Development and structure of germ cells. Sexual reproduction of plants A brief listing of the adaptations of gymnosperms and angiosperms to life on land

Gametogenesis(from Greek. gamete- wife, gametes- husband and genesis- origin, occurrence) is the process of formation of mature germ cells.

Since sexual reproduction most often requires two individuals - female and male, producing different sex cells - eggs and sperm, then the processes of formation of these gametes should be different.

The nature of the process also largely depends on whether it occurs in a plant or animal cell, since in plants only mitosis occurs during the formation of gametes, while in animals both mitosis and meiosis occur.

Development of germ cells at plants. In angiosperms, the formation of male and female germ cells occurs in different parts of the flower - stamens and pistils, respectively.

Before the formation of male germ cells - microgastogenesis(from Greek. micros- small) - happening microsporogenesis, that is, the formation of microspores in the anthers of the stamens. This process is associated with the meiotic division of the mother cell, which results in four haploid microspores. Microgametogenesis is associated with a single mitotic division of the microspore, giving a male gametophyte of two cells - a large vegetative(siphonogenic) and shallow generative. After division, the male gametophyte is covered with dense shells and forms a pollen grain. In some cases, even in the process of pollen maturation, and sometimes only after being transferred to the stigma of the pistil, the generative cell divides mitotically with the formation of two immobile male germ cells - sperm. After pollination, a pollen tube is formed from a vegetative cell, through which spermatozoa penetrate into the ovary of the pistil for fertilization (Fig. 2.55).

The development of female germ cells in plants is called megagametogenesis(from Greek. megas- big). It occurs in the ovary of the pistil, which is preceded by megasporogenesis, as a result of which four megaspores are formed from the mother cell of the megaspore lying in the nucellus by meiotic division. One of the megaspores divides mitotically three times, giving the female gametophyte, an embryo sac with eight nuclei. With the subsequent isolation of the cytoplasms of the daughter cells, one of the resulting cells becomes an egg, on the sides of which lie the so-called synergids, three antipodes are formed at the opposite end of the embryo sac, and in the center, as a result of the fusion of two haploid nuclei, a diploid central cell is formed (Fig. 2.56).

Development of germ cells at animals. In animals, two processes of the formation of germ cells are distinguished - spermatogenesis and oogenesis (Fig. 2.57).

spermatogenesis(from Greek. sperm, spermatos- seed and genesis - origin, occurrence) is the process of formation of mature male germ cells - spermatozoa. In humans, it occurs in the testes, or testes, and is divided into four periods: reproduction, growth, maturation and formation.

AT breeding season primordial germ cells divide mitotically, resulting in the formation of diploid spermatogonia. AT growth period spermatogonia accumulate nutrients in the cytoplasm, increase in size and turn into primary spermatocytes, or spermatocytes of the 1st order. Only after that they enter meiosis ( ripening period) which results in the formation of two secondary spermatocyte, or spermatocyte of the 2nd order, and then - four haploid cells with quite a lot of cytoplasm - spermatids. AT formation period they lose almost all of the cytoplasm and form a flagellum, turning into spermatozoa.

spermatozoa, or gummies,- very small mobile male sex cells with a head, neck and tail (Fig. 2.58).

AT head, apart from the core, is acrosome- a modified Golgi complex, which ensures the dissolution of the membranes of the egg during fertilization.

AT neck there are centrioles of the cell center, and the basis ponytail form microtubules that directly support the movement of the spermatozoon. It also contains mitochondria, which provide the sperm with ATP energy for movement.

Ovogenesis(from Greek. UN- an egg and genesis- origin, occurrence) is the process of formation of mature female germ cells - eggs. In humans, it occurs in the ovaries and consists of three periods: reproduction, growth and maturation. Periods of reproduction and growth, similar to those in spermatogenesis, occur even during intrauterine development. At the same time, diploid cells are formed from the primary germ cells as a result of mitosis. oogonia, which then turn into diploid primary oocytes, or oocytes of the 1st order. Meiosis and subsequent cytokinesis occurring in ripening period, are characterized by uneven division of the cytoplasm of the mother cell, so that as a result, at first one is obtained secondary oocyte, or oocyte of the 2nd order, and first polar body and then from the secondary oocyte, the ovum, which retains the entire supply of nutrients, and the second polar body, while the first polar body is divided into two. Polar bodies take away excess genetic material.

