When studying the seeds and fruits of weeds, it is necessary to clearly present the structure of all the organs (flower, pistil, ovule), in the process of development of which they are formed. Characterization of the morphological features of the reproductive organs begins with a description of the reproductive organs of the angiosperms (Angiospermae), to which all weed species included in this identifier belong. In presenting the general principles of morphology, only leading information is given, which cannot be dispensed with when studying and determining the beginnings of plants.
The plant has three main reproductive organs:
- Flower (we will talk about it in this article)
Consider the first of them - a flower.
The flower is a shortened modified reproductive shoot, the purpose of which is the generative propagation of plants. It is carried out by a pestle and stamens, indirectly the perianth serves the same purpose.
A flower does not always have all of these parts. So, perianth is often absent. Flowers with stamens and pestles are called bisexual. Flowers with only stamens or pistils are same-sex. Unisexual stamen flowers are male, pistil flowers are female.
Receptacle - the shortened stem part of the flower, on which the remaining parts of the flower are arranged with a certain regularity in a spiral or (more often) close-knit rings (circles).
The receptacle can be convex, flat and concave. Depending on its structure, the position of the ovary and the place of formation of other parts of the flower in relation to the ovary, the flower can be subpestic, near-pestal and superpestuous.
Perianth consists of tepals and is called simple if its leaves are identical in shape and color, and double if the outer leaves differ from the inner ones.
The outer leaves of the double perianth, usually grassy, are called sepals, and together - a cup. The inner tepals, usually brightly colored, are the petals, and together they make up the corolla.
The leaves of the cup and corolla can be free or fused to each other to varying degrees. In the first case, the calyx is free-leafed, and the corolla is free-laced, in the second case, the calyx is spinnately-leafed, and the corolla is spineless. On the spine-leaf cup and spine-corolla, one can distinguish the lower part - the tube and the upper - teeth, which form the limb at the spine-corolla, the border part between the corolla tube and the limb is called the pharynx,
In the structure of the flower, beam or bilateral symmetry is usually observed, asymmetric flower is less common. In the first case, the flower is regular, or actinomorphic ; while the corolla petals, as well as the sepals, are similar to each other, as a result of which several symmetry planes can be drawn through the axis of such a flower. In an irregular or zygomorphic flower, tepals differ in shape and size, so the flower can be divided only in one direction into two symmetrical halves ( Lamiaceae, Delphinium , etc.). In an asymmetric flower, it is impossible to draw a single plane of symmetry.
Following the tepals, stamens are located inside the flower . Their number is different in different plant species — from one to many dozens. The totality of all the stamens of the flower is called androecium .
The stamen consists of anther, binder and stamen filament, which is absent only in sedentary anthers. Stamens in some plants can grow together with threads in a common stamen tube ( Malvaceae ) or anthers ( Cucurbitaceae ).
The anther usually consists of two pairs of parallel folded pollen bags, interconnected by a binder, to which the stamen filament is attached. Inside each anther there is a mass of pollen consisting of pollen grains.
In the central part of the flower there is a pistil formed by one, two or many leaves - carpels . The whole set of flower carpels is called gynoecium . It happens to be leafy, or syncarpous, when all of its carpels, fused together by edges, form one pistil, and dicotyledonous, or apocarpous, when each carpel forms a separate pistil. Depending on the number of carpels, the apocarpous gynoecium may consist of one pestle and be monomeric or of many pestles and be polymer.
From the point of view of evolutionary morphology, the most primitive is the apocarpous gynoecium . From it, through the jointing of carpels, a syncarpous gynoecium emerged, which gradually changed during the evolution of angiosperms.
Pestle is a kind of closed organ inherent only in angiosperms. The carpels forming it are fused together in various ways: completely into one pistil (from the ovary to the stigma), only with ovaries or ovaries and columns.
The number of carpels participating in the formation of the pistil can be determined by the number of medium veins on the walls of the ovary, by the number of free columns and lobes of the stigma. Since the carpel is an organ of leaf origin, the pistil formed by one carpel is a leaf folded into a longitudinal tube with the edges connected to the seam.
Usually the pestle consists of three parts : the lower expanded and hollow - the ovary , the middle narrowed - the column and the upper glandular and more or less villous - stigmas . A typical form of pestle is bottle. The ovary cavity contains one, two or more ovules (ovules).
Elevations of the tissue of the inner cavity of the ovary, which serve as the site of attachment of the ovules, are called seedling. The position of the ovules in the ovary depends on its structure. In the ovary formed by only one carpel, the ovule can be attached to the fused edges of the carpel - the abdominal suture or to the midline of the carpel - the dorsal suture. And in that and in another case, the placenta is called wall or wall . However, the ovule is sometimes attached to a column that rises in the center of the ovary, or directly to the bottom of the ovary; then the placenta is called central or main.
