SPORE WALL DEVELOPMENT IN ANDREAEA (MUSCI: ANDREAEOPSIDA)

The spore wall of Andreaea rothii (Andreaeopsida) is unique among mosses studied by transmission electron microscopy. The exine of other mosses is typically initiated on trilaminar structures of near unit membrane dimensions just outside the plasma membrane. The exine of Andreaea is initiated in the absence of such structures as discrete globules within the coarsely fibrillar network of the sporocyte wall. The sequence of wall layer development, nevertheless, is essentially like that of other mosses. The intine is deposited within the exine and the perine accumulates on the surface of the exine during the latter stages of spore maturation. The mature spore is weakly trilete and inaperturate. The wall consists of three layers, the inner intine, the spongy exine consisting of loosely compacted irregular globules of sporopollenin, and an outer layer of perine. The perine differs ultrastructurally from the exine only in its greater degree of electron opacity. This ultrastructural evidence of departure from the fundamental pattern of exine development in mosses supports the taxonomic isolation of Andreaea from mosses of the Sphagnopsida and Bryopsida.     

SPORE WALL FORMATION AND CHLOROPLAST DEVELOPMENT DURING SPOROGENESIS IN THE MOSS FISSIDENS LIMBATUS

The sequence of wall formation in spores of Fissidens limbatus Sullivant is as follows: The exine is formed around the protoplasts after the sporocyte has undergone meiosis. The fully enlarged spores then become coated by the perine; this is followed by intine formation. The source of the intine and exine appears to be from within the spore, but the perine is of an apparent exogenous origin. Ornamentation of the spore is due solely to deposition of the perine. Each spore originally has a single plastid. Plastids increase in number by fission, resulting in mature spores with numerous plastids with well differentiated lamellae.     

THE POLLEN WALL OF CANNA AND ITS SIMILARITY TO THE GERMINAL APERTURES OF OTHER POLLEN

The pollen wall of Canna generalis Bailey is exceptionally thick, but only a minor part of it contains detectable amounts of sporopollenin. The sporopollenin is in isolated spinules at the exine surface and in the intine near the plasma membrane. There is no sporopollenin in the > 10 μ thick channeled region between spinules and intine. We suggest that the entire pollen wall of C. generalis is similar to the thick intine and thin exine typical for germinal apertures in many pollen grain types. Considered functionally, the Canna pollen wall may offer an infinite number of sites for pollen tube initiation and would differ significantly from grains that are inaperturate in the sense of an exine lacking definite germinal apertures.     

ULTRASTRUCTURE OF SPOROGENESIS IN THE MOSS,AMBLYSTEGIUM RIPARIUM I. MEIOSIS AND CYTOKINESIS

Emphasis is placed on three aspects of meiosis in the moss Amblystegium riparium (Hedw.) BSG: 1***) nature of the sporogenous layer; 2) prophasic microtubules and polarity; and 3) cleavage pattern. Spore tetrads develop while still encased by archesporial cell walls. The cellular nature of the sporogenous layer differs from the more usual occurrence of free sporocytes released into a common spore sac. Two important events mark the establishment of sporocyte polarity during meiotic prophase: 1) migration of the four plastids to the distal tetrad poles (telophase II poles); and 2) ingrowth of the sporocyte wall in eventual cleavage planes between the tetrad poles. An extensive, plastid-based microtubule system is associated with organelle migration during the establishment of sporocyte polarity in meiotic prophase. Disruption of the nuclear envelope in prometaphase I occurs at sites opposite the four plastids where microtubules extend from plastid envelope to nuclear envelope. Formation of a cell plate following the first meiotic division results in a dyad, whereas in many mosses meiosis is completed in the undivided sporocyte and is followed by simultaneous cleavage into a spore tetrad. Spore cleavage is accomplished by vesicular coalescence resulting in septa that coincide with the prophasic wall ingrowths.     