In humans, eggs are produced with an interval of 28-29 days. The cycle associated with the maturation and release of eggs is called the menstrual cycle.

Egg- large female sex cell, which carries not only a haploid set of chromosomes, but also a significant supply of nutrients for the subsequent development of the embryo (Fig. 2.59).

The egg in mammals is covered with four membranes, which reduce the likelihood of damage to it by various factors. The diameter of the egg in humans reaches 150-200 microns, while in an ostrich it can be several centimeters.

An adult plant, like all living organisms, is capable of reproducing new organisms of the same species as the plant itself. reproduction is an increase in the number of similar organisms. Reproduction is one of the properties of life, it is inherent in all organisms. Due to reproduction, the species can exist for a very long time.

Plants are capable of sexual and asexual reproduction.

AT asexual reproduction only one individual is involved, and it occurs without the participation of germ cells. At the same time, the daughter organisms are identical in their properties with the parent organism. In plants, asexual reproduction is represented by vegetative reproduction and reproduction by spores.

Reproduction by spores is found in algae, mosses, ferns, horsetails and club mosses. Spores are small cells covered with a dense membrane. They are able to endure adverse environmental conditions for a long time. When they get into favorable conditions, they germinate and form plants.

At sexual reproduction the fusion of female and male sex cells occurs. Daughter organisms differ from parent organisms. The process of cell fusion is called fertilization.

Sex cells are also called gametes. The female gametes are eggs, male - sperm(motile, in seed plants) or spermatozoa (motile, in spore plants).

As a result of fertilization, a special cell appears - zygote- which contains the hereditary properties of the egg and sperm. The zygote gives rise to a new organism.

Although the daughter organism is similar to the parents, it always has some new features that none of the parent organisms have. This is the main difference between sexual and asexual reproduction. Thus, sexual reproduction provides a group of organisms of the same species with different properties. This increases the group's chances of survival.

In flowering plants, fertilization is quite difficult. He is called double fertilization, since not only the egg is fertilized, but also another cell.

Sperms are formed in pollen grains, which, in turn, mature in the anthers of stamens. The ovules are produced in the ovules, which are located in the ovary of the pistil. Seeds develop from ovules after fertilization of the egg by sperm.

For fertilization to occur, the plant must be pollinated, that is, the pollen must land on the stigma of the pistil. When a grain of pollen falls on the stigma, it begins to germinate through the stigma and style into the ovary, forming a pollen tube. At this time, two spermatozoa are formed in the dust grain, which move towards the tip of the pollen tube. The pollen tube penetrates the ovule.

In the ovule, one cell divides and elongates to form the embryo sac. It contains an egg and another special cell with a double set of hereditary information. The pollen tube grows into this embryo sac. One sperm fuses with the egg, forming a zygote, and the other with a special cell. The embryo of a plant develops only from the zygote. From the second fusion, nutritional tissue (endosperm) is formed. This provides the embryo with nutrition during germination.

Double fertilization occurs only in flowering (angiosperms) plants. It was discovered in 1898 by S.G. Navashin.

The vital activity of a living organism is impossible without reproduction. Through reproduction, an increase in the number of individuals in flora. There are three ways of plant reproduction - vegetative, asexual and sexual.
sexual reproduction fundamentally different from vegetative and asexual. The sexual process in the plant world is extremely diverse and often very complex, but essentially boils down to the fusion of two sex cells (gametes) - male and female.

