cell plates? The only suggestion has been that the triradiate approximately pyramid-like configuration of the microspores just after they have separated by furrowing physically influences the number and location of the three apertures commonly found in dicot pollen Heslop-Harrison, ; Knox, Cytokinesis in honeysuckle A—C,G and impatiens D—F,H,I microspore mother cells following meiosis, showing microtubules by immuno-fluorescence.

Coenocytic tetrads; microtubules radiate equally from the four nuclei slightly flattened; nuclei in reality are tetrahedral. Cytokinesis begins at periphery of cytoplasm midway between nuclei where opposing systems of microtubules are in contact.

Two focal planes showing microtubules radiating from elongated nuclei to form four brushlike arrays with cleavage planes defined by interaction of opposing microtubules. Advanced cytokinesis; tetrahedrally arranged microspores are separate except in innermost portion where opposing microtubules are still in contact. Spore tetrad in end view; positions of nuclei and brush-like microtubules radiating from nuclear envelopes reflect the postmeiotic pattern.

One separated microspore with elongate nucleus and radiating microtubules. From Brown and Lemmon Chapter 4: Details of Stages question of why a pollen grain needs more than one aperture for germination. It should be mentioned here that in some species among various scattered families the four microspores and resulting pollen never separate from each other.

Even larger aggregations than tetrads may occur, up to the entire pollen complement of a pollen sac. In such plants, which typically have very reliable animal pollinators or are self-pollinating, packages of 4, 8, 16, 32, or even more pollen are shed instead of single pollen grains.

These are not considered here because they are rare among cultivated plants orchids are the most important exception. Knox has described in considerable detail the various kinds of pollen aggregates. This is an easy question to ask, but it poses severe technical problems because observation involves killing and processing the very cells being observed; thus one cell cannot be followed through meiosis.

By laborious sampling methods, estimates have been obtained for about 70 animal and plant species. Among the 39 species, hybrids, varieties, and cultivars of angiosperms that have been studied, the time range of meiosis is from 18— hours, or 0. Here are examples from cultivated plants, as selected from Bennett Petunia hybrida 18 hours Beta vulgaris 24 hours Pisum sativum 30 hours Hordeum vulgare 39 hours Secale cereale 51 hours Vicia faba 72 hours Allium cepa 96 hours Lilium henrii hours Meiosis in the olive tree Olea europaea, Oleaceae is also reported to take about hours Fernandez and Rodriguez-Garcia, Some other species in the lily family take longer than lily and olive.

Why there is such a range of duration times is still fundamentally unknown, but it is known that meiosis 43 can be affected by environmental conditions, most notably temperature; some plants e. Also, most polyploid species, for unknown reasons, take less time for meiosis than their diploid counterparts. Meiosis is always much slower than mitosis in the same plant, however, mostly because of the lengthy prophase of the first meiotic division.

Although more than two types of tapetum have been described, two broad categories are recognized by most workers. These two contrasting tapetal behaviors have been reviewed in great detail elsewhere—e. Information about the tapetum presented here is of a more general nature.

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It is by far the most common type. It is known from only 32 families, of which 21 are monocots Davis, ; Bhandari, Two important exceptions to these tapetal distribution patterns deserve mention. The grasses monocot family Poaceae have been reported to have only a parietal tapetum Kirpes et al. Why two types of tapetum have evolved is unknown.

Some have suggested that the invasive tapetum provides contact with all 44 Flowering Plant Embryology microspores, but this postulated advantage seems unlikely considering the tiny volume of even the largest pollen sacs and the miniscule distances needed for diffusion even from a parietal tapetum.

Parietal tapetal cells typically have two nuclei, but additional nuclei per tapetal cell are also known Wunderlich, ; as an example, common dandelion Taraxacum officinale, Asteraceae has up to 16 nuclei per tapetal cell.

Multinucleate tapetal cells are a highly unusual kind of plant cell. Also unusual are tapetal cells that have only a single conspicuously enlarged nucleus with endoreplicated DNA, which is DNA replicated without mitosis.

In the large legume family the tapetal cells in the subfamily Caesalpinioideae have two to many nuclei, whereas in the subfamilies Mimosoideae and Papilionoideae, which include most cultivated legumes, tapetal cells are reported to always remain uninucleate and often with endoreduplicated DNA Buss and Lersten, Most other investigators have also reported DNA levels above 4C. A study on onion reported that nuclear size and DNA content of tapetal cells increased during meiosis Castillo, These examples of proliferation of tapetal DNA by one mechanism or another suggest strongly that these cells are involved in intense metabolic activity followed by cell death.

Similar behavior is known in both endosperm cells Chapter 9 and in embryo suspensor cells during early stages of embryo growth Chapter This unusual tapetal nuclear behavior is conspicuous, but other changes also occur both in the cell wall and in the cytoplasm.