MICROTUBULES ASSOCIATED WITH SIMULTANEOUS CYTOKINESIS OF COENOCYTIC MICROSPOROCYTES

Microtubule arrays associated with simultaneous cytokinesis in the coenocytic microsporocytes of Lonicera japonica and Impatiens sultani were studied by indirect immunofluorescence. The future division planes are not predicted prior to meiosis by either a preprophase band of microtubules or cytoplasmic lobing. Cleavage planes appear to be determined by position of the four haploid nuclei and the development of postmeiotic microtubule systems. Perpendicular second division spindles in Lonicera result in tetrahedrally arranged tetrads while parallel spindles in Impatiens result in tetragonal arrangement. Immediately following meiosis bands of microtubules, the secondary spindles, develop between both sister and nonsister nuclei. These arrays give way to systems of microtubules that radiate equally from each of the four nuclei in the coenocytic sporocyte. Simultaneous cytokinesis is initiated by centripetal wall deposition at the periphery of the sporocyte and proceeds along planes marked by interaction of the opposing arrays of nuclear-based microtubules.     

Studies on Timmiella barbuloides (Brid.) Moenk., IV. SEM and TEM characterization of spore wall and first germination stages

Abstract

The spore wall morphology of Timmiella barbuloides (Pottiales, Musci) is described. The spores are catalept, with an ornamentation pattern consisting of unevenly spaced, shortly pedunculated pilum-and gemma-like processes. The spore coat consists of three, unevenly thick layers: intine, exine, and perine. The exine is not involved in wall ornamentation, the processes consisting of perine only. The leptoma, a spore coat area involved in germination, consists of an intine markedly thickening in an area of thinning exine and, outside, with a spore coat area where perinous processes become sparse. On the basis of observations and of the data reported in recent literature the classical definition of the leptoma is modified. It is considered to be a structurally specialized, but not necessarily thin, area.     

Chemical Oxidation (Using Ozone) of the Spore Wall of Lycopodium Clavatum

Spore walls of Lycopodium clavatum, after oxidation using ozone for varying times and treatment with dilute alkali, showed a striking loss of the outer layer (muri) of the exine. Almost all trace of the muri and undetermined portions of the remaining exine wall were removed after 20 hours' oxidation. The exine wall material seems to be evenly removed. The only degradation patterns observed are rare pits after 20 hours oxidation. A degradation-related colour change of the spore wall occurs after 11 to 16 hours' oxidation.     

Sporoderm in Populus and Salix

Four phenomena were observed in a study of Populus tremula and P. tremula f. gigas microspores from before microspore mitosis through mature pollen which may have general significance in the ontogeny of pollen grains: 1) The exine and orbicules (Ubisch bodies) were covered by membranes. 2) The exine and the tapetal surfaces where orbicules form were covered by a polysaccharide (PAS positive) coat until after microspore mitosis; subsequently the tapetum became plasmodial. 3) Material having the staining characteristics of the nexine 2 (endexine in the sense of Fægri) accumulated on membranes in microspores in the space between the exine and the plasma membrane. That material was almost completely gone from the wall in mature pollen. The membranes on which material had accumulated migrated through the exine. Following passage through the exine these membranes were seen as empty fusiform vesicles in micrographs of anthers prepared by commonly used methods. 4) At about microspore mitosis when the cellulosic intine begins to form, microtubules about 240 A in diameter occurred near the plasma membrane and generally parallel with it. Positive acid phosphatase reactions in tapetal cells together with the morphology of orbicules and other tapetal organelles suggest that the wall of orbicules, which is like the pollen exine, may form as a residual product of a lysosome system.

Sections of mature Salix humilis pollen were compared with Populus.     

Development of structure-less pollen wall inCeratophyllum demersum L. (Ceratophyllaceae)

Developmental process of structure-less exine is studied in a hydrophilous plant,Ceratophyllum demersum L., with electron microscopy. The plant shows a characteristic feature in tetrad formation. A callose wall is not synthesized and exine initiation does not occur during the tetrad stage. After release of microspores, a trilaminar layer with two electron-dense lines is formed in the surface of each microspore. The trilaminar layer develops to a thin structure-less exine that is considered to consist of only an endexine. The unusual exine would be an adaptive feature for submersed pollination in fresh water.     