Gametes occur in certain cells or organs of plants. In some cases, the gametes are the same in size and shape, and both are motile due to the presence of flagella (isogamy); sometimes they differ somewhat from each other in size (heterogamy). But more often - with the so-called oogamy - the sizes of gametes are sharply different: the male gamete, called the spermatozoon, is small, mobile, and the female - the egg - is immobile and large.
The process of fusion of gametes is called fertilization. Gametes have one set of chromosomes in their nucleus, and in the cell formed after the fusion of gametes, which is called a zygote, the number of chromosomes doubles. The zygote germinates and gives rise to a new plant.

The sexual process is carried out in plants at a certain time and at a certain stage of its development, during which the plant can also reproduce asexually (with the formation of spores) and vegetatively.
Sexual reproduction arose in the plant world in the process of evolution.

Sexual reproduction of plants (reproduction by seeds) is used in horticulture when growing rootstocks (seedlings), rarely when propagating some stone fruits (apricot, peach, sometimes cherry) and on a large scale when breeding new varieties by crossing.

Fertilization occurs in the flower of the plant. Flowers appear when the plant is sufficiently developed and reaches a certain period of life. A flower is a shoot with shortened internodes, the leaves of which have changed and turned into separate parts of the flower attached to the receptacle, which is a shortened stem. The flower usually has a pedicel, which is the lower part of the flower shoot.

For seed plants, it is characteristic that megaspores, which are formed one at a time in megasporangia, remain with them on the mother plant; the germination of megaspores, the development of the female gametophyte, fertilization by male gametes developing in the germinating microspore, one way or another transferred to the megasporangia or to the leaf producing it - the megasporophyll, takes place there. Immediately after fertilization, the development from the zygote of a new plant, the sporophyte, begins, and, unlike ferns and others, the preserved and modified megasporangia turns into a seed containing the embryo and nutrient reserves for its further development. This seed, having separated from the mother plant, in the majority, after a certain period of dormancy (a break in development), germinates into a new plant. For dispersal, distribution, plants are, therefore, not spores, as in typical spore plants, but seeds; there is no asexual reproduction by spores, the alternation of generations is not clearly expressed and is revealed only by comparative morphological and cytological studies.

The sporophylls of angiosperms, closely crowded at the ends of the shoots and in the majority surrounded by still metamorphosed apical leaves, form a flower together with them; we can characterize it as a short shoot, the leaves of which are metamorphosed in connection with sexual reproduction taking place here in the flower. Sporophylls sharply differentiate into microsporophylls producing microspores and megasporophylls producing megaspores; on superficial acquaintance, it seems that they perform sexual functions. Due to the fading of the alternation of generations and the strong reduction of gametophytes that do not lead an independent lifestyle, it turns out that the plant itself, the sporophyte, reproduces sexually. Therefore, often, but inaccurately, a flower is called the organ of sexual reproduction of plants, microsporophylls - male genital organs, megasporophylls - female genital organs. From the point of view of comparative morphology and homologation of individual parts of the flower, this is incorrect.

The terminology of the individual parts of the flower was developed at a time when the homologation of flower parts with the corresponding organs of higher spore plants was out of the question (for the first time such homologation was carried out in the works of the outstanding German botanist Hofmeister in the 50s of the last century). Therefore, parts of the flower received special names, which are retained by habit and at the present time. Microsporophylls are called stamens, microsporangia are pollen nests, microspores are dust particles, megasporophylls are carpels, megasporangium is an ovule, and the female growth is an embryo sac. The apical leaves, where they surround the sporophylls, are called the perianth, subdivided in many plants into an outer, usually green calyx, and an inner, usually larger and differently colored corolla.

Sexual reproduction of angiosperms

Germination of pollen. The pollen that ripens in the anthers looks like tiny grains. Therefore, it received the name - pollen grain. Once on the stigma of the pistil, the pollen grain begins to germinate and forms a long tube - the pollen tube.

Gradually, the tube passes between the cells of the stigma, style and reaches the ovule.

