Peter R. Bell

The rôle of the tapetum in the formation of sporopollenin-containing structures during microsporogenesis in Pinus banksiana.

SummaryIn the microsporangium of Pinus the outer layer of the peritapetal membrane and the pro-orbicular cores are not only formed in a similar manner, but are composed of apparently identical materials. Precursors for this lipoidal material are produced by the tapetal protoplasts, as are the precursors of sporopollenin. Production the precursors is sequential and appears to involve different cytoplasmic structures.The sporopollenin synthesised by the tapetum condenses upon the pro-orbicular cores, the peritapetal membrane, the exine initials and, on fragmentation of the tapetum, parts of the disintegrating cytoplasm. The evident unpolarised nature of the tapetal protoplasts, and the sequential nature of the synthesis of the lipoid and the sporopollenin by them, may point to orbicule formation in gymnosperms being a necessary by-product of the development of the peritapetal membrane.

Megaspore Abortion: A Consequence of Selective Apoptosis?

A remarkable feature of megasporogenesis, as seen in the heterosporous fern Marsilea and almost all seed plants, is the regular death of three of the products of meiosis. A general cause of this phenomenon has been hard to identify. Theories invoking causal gradients in the megasporangium or tetrad lack experimental support and could not be applicable universally. However, megaspore death can be regarded as an example of apoptosis (programmed cell death), a phenomenon becoming well established as an essential element in the development of both animals and plants. The genes controlling apoptotic phenomena in animals are now being identified and their products characterized. If megaspore death in seed plants is correctly interpreted as apoptotic, a comparable genetic control can be expected. A hypothesis is presented that illustrates how such a control could have come about. Experiments by Correns on the reproduction of Mirabilis, an autogamous plant with a single ovule in each ovary, have indicated that only 75% of the ovules contain embryo sacs and that only 25% of the pollen is capable of effecting fertilization. These results are in agreement with the kind of control proposed in my hypothesis.

The development of nuclear vacuoles during meiosis in plants

Vacuoles formed by the invagination of the inner membrane of the nuclear envelope have been observed during meiotic prophase in a wide range of plants. In the angiosperm Lycopersicon their formation was found to coincide with the completion of synaptonemal complex formation, and this timing is analogous to that observed during this stage in the silkworm Bombyx. The implications of this activity in relation to the process of chromosome movement are discussed. In the gymnosperm Pinus, the heterosporous fern Marsilea and homosporous ferns Pteridium and Dryopteris the formation of nuclear vacuoles begins much earlier, coinciding with the condensation of chromatin during leptotene. They enlarge and become more elaborate as meiosis proceeds, and may eventually become detached from the nuclear envelope. It is therefore thought unlikely that theyfulfil functions connected with chromosome movement in the manner proposed for the silkworm and the tomato. During diplotene/diakinesis they contain electron-opaque granules and fibrils, and the possible origin and significance of this material is discussed.

Interaction of Nucleus and Cytoplasm during Oogenesis in Pteridium aquilinum (L.) Kuhn

By means of auto-radiography, the synthesis of deoxyribonucleic acid has been followed throughout oogenesis in Pteridium aquilinum, the deoxyribonucleic acid having first been made radioactive by feeding the gametophyte with tritiated thymidine. It has been found that in the mature egg the deoxyribonucleic acid is dispersed throughout the whole cell. The failure of the nucleus of the mature egg to stain with the Feulgen reagent at this stage is believed to be a consequence of this dispersal of the deoxyribonucleic acid. Quantitative comparisons have been made of the amounts of radioactivity in the egg and in various nuclei of the archegonium which were expected on developmental grounds to contain the same amount of labelled deoxyribonucleic acid. Evidence is presented that the egg contains twice as much deoxyribonucleic acid as the non-gametic nuclei, and it is suggested that its chromosomes have entered the prophase condition.

The cytochemistry of the walls of the spermatocytes of Ceratopteris thalictroides.

SummaryBy means of staining and fluorescence microscopy the walls of the spermatocytes of Ceratpteris thalictroides have been shown to contain callose and lipid. It is suggested that these substances form a barrier to the ingress of metabolites, and are responsible for the failure of differentiating spermatocytes to incorporate C14-cysteine.