AN ULTRASTRUCTURAL STUDY OF SPORE WALL MORPHOGENESIS IN EQUISETUM ARVENSE

Spore wall morphogenesis of Equisetum arvense was observed by transmission electron microscopy. The spore wall of E. arvense consists of four layers: intine, exine, middle layer, and elater. The exine is formed after meiosis and consists of two distinct layers. The inner portion of the exine is formed in advance of the outer layer of the exine. The middle layer is deposited after the exine. The elater can be subdivided into two distinct layers. The inner layer comprises longitudinal microfibrils that surround the spore in spiral fashion. The elater appears as thin beltlike structures at the beginning of development. Numerous microtubules were observed on the inner surface of the plasmodial plasma membrane opposite the inner layer of the elater, suggesting that these microtubules are involved with the synthesis of inner elater microfibrils. The matrix of the outer elater is formed by discharge of granules from the plasmodial cytoplasm. The intine is the last component of the sporoderm to be formed.     

Exine Development in Illicium Religiosum Sieb. et Zucc. (Illiciaceae)

The exine development in Illicium was investigated using transmission electron and field emission scanning electron microscopy. The protectum and procolumellae appear on protruding sites of the microspore cytoplasm in the early tetrad stage. The protectum takes the form of a reticulate pattern with perforations within the callosic wall. After dissolution of the callosic wall, the central part of muri rises to form tectal ridges. The developing tectum, shows an echinate appearance in sectional view and has perforations at both sides around each lumen. There are two kinds of columellae; those forming continuous rings around each lumen and others which are individual rods standing beneath the tectum. The present developmental study in Illicium showed that the initial simple reticulate pattern formed within the callosic wall develops into the complex reticulate exine pattern of the differentiating tectum during the free microspore stage. The tectum has an angular shape with perforations and is supported by the two kinds of columellae.     

Nuclear cytoplasmic domains,microtubules and organelles in microsporocytes of the slipper orchid Cypripedium californicum A. Gray dividing by simultaneous cytokinesis

Microsporocytes of the slipper orchidCypripedium californicum A. Gray divide simultaneously after second meiosis. The organization and apportionment of the cytoplasm throughout meiosis are functions of nuclear-based radial microtubule systems (RMSs) that define domains of cytoplasm - a single sporocyte domain before meiosis, dyad domains within the undivided cytoplasm after first meiosis, and four spore domains after second meiosis. Organelles migrate to the interface of dyad domains in the undivided cytoplasm after first meiotic division, and second meiotic division takes place simultaneously on both sides of the equatorial organelle band. Microtubules emanating from the telophase II nuclei interact to form columnar arrrays that interconnect all four nuclei, non-sister as well as sister. Cell plates are initiated in these columns of microtubules and expand centrifugally along the interface of opposing RMSs, coalescing in the center of the sporocyte and joining with the original sporocyte wall at the periphery to form the tetrad of microspores. Organelles are distributed into the spore domains in conjunction with RMSs. These data, demonstrating that cytokinesis in microsporogenesis can occur in the absence of both components of the typical cytokinetic apparatus (the preprophase band of microtubules which predicts the division site and the phragmoplast which controls cell-plate deposition), suggest that plant nuclei have an inherent ability to establish a domain of cytoplasm via radial microtubule systems and to regulate wall deposition independently of the more complex cytokinetic apparatus of vegetative cells.     

Pollen morphology and pollen-wall proteins (localization and enzymatic activity) inSesamothamnus lugardii (Pedaliaceae)

The pollen grains ofSesamothamnus lugardii Stapf (Pedaliaceae of subdesert regions of SE tropical Africa) are associated in acalymmate tetrads (cross wall cohesion), with a tectate and perforate exine and 8–12 colpi. The pollen wall consists of an ectexine with a complete, perforate and ample tectum, columellated infratectum and clearly interrupted and fragmented foot layer. The endexine is built of scanty lamellae and granules. The intine is bistratificate, with a homogeneous, fibrillate layer (endintine or intine-2) and a heterogeneous, more lax and channeled layer (exintine or intine-1). Test for glycoprotein is particularly positive in the homogeneous internal intine and channels of external intine. On the other hand acid phosphatase has been localized in the exine and channeled external intine layers. These observations confirm the general interpretation of the distribution of wall compounds.     