Single-ovary ovaries can also have pistils, consisting of several carpels grown together, and the position of the seedling here also varies. When the carpels are fused only by their edges, the ovules can be located along the fusion lines of the edges of the carpels or along the dorsal sutures; further, the edges of the carpels can be wrapped and put forward somewhat inside the ovary, dragging the ovules along with them. The placenta of such ovaries are called wedges . In addition, like the ovary of a pestle formed by one carpel, a single-nested ovary of a pestle, arising from several carpels, can also have a central placenta.
Sometimes carpels protrude so much into the ovary that they grow together in the middle with their edges, and the walls adjoining each other grow one against the other, forming complete partitions. Here we have an ovary divided into several non-interconnected nests — multi-nest. According to the number of nests, such ovaries are called two-, three-nested. If the partition is incomplete, that is, at least somewhere between the nests there is a small message, the ovary is called single-socket .
Only those partitions are called true , which are the edges of the carpels wrapped inside. Partitions formed by later outgrowths of tissue are classified as false. The presence of these outgrowths explains why the ovary of a single-nested pestle sometimes becomes two-celled, in a two-celled pestle, four-celled, etc.
The ovary in relation to other parts of the flower may be located differently depending on the shape of the receptacle. If the ovary is on a flat or convex receptacle, and the stamens and perianth are attached around its base or lower, then the ovary is called free or upper. When the ovary is enclosed in a concave receptacle fused with it, and the stamens and perianth are attached above it, the ovary is called adherent or inferior. If only the lower part of the ovary is enclosed in a concave receptacle, and the stamens and perianth are attached around its middle, then the ovary is lower.
There can be one or several columns in the pistil, usually their number corresponds to the number of carpels. If the column is one, then it can be simple or dividing into blades.
The column contributes to the extension of the stigma and the passage of pollen tubes into the ovary. Inside the column there is a channel that conducts the pollen tube deep into the ovary, or a special loose tissue that feeds the pollen tube. The following types are highlighted depending on the structure of the column:
open - with a channel lined with ordinary epidermis; half-closed - with a channel lined with glandular cells; closed - with a channel made of conductive tissue, along the intercellular spaces of which pollen tubes move.
The first type of columns prevails among representatives of monocotyledonous, the last two types are characteristic of dicotyledons. The column usually comes from the top of the ovary, less often it is attached to the side (called lateral) or at the very base of the ovary (then called gynobasic).
After fertilization, the column usually disappears, less often stored on the fruit.
Stigma is the part of the pestle that traps pollen. The pollen is held by the stigma due to the glandular papillae, hairs and tubercles, usually visible only under a magnifying glass. Once on the stigma, pollen begins to germinate and, forming a pollen tube, penetrates the ovary.
If there is no column or the column is very short, then the stigma is called sessile . Stigmas in size and shape can be very different. Rarely is the stigma so weakly marked that it represents a sticky area. If pollen is transported by the wind in plants, the stigma surface is greatly increased, which facilitates the capture of pollen. Usually the number of corners, or lobes, stigmas is equal to the number of carpels forming a pistil, or twice as much.
The ovule (ovule) is the germ of the future seed, which forms in the ovary of the pestle. The place of its laying is called the placenta or placenta.
Initially, the ovule appears in the form of a microscopically small tubercle, in which the external and internal differentiation of the cells immediately begins. Almost simultaneously with the development of the central part of the ovule, called the nucleus or nucellus ( nucellus ), integuments or integuments develop. The ovule often has two integuments - the outer and the inner, as is observed in most monocotyledons, and of the dicotyledons - in many dicotyledons. Usually, one cover is characteristic of the ovule of the spine-dicotyledonous dicotyledons. Less often, within one and the same genus, one or two integuments ( Delphinium ) are found; even less often, they are completely absent (naked ovule). As an extremely rare deviation, ovules with three integuments can be found when the third integument is formed as a result of the splitting of the outer integument or is a new formation that has arisen from the base of the ovule.
At the top of the ovule, its integument does not close, forming a channel for the penetration of pollen tubes into the embryo sac. This canal is called a pollen feeder, ejaculatory or micropyle .
There are various structures of micropyles : in some, these are narrow hollow channels, in others they are filled with elongated cells that take the form of hairs ( Asteraceae ), in the third, with glandular hairs, obviously serving to feed pollen tubes. In cases where the integument of the ovule completely closes or the nucellus protrudes from the integument, there is no micropile.
The ovule is attached to the placenta with the help of a stalk called the placenta or funiculus . The latter can be either very elongated or shortened so much that it seems sedentary, for example, in cereals ( Roaseae ), some asteraceae ( Asteraceae ), etc.
An ovule serves not only to strengthen the ovule, but also to carry nutrients from the ovary. The lower part of the ovule at the place of its transition into the placenta is called the chalase. The place of attachment of the ovule to the placenta, noticeable on the surface of the seed, is called the scar.