Unlike the pollen of insect pollinated plants, which has a variety of spines and outgrowths, the pollen of wind pollinated plants is small, light, and smooth. How does it stay on the stigma of the pistil and not be blown away by the wind, not thrown off by insects scurrying around in the flower? It turns out that the stigma of the pestle secretes a sticky, sugary substance that causes the pollen to stick to the pestle. It is also believed that the pistil secretes a certain substance that is specific to the pollen of a given plant species and that prevents the development of foreign pollen.

Not only does the pistil influence the germination of pollen, but the pollen also influences the pistil. The germinating pollen also releases special substances that cause the ovary and other parts of the flower to grow into fruit. Therefore, in many plants, fruit growth is the better, the more pollen gets on the stigma.

The structure of the ovule and fertilization. So, in the ovary of the pistil there is one or more ovules. Outside, the ovule is surrounded by covers that do not close in one place, forming a pollen entrance. Inside the ovule is the embryo sac, which contains several cells. The most important are the central cell and the ovum.

Sex cells are called gametes. Accordingly, the egg is the female gamete, and the sperm is the male gamete.

When the pollen tube enters the embryo sac through the pollen passage, one of the sperm fuses with the egg. The fusion of two sex cells - the egg and the sperm - is called fertilization. As a result of fertilization, a zygote is formed (from the Greek zygote - paired). The second sperm fuses with the central cell. It turns out that two identical sperm merge with two completely different cells. This process occurs only in flowering plants. The Russian scientist S. G. Navashin discovered, described and explained this process. He called it double fertilization.

Formation of seed and fruit. After fertilization, the zygote divides many times and forms an embryo. In the embryo, the embryonic root, embryonic stem and bud (shoot) are clearly distinguishable. If there were many ovules in the ovary, then there will be many seeds in the fruit.

Simultaneously with the formation of seeds, the wall of the ovary also grows. A fruit is formed from it, or, more correctly, the walls of the fetus - the pericarp. In flowers with several pistils, the ovaries of each pistil grow. They can remain free, or they can grow together. In many plants, other parts of the flower (apple, strawberry) also participate in the formation of fruits.

The central cell, having merged with the sperm, also divides many times and forms the endosperm. The endosperm is a special tissue in the cells of which the reserves of nutrients necessary for the development of the embryo accumulate. From the integument of the ovule, a seed coat is formed, which protects the embryo from external influences.

On some seeds, one can also discern a trace of pollen entering the ovule. To do this, you can soak the bean seed, and when it swells, lightly press with your fingers. A drop of water will appear from a small hole. This is where the pollen outlet was.

Spermatophyta (Greek sperma - seed) is the most prosperous group of land plants. In this section we will focus on those adaptations of the seed plants that have made them flourish, and, in addition, we will compare them with the lower organized groups that we have already considered.

Seed plants appear to have evolved from extinct seed ferns. If we recall Selaginella (as one of the representatives of ferns), then it should be noted that it has essentially the same life cycle as that of seed plants; the only difference is that in Selaginella the female gametophyte is autotrophic, while in seed plants it loses autotrophy. However, let's forget about Selaginella and try to compare the life cycle of seed plants and isosporous ferns (common ferns).

One of the main difficulties that plants face on land is related to the vulnerability of the gametophyte generation. So, for example, in ferns, the gametophyte is a tender growth that forms male gametes (spermatozoa) that need water to reach the egg. But in seed plants, the gametophyte is protected and very much reduced. Only by comparing the life cycles of seed plants and more primitive plants can one understand that seed plants also retain alternation of generations. Seed plants have three very important advantages: 1) heterosporousness, 2) seed production, and 3) the appearance of non-swimming male gametes.

Heterospore

A very important step on the path of evolution from ferns to seed plants was the appearance of plants that form spores of two types - microspores and megaspores. Such plants are called heterosporous; they were discussed in sect. 3.4. In table. Figure 3.6 provides a brief glossary of terms related to sporulation in the life cycle of heterosporous plants (see also Figure 3.26). All seed plants are heterosporous.

