PLANT MORPHOLOGY - T. A. Sautkina - 2012

CHAPTER 6. PLANT REPRODUCTION

6.4. Reproduction of Higher Plants as a Reflection of the Life Cycle

As follows from the analysis of life cycles in the vast majority of plants, strictly speaking, there is neither purely asexual nor sexual reproduction, but two processes invariably take place: spore formation and the sexual process. As it turns out, reproduction in most plants—that is, the Replication of their own kind—occurs only As a result of the combination of these two qualitatively distinct reproductive processes. In other words, in spore-bearing plants dominated by the gametophyte, reproduction begins with the sexual process, followed by spore formation. Conversely, if the sporophyte dominates The life cycle, spore formation is observed first, followed by the sexual process.

In higher spore-bearing plants, spore formation and the sexual process are temporally separated, while gametophytes and sporophytes are spatially segregated. Consequently, Two Types of reproduction can be distinguished in higher spore-bearing plants: gameto-sporous and sporo-gametous. We believe these terms, introduced by us, adequately reflect the Features of the developmental and reproductive cycles of most evolutionarily advanced Algae and all higher spore-bearing plants.

In seed plants (gymnosperms and angiosperms), spore formation and the sexual process follow one another directly (although there may be a considerable time interval between them) and occur within the confines of the sporophyte. As a result, a special type of reproduction emerges in seed plants: seed reproduction.

6.4.1. Gameto-sporous reproduction

Gameto-sporous reproduction is characteristic of plants in which the gametophyte dominates the life cycle, whereas the sporophyte is strongly reduced and develops upon the gametophyte. Among higher plants, gameto-sporous reproduction is typical only of bryophytes.

Gameto-sporous reproduction, characteristic of bryophytes, can be schematically represented as follows. On the gametophyte, which possesses a thalloid or leafy-stemmed Structure, sex Organs—antheridia and archegonia—are formed. Since bryophytes include both monoecious and dioecious species, sex organs develop either on a single individual or on separate individuals. Gametangia feature a single-layered multicellular wall.

Antheridia, which are elliptical structures, contain spermatogenous tissue at early Selection/3.html">Stages of development, from the Cells of which spermatozoa are formed as a result of mitotic division.

Archegonia represent a flask-shaped structure differentiated into an expanded part—the venter of the archegonium—and a narrowed part—the neck of the archegonium. An egg Cell and a venter canal cell develop within the venter, while neck canal cells develop within the neck. The latter are short-lived and degenerate by the time the egg cell matures; their contents create a moist environment that facilitates the movement of spermatozoa. The maturation of spermatozoa in the antheridium and the egg cell in the archegonium occurs synchronously. The sexual process takes the form of oogamy, with Fertilization taking place in the presence of a liquid droplet medium. The resulting zygote remains dormant for some time and then begins to divide mitotically within the archegonium venter, resulting in The formation of a sporophyte on the gametophyte. In all bryophytes, it is called a sporogonium and represents a variously differentiated "capsule". The most important part of the sporogonium is the sporangium. The shape of sporangia, mechanisms for their dehiscence, and spore dispersal Methods vary among different bryophyte species. A young sporangium is filled with sporogenous tissue, whose cells undergo reduction division to form spore tetrads (meiospores).

Subsequently, the tetrads break down, the spores become isolated, and after the sporangium dehisces, they are shed and dispersed by the wind. The spores of bryophytes, like those of higher spore-bearing plants, are covered by two walls: an outer one, the exospore, and an inner one, the endospore. The exospore consists of Cellulose impregnated with sporopollenin, making the outer wall highly durable and reliably protecting the spore from adverse conditions. The endospore is a semipermeable membrane surrounding the living Contents of the spore. The exospore features an aperture through which a protonema—representing the early developmental stage of the gametophyte—bulges during spore germination. It can be filamentous or plate-like. Buds form on the protonema, from which thalloid or leafy-stemmed gametophytes develop (see Fig. 141).

Thus, as a result of the combination of two processes—the sexual process and spore formation—offspring are produced that are identified as specific species of bryophytes. Bryophytes are the only group of higher plants in which the spore fulfills not only the function of dispersal but also the function of species propagation.

6.4.2. Sporo-gametous reproduction

Sporo-gametous reproduction is a widespread type of reproduction in higher spore-bearing plants in which the sporophyte dominates the life cycle.

In sporo-gametous reproduction, which is characteristic of both homosporous and heterosporous clubmosses, horsetails, and ferns, The process of propagation begins with the formation of sporangia on the sporophytes. Sporangia develop on specialized leaves—sporophylls (see Fig. 134)—or on trophic leaves (see Fig. 136) that combine the photosynthetic function with the reproductive one.

In horsetails, sporangia are formed on sporangiophores of cauline origin (see Fig. 135). The structure, dimensions, modes of formation, and dehiscence mechanisms of sporangia vary.

