General characteristics. The first gymnosperms appeared at the end of the Devonian period about 350 million years ago; they probably descended from ancient ferns that became extinct at the beginning of the Carboniferous period. In the Mesozoic era - the era of mountain building, raising the continents and dry climate - the gymnosperms reached their peak, but already from the middle of the Cretaceous lost their dominant position to the angiosperms. As part of gymnosperms, classes are distinguished:

ginkgo (Ginkgophyta);

Gnetoform (Gnetophyta);

Cycads (Cycadophyta);

*Coniferous or Pine

The department of modern gymnosperms includes more than 700 species. Despite the relatively small number of species, gymnosperms have conquered almost the entire globe. In the temperate latitudes of the Northern Hemisphere, they form coniferous forests, called taiga, over vast expanses.

Modern gymnosperms are represented mainly by trees, much less often by shrubs and very rarely by lianas; there are no herbaceous plants among them. The leaves of gymnosperms differ significantly from other groups of plants not only in shape and size, but also in morphology and anatomy. In most species, they are acicular (needles) or scaly; in some representatives they are large (for example, in the amazing Velvichia, their length reaches more than 2-3 m), pinnately dissected, bilobed, etc. The leaves are arranged singly, two or more in bunches.

The vast majority of gymnosperms are evergreen, mono- or dioecious plants with a well-developed stem and root system formed by the main and lateral roots. They spread by seeds, which are formed from ovules. The ovules are naked (hence the name of the department), located on megasporophylls or on seed scales collected in female cones.

In the cycle of development of gymnosperms, there is a successive change of two generations - the sporophyte and the gametophyte with the dominance of the sporophyte. The gametophytes are greatly reduced, and the male gametophytes of the holo- and angiosperms do not have antheridia, which sharply differs from all heterosporous seedless plants.

Gymnosperms include six classes, two of which have completely disappeared, and the rest are represented by living plants. The most preserved and most numerous group of gymnosperms is the class Conifers, numbering at least 560 species that form forests in vast expanses of Northern Eurasia and North America. The largest number of species of pine, spruce, larch is found along the coasts of the Pacific Ocean.

Coniferous class. All conifers are evergreen, rarely deciduous (for example, larch) trees or shrubs with needle-like or scale-like (for example, cypress) leaves. Needle-shaped leaves (needles) are dense, leathery and hard, covered with a thick layer of cuticle. The stomata are immersed in depressions filled with wax. All these features of the structure of the leaves provide a good adaptation of conifers to growth in both arid and cold habitats.

Conifers have erect trunks covered with scaly bark. On the transverse section of the stem, developed wood and less developed bark and pith are clearly visible. The xylem of conifers is 90-95% formed by tracheids. Cones of conifers are dioecious; plants - more often monoecious, less often - dioecious.

The most widespread representatives of conifers in Belarus and Russia are Scotch pine and Norway spruce, or European. Their structure, reproduction, alternation of generations in the development cycle reflects characteristics all conifers.

Scotch pine- monoecious plant (Fig. 9.3). In May, bunches of greenish-yellow male cones 4-6 mm long and 3-4 mm in diameter form at the base of young pine shoots. On the axis of such a cone are multilayer scaly leaves, or microsporophylls. On the lower surface of the microsporophylls are two microsporangia - pollenbag , where pollen is produced. Each pollen grain is provided with two air sacs, making it easier for the pollen to be carried by the wind. There are two cells in the pollen grain, one of which subsequently, when it hits the ovule, forms a pollen tube, the other, after division, forms two sperm cells.

Rice. 9.3. Development cycle of Scots pine: a - branch with cones; b- female cone in section; c - seed scales with ovules; G - ovule in section; e - male cone in the context; e - pollen; well - seed scales with seeds; 1 - male cone; 2 - young female cone; 3- bump with seeds; 4 - cone after rash of seeds; 5 - pollen input; 6 - cover; 7 - pollen tube with sperm; eight - archegonium with ovum; nine - endosperm.

On other shoots of the same plant, reddish female cones are formed. On their main axis there are small transparent covering scales, in the axils of which large thick, subsequently lignified scales sit. On the upper side of these scales there are two ovules, each of which develops female gametophyte - endosperm with two archegoniums with a large egg in each of them. At the top of the ovule, protected from the outside by the integument, there is a hole - the pollen inlet, or micropyle.