Release of proteins from the pollen wall of Linum grandiflorum

Summary The emission of proteins from the pollen wall of Linum grandiflorum stained with Coomassie blue was followed directly in moistened grains as well as in pollen prints. Within the first minute of the grain being moistened exine-borne proteins emerged from both inter-apertural and apertural sites; subsequently, proteins of a different nature were discharged from the apertures only. In a fraction of the grains the release of intine proteins was not preceded by that of exine proteins. Pin and thrum pollen did not differ in terms of mode or site of this protein emission. The presence and emergence of exine proteins from the apertures is explained by the process of infolding of the colpal wall at desiccation and its expansion at rehydration, which causes an initial trapping and subsequent re-exposure of surface materials. This explanation may also account for the occurrence of poral sporophytic proteins in the pollens of many dictoyledons.     

ULTRASTRUCTURE OF SPOROGENESIS IN A MOSS,DITRICHUM PALLIDUM. III. SPORE WALL FORMATION

Ultrastructural evidence indicates that marked cytoplasmic polarity occurs during wall and aperture ontogeny in spores of the moss (Musci), Ditrchum pallidum (Hedw.) Hampe. Shortly after cytokinesis, an extensive system of microtubules underlies the entire distal spore surface where exine deposition is initiated. These microtubules appear to be focused on the plastid. The apposition of slips nearly of membrane dimension contributes to the forming exine. As the lamellate exine thickens and extends to the proximal surface, the plastid and associated nucleus migrate to the proximal surface where an elaborate system of microtubules involved in aperture development is generated. The exine gradually loses its stratiform character, becoming homogenous and eventually papillate. At maturity, the spore wall consists of four layers, the outermost perine, the exine, a separating layer, and the intine. The aperture is a complex, localized modification of these layers on the proximal surface. It consists of a pore containing a fibrillar material surrounded by a thin annulus.     

ABCG26-Mediated Polyketide Trafficking and Hydroxycinnamoyl Spermidines Contribute to Pollen Wall Exine Formation in Arabidopsis

Pollen grains are encased by a multilayered, multifunctional wall. The sporopollenin and pollen coat constituents of the outer pollen wall (exine) are contributed by surrounding sporophytic tapetal cells. Because the biosynthesis and development of the exine occurs in the innermost cell layers of the anther, direct observations of this process are difficult. The objective of this study was to investigate the transport and assembly of exine components from tapetal cells to microspores in the intact anthers of Arabidopsis thaliana. Intrinsically fluorescent components of developing tapetum and microspores were imaged in intact, live anthers using two-photon microscopy. Mutants of ABCG26, which encodes an ATP binding cassette transporter required for exine formation, accumulated large fluorescent vacuoles in tapetal cells, with corresponding loss of fluorescence on microspores. These vacuolar inclusions were not observed in tapetal cells of double mutants of abcg26 and genes encoding the proposed sporopollenin polyketide biosynthetic metabolon (ACYL COENZYME A SYNTHETASE5, POLYKETIDE SYNTHASE A [PKSA], PKSB, and TETRAKETIDE α-PYRONE REDUCTASE1), providing a genetic link between transport by ABCG26 and polyketide biosynthesis. Genetic analysis also showed that hydroxycinnamoyl spermidines, known components of the pollen coat, were exported from tapeta prior to programmed cell death in the absence of polyketides, raising the possibility that they are incorporated into the exine prior to pollen coat deposition. We propose a model where ABCG26-exported polyketides traffic from tapetal cells to form the sporopollenin backbone, in coordination with the trafficking of additional constituents, prior to tapetum programmed cell death.     