The shape of the ovule is usually oval or more or less round. According to the position of the ovule relative to the placenta, the following main types are distinguished.
A direct, or atropic, ovule is formed with a uniform growth of integument in all directions, as a result of which nucellus is a continuation of the placenta. A similar structure of the ovule is observed infrequently, for example, in buckwheat ( Polygonaceae ), nettle ( Urtica ), etc.
The inverted, or anatropic, ovule , while remaining straightened, is bent 180 ° and, as a result, is laterally grown towards the elongated ovule. At the place where this fusion ends, a secondary scar forms, which lies near the vas deferens. The primary scar, remaining in its place under the chalase, is opposite to the ejaculation. Between the primary and secondary scars there is an elevation called the suture . Inverted ovule is most common among angiosperms; it is found in monocotyledonous and dicotyledonous, in dicotyledonous and spine-lobed.
A curved, or campyllotropic, ovule is formed when one side of it develops more than the other. At the same time, the core of the ovule and its covers are bent, and the ejaculation approaches the chalase and the scar, being away from them. It is observed in cruciferous ( Brassicaceae ), clove ( Caryophyllaceae ), nightshade (Solanaceae ), etc.
When studying the structure of the ovule, it is not always possible to attribute them to one of these three categories : there are intermediate forms, such as half-reverse, half-curved, etc.
The number of ovules in the ovary, their shape, the presence or absence of integument in the ovules, and the structure of the nucellus are of important systematic and phylogenetic importance.
The embryo sac inside the nucleus of the ovule is the most complex and vaccine part of it , since later, after fertilization, the germ of the future plant, the embryo, develops in it. The formation of the embryo sac begins with the isolation of one of the nucellus cells (against the micropyle) in the primary archespore cell.
A mature germ sac usually consists of six cells and two polar nuclei. In many species, the polar nuclei merge even before fertilization and a secondary, or central, nucleus appears, located in the central cell of the embryonic sac.
The egg apparatus in a mature embryo sac consists of an egg and two synergies. This complex of cells has very peculiar morphological and physiological features. The egg usually occupies a middle position in the egg apparatus, it is larger than the synergid, has an elongated or pear-shaped, contains a large nucleus located in the lower part of the cell, and a dense cytoplasm. Synergides are sister cells, in structure and size are pretty similar. They play a significant role in the movement of pollen tubes to the embryo sac and egg apparatus, and also contribute to the dissolution of pollen tubes.
Near the egg apparatus is the central cell of the embryo sac, which includes two polar nuclei.
In the chalazal end of the embryonic sac, a complex of antipodes of three cells develops, which has great variability.
Fertilization in plants is carried out using pollination, germination of pollen tubes and fusion of generative cells.
Most angiosperms have bisexual flowers, in which stamens and pistils are usually planted at different times, which ensures cross-pollination. The latter in flowering plants occurs mainly during the transfer of pollen by wind or insects.
Pollen grains falling on the stigma of the pestle begin to sprout. The stigma at this time releases a solution of sugar (with an admixture of nitrogenous substances), which sprouting pollen tubes feed on.
The direction of growth of pollen tubes is determined by the phenomenon of chemotropism , that is, their desire for chemicals that are present in the ovary and ovule.
At this time, the contents of the pollen grain, consisting of two sperm and a vegetative nucleus or from a generative cell and a vegetative nucleus, pass into the pollen tube. In the latter case, sperm are formed already in the pollen tube. Approaching the ovule, the pollen tube in most cases penetrates through the micropile into its embryo sac.
Angiosperms are fertilized as follows . The first sprouted pollen tube, falling into the embryo sac, is poured into one of the synergides, the contents of which penetrate into the area of the egg apparatus, usually into the gap between the egg cell and the central cell. After an outpouring of a pollen tube, sperm synergy enters the plasma of the embryo sac.Then one of the sperm penetrates the egg and merges with its nucleus, and the other sperm merges with the nucleus of the central cell. As a rule, this is preceded by the fusion of polar nuclei and the formation of the nucleus of the central cell of the embryo sac.
Subsequently , the embryo is formed from the fertilized egg ( zygote ) , and the endosperm is formed from the fertilized core of the central clove .
This complex reproductive process, which is of universal importance for angiosperms, has been called double fertilization. It was discovered and studied in 1898 by academician SG Navashin. The biological meaning of double fertilization in angiosperms is that endosperm as a nutrient material for the embryo develops earlier than the embryo.
Only after the endosperm is largely formed does embryo development begin.
In cases where fertilization does not occur, the endosperm does not develop, since the mother's body must spend a lot of energy and nutrient material on its formation. With underdevelopment and abnormalities in the development of the endosperm, the ovule usually turns into an inferior seed.