At Cytology/cytology/16.html">Early stages of development, sporangia are filled with sporogenous cells. Spores are formed as a result of the reduction division of sporogenous cells (meiospores). As in bryophytes, the spores are coated with two walls, and only in horsetails are there remnants of a third wall (epispore) that form so-called elaters, which facilitate joint spore dispersal. (Some horsetails may exhibit physiological heterospory when spores encounter varying environmental conditions.)

In homosporous species of clubmosses, horsetails, and ferns, bisexual prothalli (gametophytes) are formed, reaching no more than 0.5 cm in diameter (see Figs. 134–136). The prothalli exist independently of the sporophyte. They can be aerial and green—thus feeding autotrophically—or subterranean and colorless. Such prothalli establish a Symbiosis with Fungi and feed mycotrophically.

In heterosporous species (genera Selaginella, Isoëtes, Marsilea, Salvinia), microspores and megaspores are formed, which differ in morphological and physiological-biological properties. Unisexual prothalli develop from them: male prothalli from microspores, and female prothalli from megaspores. These prothalli are highly reduced, especially the male ones. Often, male prothalli consist of only a single vegetative cell and a single antheridium. The greater reduction of the male prothallus compared to the female one can be explained by the fact that its primary function is solely The production of male Gametes—spermatozoa—whereas the female prothallus must also ensure the survival of the zygote and The Development of the embryo, since the latter is initially incapable of independent existence (see Fig. 142).

The sexual process is oogamy. For the successful locomotion of spermatozoa and the fertilization process, the presence of a liquid droplet medium is mandatory. The zygote, arising after fertilization within the archegonium, soon begins to develop into the embryo of the new generation, i.e., a new sporophyte. Initially, it is nourished at the expense of the prothallus's nutrient reserves, and subsequently, after the differentiation of embryonic organs—the embryonic ROOT, stem, and leaves—it transitions to independent autotrophic Nutrition.

In higher spore-bearing plants, the asexual (spore formation) and sexual processes are separated not only spatially but also temporally. It may take from 1 to 3 weeks for a prothallus to develop from a spore. The prothallus can exist from 10 to 15 years prior to fertilization. If fertilization of the egg cell does not occur, the prothallus dies.

Thus, offspring identifiable as particular species are produced exclusively as a result of the combination of two processes: spore formation and the sexual process.

6.4.3. Seed reproduction

Seed reproduction is characteristic only of representatives of two divisions—Gymnosperms and Angiosperms (or flowering plants). Representatives of these divisions exhibit heterospory, but their microsporangia and megasporangia differ significantly from the sporangia of higher spore-bearing plants.

Microsporangia possess a multilayered wall that reliably protects the microspores (meiospores) developing as a result of Meiosis, as well as the microsporangia derivatives formed from them—highly reduced male gametophytes, namely pollen grains (pollen).

Gymnosperms and angiosperms develop specialized structures known as ovules, the central part of which—the nucellus—represents a macrosporangium (megasporangium). Within the nucellus, as a result of

meiosis, a single tetrad of macrospores is typically formed. The female gametophytes of gymnosperms and angiosperms differ in structure, yet in both taxa they develop inside the ovular nucellus and are well-protected. In gymnosperms, two archegonia form on the female gametophyte, each containing a single egg cell. In angiosperms, due to the severe reduction of the female gametophyte, archegonia are absent; instead, the egg cell develops within a specialized female gametophyte known as the embryo sac.

In gymnosperms and angiosperms, both processes—spore formation and sexual reproduction—follow one another very rapidly, that is, practically without interruption or with a very short interval. In other words, whereas in spore-bearing plants the Asexual and sexual processes Complement each other, in seed plants they become inseparable, giving rise to a distinct type of reproduction: seed reproduction.

The seed is a complex structure consisting of an embryo (the rudiment of a new Organism), a supply of nutrients, and a seed coat (spermoderm) that serves a protective function.

Seeds enable both the dispersal and propagation of seed plants. Seed reproduction offers A number of advantages over gametophyte-spore and spore-gametophyte types of reproduction. Firstly, the Sexual process in gymnosperms and angiosperms occurs via siphonogamy (spermatozoids or sperm cells are delivered to the egg cell by a pollen tube), which allows fertilization to take place independently of liquid Water. Secondly, embryo development in most species is completed on the parent plant, ensuring optimal development and enhanced viability. The seed contains a substantial nutrient reserve, making the embryo relatively independent of external environmental conditions during early stages of development. And finally, thirdly, the seed embryo remains dormant and is protected by seed coats, which further increases its viability.

Gymnosperms evolved complex structures—male and female cones (strobili)—in which microsporangia and macrosporangia are formed and all processes related to seed production take place, whereas angiosperms developed a specialized reproductive organ: the flower.