The male gametophyte develops from the microspore, and the female gametophyte develops from the megaspore. In both cases, the gametophyte is very much reduced and does not emerge from the spore. The exception is the free-living independent gametophyte of isosporous plants such as Dryopteris. The spore protects the gametophyte from drying out, which is an important adaptation for life on land. Gametophytes are not capable of photosynthesis, so they need the nutrient reserves accumulated in spores by the previous sporophyte generation. As we shall see later, the ultimate reduction of the gametophyte is observed in flowering plants.

Megaspores are formed in megasporangia on megasporophylls, and microspores - in microsporangia on microsporophylls. In seed plants, the structure equivalent to the megasporangium is called ovule. Inside the ovule, only one megaspore develops, or one female gametophyte, which is called embryo sac. The structure equivalent to a microsporangium is called pollen sac. The pollen sac contains many microspores called pollen grains or specks of dust.

seed evolution

In seed plants, the megaspores do not separate from the sporophyte. In contrast to the picture that we observe in more primitive heterosporous organisms, megaspores remain inside the ovules (megasporangia) attached to the sporophyte. Inside the megaspore, a female gametophyte (embryo sac) develops and one or more female gametes, or eggs, are formed. After fertilization of the female gamete, the ovules are already called seed. Thus, a seed is a fertilized ovule. The ovule, and later the seed, has a number of advantages:

1. The female gametophyte is protected by the ovule, entirely dependent on the parent sporophyte, but much less susceptible to dehydration than the free-living gametophyte.

2. After fertilization, a supply of nutrients is formed, which the gametophyte receives from the parent sporophyte plant, from which it is still not separated. This reserve is used by the developing zygote (next sporophyte generation) after seed germination.

3. Seeds are adapted to withstand adverse conditions and can remain dormant until conditions are favorable for germination.

4. Seeds can develop various adaptations to facilitate their distribution. The seed is a complex structure in which the cells of three generations are assembled - the parent sporophyte, the female gametophyte, and the embryo of the next sporophyte generation. In its most general form, this is shown in Fig. 3.34. The parent sporophyte provides the seed with everything it needs for life, and only after the seed has fully matured, i.e., has accumulated a supply of nutrients for the sporophyte embryo, does it separate from the parent sporophyte.

Evolution of non-swimming male gametes and water-independent fertilization

For sexual reproduction of plants, which we have already considered, it is necessary that the spermatozoon can swim up to the egg, that is, water is needed. Therefore, seed plants face certain problems. In order for fertilization to occur, male gametes must reach female gametes, and, as we have already said, male and female gametes develop separately, and besides, female gametes also remain inside the sporophyte ovules. Male gametes are produced by male gametophytes inside microspores, or pollen grains. They do not turn into floating spermatozoa, but remain immobile and are transported together with pollen grains from pollen sacs (microsporangia) to the ovules. This transfer of pollen is called pollination. At the last stage of pollination, pollen tube, which grows towards the ovule; through this tube, immobile male gametes reach the egg, and fertilization occurs. Sperms do not need water at any of these stages. Only in some primitive seed plants, such as the cycads, do spermatozoa emerge from the pollen tubes, indicating a definite relationship with non-seed plants. On fig. 3.34 compares the life cycles of seed and some non-seed plants. The origin of seeds and the relationship between sporophyte and gametophyte generations are highlighted. Pollination may not provide any benefit, since this process is purely random and difficult to achieve, and education a large number pollen is biologically unfavorable. It is believed that initially pollination occurred only with the help of wind. However, already at the dawn of the evolution of flowering plants, the first flying insects appeared (about 300 million years ago, in the Carboniferous period). The possibility of more efficient pollination by insects immediately arose. One of the groups of seed plants - flowering plants - most successfully uses this opportunity.

3.12. The chances of wind-borne pollen grains (microspores) surviving and producing are much less than Dryopteris spores. Why?

3.13. Explain why megaspores are large and microspores are small.

3.5.1. Main features and systematics of Spermatophyta

The main features and systematics of Spermatophyta are presented in Table. 3.8.