The tapetal wall might begin to disappear as early as meiosis, and at some post-meiotic stage it is common for at least its inner tangential wall facing the pollen sac to disappear. Tapetal cells of many plants eject the contents of small membrane-bound vesicles onto this exposed cell membrane, which then form discrete granular bodies Fig. The orbicules gradually become coated with sporopollenin, which supports the hypothesis of some workers that the tapetum secretes sporopollenin and contributes to the exine of the pollen wall.

Some of this sporopollenin is hypothesized to Figure 4. Citrus limon vacuolate microspore right abuts naked tapetal cell membrane left , which is studded with orbicules dark spots ; developing pollen wall consists of tectum of exine X supported by columnar bacula B arising from basal nexine layers 1, 2 of the exine; intine I has just begun to be deposited.

Transmission electron micrograph. From Horner and Lersten Details of Stages 45 gested that these extremely small particles could be important in pollinosis, which is a serious allergenic reaction in the lower part of the lungs.

Dickinson concluded that, In fact, apart from meiosis itself, the sequence of events taking place in these two groups of cells [i. They undergo precisely parallel phases of synthesis of protein, lipid, and polymers.

Indeed, the tapetal cells of some species even attempt to form some kind of patterning on their surfaces [this a reference to the orbicules and orbicular wall mentioned earlier]. In view of the complexity of these events, and of the fact that the tapetal tissue is only one cell in depth, it is indeed remarkable that the distinction between these two tissues is maintained.

Figure 4. Sorghum tapetum at vacuolate pollen stage; orbicules with sporopollenin cores arrow mimic pollen exine upper right ; fibrillar material F subtends orbicules in the now degenerating tapetal cell wall. From Christensen et al. Tapetal orbicules are commonly featureless spheroids consisting of a lipid core Steer, a,b but in some species they take on the appearance of the patterned exine of the pollen wall.

This is especially pronounced in many grasses, where sporopollenin accumulates between the originally separate orbicules to form a continuous tapetal membrane Banerjee, ; Christensen et al. The tapetal membrane therefore resembles the outer pollen wall in both physical appearance and time of formation. Tapetal orbicules have a possibly practical significance because they occur on the tapetum of some allergenic plants. Vinckier and Smets illustrated several examples and sug- Dickinson implies here that the tapetum may be some kind of sterilized sporogenous cell layer.

Many insect-pollinated plants eject tapetal substances into the pollen sac that coat the outside of pollen grains. The two recognized types of such tapetal products are pollenkitt and tryphine Dickinson and Lewis, It causes pollen grains to stick to each other and to their insect pollinators. Its chief components, however, are various neutral non-polar lipids, according to a survey of the pollen of 69 species of 28 families Dobson, Tryphines are said to help pollen stick to the stigma, but it is often difficult to distinguish between tryphines and pollenkit.

An example of tryphine was shown in Chapter 3 from radish and mustard see Fig. An example of pollenkitt, occurring as large globules in the tapetal cells of flowering ash, Fraxinus excelsior Oleaceae , can be seen just before these cells rupture Fig.

An appreciation of the amount of pollenkitt 46 Flowering Plant Embryology Figure 4.

Embryology of Angiosperms

Transmission electron micrograph of flowering ash pollen sac; degenerating tapetal cells at left are about to rupture and deposit pollenkitt conspicuous dark oily bodies on pollen grains at right and bottom.

From Hesse Whether the tapetum has the same multiple functions in all species is harder to defend. In the s it was suggested that chromatin actual genetic material moves physically from tapetum to microspores. This proved impossible to defend after a brief period of notoriety.

A study by Moss and HeslopHarrison on maize anthers, as well as several other studies, failed to detect anything moving from tapetum to developing pollen.

What can be said more specifically about tapetal function? There is direct evidence that pollenkitt and tryphines, substances of tapetal origin, coat the pollen of many species, as shown in the previous section and in the mustard family example described in Chapter 3. Certain proteins in this tapetal exudate may become recognition factors that contribute to a compatible pollen germination response on the stigma.

In such species the sporophytic parent, of which the tapetum is a component, therefore indirectly helps the gametophytic pollen grain to gain acceptance by the stigma of the carpel. This tapetal function, however, benefits only mature pollen, its transport, and its subsequent interaction with the stigma. The degeneration of the tapetum, either by dramatic rupture or gradual Chapter 4: Details of Stages 47 Figure 4.

Interior of witch-hazel pollen sac showing mature clumped pollen grains with pollenkitt coating. Scanning electron micrograph. Reznickova and Dickinson reconciled these two func- 48 Flowering Plant Embryology tions by providing evidence, at least for lily, that some of the accumulated tapetal lipids are deposited as pollenkitt and some are hydrolyzed and subsequently transferred to vacuolate pollen grains, where they are converted to food reserves.

Stieglitz produced convincing evidence for a third tapetal function. She was able to separate the tapetum from the rest of the anther and extract the enzyme from it.