Some characteristics of pollen wall cytochemistry and ultrastructure in Japanese larch (Larix leptolepis Gord.)

Summary The mature pollen of Larix leptolepis Gord. (Conifer) contains five different cell types, and the plasma membrane of the vegetative cell is continuous and organized. The pollen wall is composed of two morphologically and cytochemically distinct domains: the exine and the intine. In the multilayered exine, the ektexine appears granular and the endexine, lamellar. The intine is thick and bilayered with a microfibrillar structure occupying its inner portion. Cytochemical reactions of the exine and the intine are similar to those found in angiosperms. Pollen wall involvement in the male female recognition system is discussed with respecl to the angiosperms.     

POLLEN WALL AND TAPETAL ORBICULAR WALL DEVELOPMENT IN SORGHUM BICOLOR (GRAMINEAE)

Pollen wall development in Sorghum bicolor is morphologically and temporally paralleled by the formation of a prominent orbicular wall on the inner tangential surface of the tapetum. In the late tetrad stage, a thin, nearly uniform primexine forms around each microspore (except at the pore site) beneath the intact callose; concurrently, small spherical bodies (pro-orbicules) appear between the undulate tapetal plasmalemma and the disappearing tapetal primary wall. Within the primexine, differentially staining loci appear, which only develop into young bacula as the callose disappears. Thus, microspore walls are devoid of a visible exine pattern when released from tetrads. Afterwards, sporopollenin accumulates simultaneously on the primexine and bacula, forming the exine, and on the pro-orbicules, forming orbicules. Channels develop in the tectum and nexine, and both layers thicken to complete the microspore exine. Channeled sporopollenin also accumulates on the orbicules. A prominent sporopollenin reticulum interconnects the individual orbicules to produce an orbicular wall; this wall persists even after the tapetal protoplasts degenerate and after anthesis. While the pollen grains become engorged with reserves, a thick intine, containing conspicuous cytoplasmic channels, forms beneath the exine. Fibrous material collects beneath the orbicular wall. The parallel development and morphological similarities between the tapetal and pollen walls are discussed.     

SPORE WALL ULTRASTRUCTURE IN THE LIVERWORT ATHALAMIA HYALINA

Mature spores of Athalamia hyalina (Marchantiales, Cleveaceae) were examined with both scanning and transmission electron microscopes. Single, hollow, dome-like projections, sometimes having small pores and a coarsely granular surface texture, stud the spore surface, usually in a pattern of concentric circles. In section, the spore wall has an intine and two-layered exine. Intine-like material separates some lamellae of the inner exine, which is joined to the outer exine around the dome bases. Inner exine lamellae are composed of thin (5–6 nm), closely parallel membrane-like subunits. The outer exine is formed from a single large highly modified and doubly-coated lamella, the undulations of which form the surface domes. Dome cavities often are filled with a loose network of granular material.     

EVOLUTION OF EXINE STRUCTURE IN THE POLLEN OF PRIMITIVE ANGIOSPERMS

Diversity in the structure of the exine in 35 families of the ranalean complex is compared through a series of representative scanning electronmicrographs, and evolutionary trends in exine structure of primitive angiosperms are outlined, along with discussion of the significance of these data for understanding the evolution of exine structure in flowering plants as a whole. In order to reduce ambiguity in the palynological literature, it is suggested that persons undertaking light microscope studies of unstained, acetolyzed pollen grains adopt the morphological terms sexine-nexine in describing pollen wall layers while restricting their use of the chemically defined terms ektexine-endexine largely to pollen studies carried out with the transmission electron microscope. This study emphasizes that a clear understanding of the palynological concept of structure versus sculpturing is a necessary prerequisite for the taxonomic/ phylogenetic use of pollen wall morphology. Finally, data from investigation of a number of ranalean families of primitive angiosperms support the conclusion that the direction of a recurrent and major evolutionary trend in exine structure of flowering plants proceeds from pollen that is tectate-imperforate to tectate-perforate pollen to semitectate pollen, and more rarely, to pollen grains that are intectate.