This table considers two groups of seed plants - gymnosperms and angiosperms. The last group is usually called flowering plants. In gymnosperms, the ovules, and then the seeds, are located on the surface of special leaves, which are called megasporophylls or seed scales. These scales are collected in cones. In angiosperms, the seeds are closed, which even better protects the gametophyte and the zygote that is then formed. The structures that contain the seeds are called carpels. The carpels are thought to be equivalent to megasporophylls (leaves) folded so that they cover the ovules (megasporangia). There may be one carpel or there may be many.

The hollow base of a carpel, or group of carpels fused together, is called ovary. In the ovary are the ovules. After fertilization, the ovary is called fruit, and the ovules seeds. Either fruits or seeds (sometimes both) often have different dispersal devices.

On fig. 3.35 as simple circuits various spore-bearing structures of vascular plants are shown for comparison. The comparison will help you understand some of the terms that have been used in the presentation of this material.

3.5.2. Class Gymnospermae - gymnosperms, e.g. conifers, cycads, yews, ginkgos

The main features of Gymnospermae are listed in Table. 3.8.

Gymnosperms are a thriving group of plants distributed throughout the globe; Gymnosperm forests make up about a third of all forests on the planet. Among the gymnosperms are trees or shrubs, mostly evergreen with needle-like leaves. The largest group is conifers, which include trees that grow in high latitudes and spread north further than all other trees. Conifers are of great economic importance, primarily as a source of ornamental wood, which is used not only to obtain sawn and timber, but also to obtain resin, turpentine and wood pulp. Conifers include pines, larches (with needles falling for the winter), fir, spruce and cedar. Consider a typical coniferous tree, the Scotch pine (Pinus sylvestris).

Pinus sylvestris is distributed in Central and Northern Europe, the USSR and North America. It has also been introduced to Great Britain, but under natural conditions it grows only in Scotland. Pines are grown both for decorative purposes and for timber and lumber production. It is a beautiful majestic tree up to 36 m high with a characteristic exfoliating pinkish or yellowish-brown bark. Pines most often grow on sandy or poor mountain soils, and therefore their root system usually spreads over the surface and branches heavily. Appearance pines is shown in fig. 3.36.

Each year, a new whorl of branches grows from the whorl of lateral buds at the top of the trunk. The characteristic conical appearance of Pinus and other conifers is due to the fact that the whorls of younger (and shorter) branches at the top from top to bottom are gradually replaced by older (and longer) ones. With age, the lower branches die off and fall off, so the trunks of old trees are usually devoid of branches (Fig. 3.36).

The main branches and trunk continue to grow from year to year due to the growth of the apical bud. Therefore, they say that conifers are characterized unlimited growth. Scale-like leaves are arranged in a spiral; in the axils of such leaves there are buds from which very short branches (2-3 mm long) develop, called short shoots. These are the stems limited growth, on top of which grows two leaves. As soon as the shoot grows, the scale-like leaf at its base disappears, and a scar remains in its place. Leaves are like needles, which reduces the surface area of ​​the leaf, and hence the loss of water. In addition, the leaves are covered with a thick, waxy cuticle and the stomata are deeply embedded in the leaf tissue, another adaptation for water conservation. Evergreen xeromorphic adaptations ensure minimal water loss during cold seasons when water freezes and is difficult to extract from the soil. After two or three years, the shortened shoots fall off along with the leaves, and another scar remains in their place.

The tree is a sporophyte and is heterosporous. In spring, both male and female cones form on the tree. The diameter of male cones is about 0.5 cm; they are rounded and arranged in clusters at the base of new shoots under the apical bud. They are formed in the axils of scaly leaves instead of shortened shoots. The female cones appear in the axils of scaly leaves at the end of new strong shoots at some distance from the male cones and are arranged more randomly. The full development of cones takes three years, so all cones are of different sizes, and cones from 0.5 to 6 cm in size can be found on one tree. Young buds are green, turning brown or reddish brown in their second year. Both male and female cones consist of sporophylls tightly pressed together, arranged in a spiral around a central axis (Fig. 3.36).