In late spring or early summer, mature pollen is carried by the wind and falls on the ovule. Through the micropyle, pollen is drawn into the ovule, where it grows into the pollen tube, which penetrates to the archegonia. The two spermatozoa formed by this time travel through the pollen tube to the archegonium. Then one of the sperm merges with the egg, and the other dies. From a fertilized egg (zygote) a seed embryo is formed, and the ovule turns into a seed. Pine seeds ripen in the second year, spill out of the cones and, picked up by animals or the wind, are transported over considerable distances.

In terms of their importance in the biosphere and role in human economic activity, conifers occupy the second place after angiosperms, far surpassing all other groups of higher plants.

They help solve huge water protection and landscape problems, serve as the most important source of wood, raw materials for the production of rosin, turpentine, alcohol, balms, essential oils for the perfume industry, medicinal and other valuable substances. Some conifers are cultivated as ornamental (firs, arborvitae, cypresses, cedars, etc.). Seeds of a number of pines (Siberian, Korean, Italian) are eaten, oil is also obtained from them.

Representatives of other classes of gymnosperms (cycads, gnets, ginkgos) are much rarer and less known than conifers. However, almost all types of cycads are decorative and are widely popular with gardeners in many countries. Evergreen leafless low shrubs of ephedra (gneta class) serve as a source of raw materials for the production of the alkaloid ephedrine, which is used as a means of stimulating the central nervous system, as well as in the treatment of allergic diseases.

General characteristics. Features of the life cycle. reproductive organs. Strobili (bumps)

Class Seed ferns - Pteridospermae

Bennettite class - Bennettitopsida

Ginkgo class - Ginkgoopsida

Class Conifers - Pinopsida

Subclass Cordaitidae - Cordaitidae

Subclass Conifers - Pinidae

Araucaria family - Araucariales

Cypress family – Cupressales

Family Taxodiaceae – Taxodiaceae

Yew Order – Taxales

Pine family – Pinales

Division Gymnosperms - Pinophyta, or Gymnospermae

Gymnosperms, like angiosperms, are the main producers of the planet's terrestrial ecosystems, differing from spore plants in that their main means of settlement are not spores, but seeds. The seed is the life stage of the sporophyte, a special formation in which the future adult sporophyte, the embryo, as well as the supply of nutrients, the endosperm, is in a compact and protected state from adverse conditions.

“Internal fertilization, the development of the embryo inside the ovule, and the emergence of a new, extremely effective unit of settlement - the seed - are the main biological advantages of seed plants, which made it possible for them to more fully adapt to terrestrial conditions and achieve a higher development than ferns and other seedless higher plants. If during reproduction by spores each time a huge number of them, usually millions, is formed, then during reproduction by seeds, their number is many times less. A seed is a more reliable unit of dispersal than a spore. The seed already contains an embryo - a tiny sporophyte with a root, bud and germinal leaves (cotyledons), a supply of nutrients and the necessary enzymatic apparatus. A seed is truly a small masterpiece of evolution ”(Takhtadzhyan A.L. Plant life. V.4. 1978. P.258).

Wood consists only of tracheids (with the exception of representatives of the Gnetovye class). The leaves are narrow (needle-shaped) or scaly, although there are genera with broad leaves.

The heyday of gymnosperms is the Mesozoic, they have reached our time in a limited variety. At the same time, modern gymnosperms are clearly divided into 2 groups: the 1st includes the class Cycads - Cycadopsida and Ginkgos - Ginkgoopsida. These are "living fossils". 2nd group - Conifers, which are the main gymnosperms.

Gnetovye stand apart - Gnetopsida, which are classified as gymnosperms with a considerable degree of conventionality.

Life cycle features

Gymnosperms have an increasing tendency to care for the gametophyte. So not only the female gametophyte does not leave the microspore shell, but the macrospore remains in the macrosporangium, so the female gametophyte does not come into contact with the external environment, maintains a constant connection with the sporophyte; the male gametophyte is even more reduced, as in heterosporous plants it develops in the shell of the microspore, the multicellular antheridia are replaced by a new formation from auxiliary vegetative prothallial (from the Greek. prothallium- "vegetative part") cells that serve gametogenic antheridial cells, giving a very small (usually 2) number of male gametes.