The occurrence of a multilayered structure in the sperm of a pteridophyte

SummaryA multilayered structure, previously recorded only in bryophytes, is reported in spermatids of three species of Equisetum. It is interpreted as comprising four layers, recalling the Vierergruppe of Marchantia spermatids. Unlike the multilayered structure of bryophyte spermatids, a rather compact organelle, that of Equisetum forms a thin strip extending almost the whole diameter of the cell. As in bryophytes the upper layer of the Equisetum multilayered structure is composed of parallel microtubules, extending far beyond the underlying layers. The microtubular band is considered equivalent to the similar structure seen in a variety of plant spermatozoids, but in Equisetum it is much longer and composed of many more microtubules than reported from any other plant spermatozoid. The morphology of the multilayered structure is related to the large size and short, broad form of Equisetum spermatozoids.

The Alternation of Generations

Publisher Summary In species with monosporic embryo-sacs the mitochondria and plastids are often concentrated in the chalazal region of the megaspore mother cell, resulting in an unequal division between the spores. This has been held to account for the promotion of the chalazal spore, but in Myosurus and some other species there is no evident inequality in the distribution of organelles. In Capsella and some Orchidaceae , the chalazal region of the wall of the mother cell and of the subsequent surviving megaspore becomes labyrinthine, leading to the suggestion that this spore alone receives nutrients adequate for development. This view is strengthened by the presence in Oenothera , in which the micropylar megaspore survives, of plasmodesmata in the corresponding micropylar portion of the wall of the mother cell. It should be noted that megaspores in seed plants are often smaller than the microspores. “Androspore” and “gynospore” would be a more accurate terminology.

The Development of the Male Gametophyte and Spermatogenesis in Taxus baccata L.

The development of the male gametophyte of Taxus baccata has been studied over a period of 20 weeks, from germination of the microspore in February to spermatogenesis in July. A few days after germination the microspore nucleus divides and a transverse wall forms at the equator cutting off the small generative cell and a large tube cell. The latter immediately begins to expand to form the pollen tube. The first division thus establishes the polarity of the gametophyte and the generative cell is regarded as proximal. The transverse wall is ephemeral, and within six weeks it has disappeared. The nucleus of the generative cell divides while still at the proximal pole. The two daughter nuclei are unequal in size, but they remain associated and together move distally. The larger nucleus eventually becomes the nucleus of the spermatogenous cell, and the smaller the sterile nucleus. The spermatogenous cell acquires a distinctive cytoplasm and becomes surrounded by a wall which arises de novo. The nucleus of the spermatogenous cell enlarges, but always remains towards one side of the cell so that at mitosis the spindle is contained within one hemisphere. After division the wall of the spermatogenous cell is ruptured and the two sperms are released as naked nuclei of equal size. The cytoplasm of the spermatogenous cell degenerates as it enters the tube, but remains recognizable until fertilization.

Phytoferritin in the reproductive cells of a fern, Pteridium aquilinum (L.) Kuhn

Electron-opaque aggregates of various forms are described from dedifferentiated plastids in the egg, spore mother cells and young spores of Pteridium. X-ray microanalysis shows them to be phytoferritin. The phytoferritin seems to be generally similar to that described from other vascular species, and fungi, but is unusual in the presence of readily detectable amounts of calcium. The aggregates of phytoferritin in Pteridium are not evidently crystalline, but crystallinity is nevertheless revealed by optical diffractometry. The existence of amorphous phytoferritin is questioned.

Evidence for the association of acyl transferases with the production of nuclear evaginations in maturing eggs of the fern Dryopteris filix-mas

SummaryUtilizing the ability of free CoA to reduce ferricyanide, evidence is presented that acyl transferase activity is associated with the nuclear envelope in the maturing egg of the fern Dryopteris filix-mas. This activity is particularly marked in the nuclear evaginations. This strengthens the view that the evaginations are distinct structures formed by localized growth of the envelope, and not transient extensions of the nucleus.