On the underside of each sporophyll of the male cone are two microsporangia, or pollen sacs. Inside the pollen sacs, meiotic division of pollen mother cells occurs and pollen grains, or microspores, are formed. Pollen grains have two air sacs that help them be carried by the wind. In May, the buds turn completely yellow due to the pollen that flies out of them in a whole cloud. At the end of summer they wither and fall off.

The sporophyll of the female cone consists of a lower covering scale and a larger upper scale that bears the ovules. On the upper surface of a large scale, there are two ovules nearby; meiotic division of the mother cell of the megaspore occurs in them and four megaspores are formed, of which only one develops further. Pollination occurs as early as the first year of cone development, but fertilization is delayed until the following spring, when the pollen tubes germinate. Winged seeds are formed from fertilized ovules. They continue to ripen during the second year and fall asleep only in the third year. By this time, the cone has become quite large, woody, and the scales are bent outward before the wind blows the seeds.

3.5.3. Class Angiospermae - angiosperms, or flowering plants

The main features of Angiospermae are listed in Table. 3.8.

Angiosperms are better than other plants adapted to life on land. They appeared in the Cretaceous period, about 135 million years ago, quickly multiplied, having mastered a variety of habitats, and soon replaced gymnosperms, occupying a dominant position among terrestrial vegetation. Some angiosperms have returned to a freshwater lifestyle, and a few species have even returned to a saltwater lifestyle.

One of the most characteristic features of angiosperms, apart from the closed seeds we have already discussed, is the appearance of flowers instead of cones. The presence of flowers allowed these plants to attract insects for pollination, and sometimes even birds and bats. The bright color of the flowers, fragrant aroma, edible pollen and nectar are all means to attract animals. In some cases, insects cannot do without flowers at all. The evolution of insects and flowers has in a number of cases been very closely linked, resulting in very different, very specific, and, moreover, mutually beneficial relationships. Flower adaptations have generally been aimed at maximizing the chances for pollen transport by insects and are therefore more reliable than wind pollination. Plants pollinated by insects do not require as much pollen as wind pollinated plants. However, many flowering plants have adapted to wind pollination.

Life cycle

The life cycle of a typical flowering plant is shown in Fig. 3.37.

The main purpose of this drawing is to compare the life cycle of a flowering plant with the life cycles of more primitive plants. The life cycle itself will be described in detail in Sect. 20.2. In essence, it almost does not differ from the cycle shown in Fig. 3.21. Pay special attention to the stages when meiosis or mitosis occurs. Gametes are formed as a result of mitosis, and spores - as a result of meiosis, as in all other plants with a change of generations. Strictly speaking, a flower is an organ of both asexual and sexual reproduction, since spores (asexual reproduction) are formed in it, inside which gametes arise (sexual reproduction). It should be noted that the pollen grain is a spore and not a male gamete, as it contains male gametes. As mentioned above, pollen grains carry male gametes to the female reproductive organs, and this eliminates the need for floating sperm.

The process of endosperm development is also shown in Fig. 3.37. Nutrient reserves are formed from the endosperm, and the very method of their formation is unique and inherent only to angiosperms.

Dicotyledonous and monocotyledonous

Angiosperms are divided into two large groups, which can be given the status of classes or subclasses, depending on which systematic scheme is used. Most often, these two groups are called monocots and dicots. In table. 3.9 lists the main features by which they differ. Few of these traits individually are of systematic importance, since there are numerous exceptions, and only a combination of several traits allows such plants to be accurately identified. According to modern ideas, monocots are a more perfect group; it is believed that they probably evolved from primitive dicots.