She also removed microspore tetrads still encased in callose from pollen sacs and divided them into two sets. One set was treated with the enzyme, and the callose dissolved. The second set went untreated and its callose did not dissolve. Another enzyme phenylalanine ammonialyase has also been found to be almost restricted to the tapetum in tulip Tulipa sp. They regarded this as evidence for tapetal involvement in phenylpropanoid metabolism in the anther because this biochemical pathway eventually produces the flavonoid pigments that are commonly deposited on the pollen wall as a component of tryphine.

This enzyme and its product, a pentahydroxychalcone, were detected by Sutfield et al. The concentration of both substances peaked when microspores began to divide to become vacuolate pollen, after which both enzyme and product dropped rapidly to almost zero by the mature pollen stage.

A recent study of lily using labeled monoclonal antibodies Aouali et al. They speculate that the pectin later helps to form and maintain the pollen tube wall after germination. The tapetum has also been postulated to produce the carotenoid precursors that are synthesized into sporopollenin, the chief component of the exine of the pollen wall, or at least that the tapetum makes a substantial contribution to the pool of sporopollenin precursors. Indirect evidence for this is the sporopollenin coating that forms around orbicules on the tapetal cell surface, as shown in the example of sorghum see Fig.

Not all agree, however. Steer a,b , in two important papers on tapetum development in oats, concluded that this hypothesis is not supported by convincing evidence. At the level of gene action, studies done mostly on mustard Brassica and petunia have identified genes in both tapetum and microspores that code for callase, lipids, flavonols, and oleiosins storage oils , all of which involve tapetal function studies cited in Kapoor et al.

These molecular studies have provided deeper and more precise insight into certain tapetal functions. All of the proven and postulated tapetal functions just described cannot be said to operate in all species, but this versatile cell layer is remarkable for its importance in several critical ways to both developing and mature pollen. More will be said about the tapetum in Chapter 7.

Details of Stages other tetrads because the callose persists. It has been suggested that isolation by callose is necessary for the transition to the gametophyte pollen grain generation to occur without influence from the surrounding sporophyte anther tissue.

It has also been suggested that callose may act as a template, or mold, that influences the configuration and pattern of the future pollen wall. A detailed study of lily microspores by Dickinson and Sheldon did not, however, support this hypothesis. Whether or not this proposed function is true, callose does persist until the microspores form at least the rudiments of what will become the pollen wall.

A young microspore, while still encased in callose, initiates the framework for its distinctive pollen wall, which will be completed much later. The mature pollen wall consists of two distinct and chemically different layers. The outermost layer is the exine, which is composed largely of a durable material called sporopollenin, a polymer that can remain extremely resistant to physical and chemical agents for millions of years under anaerobic conditions.

This is why pollen deposited in oxygen-poor strata—e. This property of inertness has also defied attempts to adequately analyze the chemical composition of sporopollenin Nepi and Franchi, The inner and far less—resistant wall layer is the intine, which is composed largely of carbohydrates, including cellulose, and is similar in some respects to the primary cell wall of a vegetative cell Nepi and Franchi, Exine and intine can each be subdivided into two or more subsidiary layers, but not everyone agrees on how many layers or what to name them, especially because such structural details differ considerably among species.

Pollen wall development has been studied extensively and intensively by light microscopy, electron microscopy, and by various other techniques down to the molecular level. The details of these topics lie mostly outside this book. Instead, a series of observations 49 on sorghum by Christensen et al. Entry into the great number of studies that provide more detailed descriptions of the pollen wall can be found in Dickinson and Sheldon , Knox , Rowley , and Scott At this early tetrad stage the microspores have just a naked cell membrane.

The first visible event is the appearance of some vaguely fibrous material called primexine, which provides a temporary matrix within which the permanent exine will develop Fig.

Denser material can be detected within the primexine as callose begins to Figure 4. Sorghum microspore at early tetrad stage right in callose C ; microspore has only a cell membrane; remnant of tapetal cell wall at left shows sporopollenin-like granules arrow. Sorghum pollen wall development: Late tetrad with first sign of primexine arrow on microspore cell membrane; callose at left. Late tetrad, with patches of primexine arrow visible between membrane right and callose left.

Late tetrad, with continuous primexine arrow. Late tetrad, with differentially stained loci now visible within primexine arrow ; callose disappearing at left. Early vacuolate microspore free of callose; loci in primexine now more prominent arrows.

Loci now evident as bacula in primexine; irregular space separates thin basal layer of primexine arrow from cell membrane. Early vacuolate microspore with growing bacula visible above darkly stained central primexine layer.Mature pollen with intine I now thicker than exine and traversed by numerous cytoplasmic channels; granular zone arrow persists between exine and intine.

Following microspore mitosis the pollen grain has a small generative cell within a large vegetative cell, food reserves begin to accumulate, and the parietal tapetal cells gradually collapse Fig.

The processes of anther dehiscence and pollen dispersal. In cranberry and blueberry also Ericaceae , the pore is at the tip of a long apical tube Fig. These two disciplines had already accumulated a substantial published literature for about years by the end of the 19th century.

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