In primitive groups of 2 antheridial cells, a haustorium (haustorial cell) develops from one cell, then it divides into 2 more cells, one of which either forms 2 spermatozoa (spermatogenic) or 2 spermatozoa (spermatogenous). The second antheridium cell remains sterile and then collapses. The process of fertilization in seed plants gets rid of the connection with the aquatic environment.

The male gametophyte, called pollen, is carried entirely by the wind to the female gametophyte, where it germinates using the nutrients of the female gametophyte (Fig. 1).

Rice. 1. Development of the male gametophyte of Scotch pine (Pinus sylvestris )

A - division of the archesporial cell; B - tetrad of microspores; B - microspore; G-F - formation of male gametophyte (pollen); G - pollen germination: 1-2 - prothallial cells, 3 - antheridial cell, 4 - vegetative cell, 5 - stalk cell, 6 - spermagene cell.

The ovule consists of a macrosporangium - the nucellus, protected by an additional cover - the integument. At the top of the ovule, the integument is not closed, its edges form an opening - the micropyle. Inside the nucellus, a female gametophyte develops, which is a colorless multicellular body, the cells of which accumulate a significant amount of reserve substances, mainly oils. At the end of the gametophyte facing the micropyle, 2 archegonia immersed in its tissue are formed, in the abdomen of each of which there is a large egg. Other more primitive Conifers may have dozens of archegoniums (Araucaria - 25, cypress - up to 200).

After fertilization, a seed is formed from the ovule. The integument turns into a seed coat, the nucellus is consumed by the developing embryo, leaving a thin film of it. The tissues of the outgrowth or endosperm grow strongly and nutrients are deposited in them (Fig. 2).

Rice. 2. Development of the sexual generation of pine.

A - pollen grain; B - formation of male gametophyte; 1 - prothallial cell; 2 - antheridial cell; 3 - air bags; B - pollen tube; 4 - generative cells (sperms); D - longitudinal section of the ovule; D - upper part of the ovule; 5 - integument; 6 - micropyle; 7 - nucellus; 8 - pollen tube; 9 - endosperm; 10 - neck of archegonium; 11 - egg; 12 - female gametophyte.

From a fertilized egg, an embryo is formed, consisting of a root, a stalk and a kidney with 2-18 cotyledons.

In living fossils, the seeds fall before full maturation and even before fertilization (the so-called "oviparous" plants), in conifers, the seed leaves the mother's organism in a state of complete readiness for the development of a daughter sporophyte plant ("viviparous"). The seed of "oviparous" plants germinate without a dormant period.

Strobili (bumps)

In gymnosperms, micro- and macrosporophylls can develop on the same (monoecious) or on different individuals (dioecious). Despite the diversity of their structure, a general pattern is visible: the older the taxon, the larger the size of macro- and especially microsporophylls, which can even be pinnate, resembling fern leaves (as in extinct bennettites).

In more developed gymnosperms, sporophylls become scaly and unite into strobili (cones), convenient for maturation, protection, and seed dispersal. Adaptations for dispersal of seeds arise due to the covers of the seed itself or parts of the seed cones. In all living gymnosperms, strobili are unisexual, male are called microstrobils, female are called macrostrobils (megastrobils). The taxonomy of gymnosperms is quite complex.

Questions for independent work

1. List the most important orders and families of the Cycadian class.

2. Give general characteristics, structure of vegetative and reproductive structures. Write down the life cycle diagram in your notebook.

3. Give a description of the Gnetovye class. What is the scope of the taxon?

4. Tell us about the structural features of the Gnetov sporophytes.

5. From what ancestors do the Gnetovs originate? What is the phylogenetic significance of the Gnetovs?

"Nervous" plants "Looking at a drop of water, an analytically thinking observer will come to the conclusion that the Pacific Ocean exists," said Sherlock Holmes. One must think that in his practical activity he did not go so far along the path of deduction - otherwise, undoubtedly, he would not

DEPARTMENT SECOND EXTERNAL DISEASES

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