Angiosperms are herbaceous(i.e. not lignified) and woody. Woody plants are shrubs and trees. In such plants, a large amount of secondary xylem (wood) is formed, which serves as an internal support for the trunk and, in addition, performs the functions of a conductive tissue. Xylem arises from the activity of cambial cells. Herbaceous plants, or herbs, rely only on the turgidity of cells and on a small amount of mechanical tissues such as collenchyma, sclerenchyma or xylem; no wonder that they themselves are not very large. Herbaceous plants either do not have a cambium at all, or, if they do, their activity is negligible. Many herbaceous plants annual, i.e. they complete their development cycle from seed to seed in one year. Some herbaceous plants produce perennial organs such as bulbs, corms, or tubers that overwinter or survive adverse conditions such as drought (Section 20.1.1). In this case, they are biennial or perennial, i.e. they either form seeds in the second year and die off, or live year after year. Shrubs and trees are perennials and can be either evergreen, i.e., they form and shed foliage all year round, and therefore there are always leaves on the plant, or deciduous, i.e., they completely shed their leaves in cold or dry times. To illustrate how diverse angiosperms are, Fig. 3.38-3.42 shows the structure of some representatives of this class.

Rice. 3.39. The structure of the flower and vegetative organs of a monocotyledonous herbaceous plant - meadow fescue (Festuca pratensis). This perennial plant, 30-120 cm tall, forms large tufts and is found throughout the UK in water meadows, pasturelands, old pastures and along roadsides. The second leaves in the figure are indicated in gray. The leaves, as a rule, are arranged in two rows alternately on one or the opposite side of the stem. A. The structure of the vegetative organs. AT node there is a meristem from which the leaf and internodes grow; not hollow unlike internodes. For leaf blade characterized by parallel venation. Ears are small pointed protrusions (not all cereals have). Stem unbranched; quickly elongates just before flowering, and then it is called straw. Second leaf vagina cylindrical and partially closes the internode between the second and third nodes. adventitious roots grow from the base of the stems; form fibrous root system without tap root. Young Escape with not yet elongated internodes; nodes are located close to each other and hidden in the vagina at the base of the shoot. The stem is formed by nodes and internodes, and the leaf is formed by a leaf blade and a sheath. B. The structure of the inflorescence. C. Details of the structure of a single open flower, or floret; two small petal-like structures (films, or lodicules) that cover the ovary are not shown

3.5.4. A brief listing of the adaptations of gymnosperms and angiosperms to life on land

We have already touched upon the problems associated with the transition from an aquatic to a terrestrial way of life in Sec. 3.3. Now that we have met representatives of all the main groups of land plants, we can return to this issue again and discuss why the gymnosperms and angiosperms have adapted so well to life on land. The main advantage of these plants over all others, of course, is related to their method of reproduction. There are three main aspects here:

1. The gametophyte generation is very reduced. The gametophyte is completely dependent on the sporophyte and is always under its protection. But in mosses and liverworts, in which the gametophyte predominates, and in ferns, which have a free-living growth, the gametophyte is not protected and dries out very easily.

2. Unlike all other plants, in which spermatozoa swim up to the eggs, angiosperms do not need water for fertilization. The male gametes of seed plants are immobile and carried by the wind or insects along with pollen grains. At the final stage of pollination, male gametes penetrate to the egg through the pollen tube, while the eggs themselves are enclosed inside the ovules.

3. Of all modern plants, only seed plants have special seed structures. The emergence of the seed became possible due to the fact that the ovules, together with all their contents, remain on the parent sporophyte.

Other characteristics of angiosperms that help them live on land are listed below. We will discuss them in more detail in the relevant sections of this book.

a) In all vascular plants, the xylem and sclerenchyma tissues are lignified and provide internal support. Many seed plants exhibit secondary growth and the deposition of large amounts of wood (secondary xylem). These plants include shrubs and trees.

b) Real roots, which are also characteristic of vascular plants, allow you to effectively extract moisture from the soil.

c) These plants are protected from drying out by the epidermis and the water-insoluble cuticle or cork formed during secondary thickening.

d) The epidermis of terrestrial organs, and especially the epidermis of leaves, is permeated with stomata, which contributes to better gas exchange between the plant and the atmosphere. e) Plants also have other adaptations for life in hot, waterless places (xeromorphic adaptations); These devices will be discussed in Sect. 18.2.3 and 19.3.2.