Spore horsetails. Horsetail. Characteristics of the horsetail department. Development cycle, representatives, meaning
horsetail
Horsetail, or articulate, or articulate. or wedge-shaped (lat. Equisetophytina) - a subdivision of higher spore plants of the fern-shaped department, previously placed in the now abolished department of horsetail (Equisetophyta).
Horsetails are characterized by the presence of shoots consisting of clearly defined segments (internodes) and nodes with whorled leaves. In this feature, modern and fossil horsetails differ sharply from all other higher spore plants and in appearance resemble some algae (characeae), gymnosperms (ephedra), or even flowering plants.
Horsetails are peculiar plants. Many vegetative characteristics bring them closer to cereals. Apparently, horsetails are the most competitive among ferns, which is explained by numerous improvements in vegetative organs: the stem of horsetails is articulate and grows in nodes (like in cereals); the epidermis is reinforced with silica; the weight of the stem is lightened due to the presence of a central cavity; there are numerous strands of mechanical tissues that increase the strength of the stem; there are air cavities that allow to establish the supply of oxygen to underground and underwater parts; real vessels develop (as in angiosperms); spores are equipped with special pushing processes (elaters), which contribute to the loosening of the spore mass, and, consequently, the transfer of spores by the wind. It is due to these structural features that horsetails, as in the distant Carboniferous period, continue to dominate in some biocenoses - mainly along the banks of reservoirs. Horsetails have one more interesting feature: they have heterosexual growths, and the development of male or female growths is predetermined by environmental conditions. In general, the worse the conditions, the greater the percentage of male growths formed. Thus, horsetails demonstrate a transitional stage from typical equisporous to typical heterosporous.
Appearance
Horsetails include both herbaceous extinct and now living plants with stems ranging in length from several centimeters to several meters, and tree-like extinct forms that reached 15 m in height and a trunk diameter of 50 cm.
Anatomy
The conducting system of the horsetail stem is represented by an actinostele or arthrostele, that is, an articulated stele consisting of sections of various structures alternating among themselves along the stem. The conducting elements of the xylem are represented by various types of tracheids, and in horsetails also by vessels. Phloem consists of sieve elements and parenchymal cells.
Origin and evolution
Horsetails appeared in the Upper Devonian and descended from the now extinct Rhyniales or some plants close to them, but they flourished in the Carboniferous period, when they were widely represented by a variety of woody and herbaceous forms. Together with lepidodendrons and tree ferns, horsetails played an important part in the formation of coal forests.
Fossil horsetails (for example, tree-like calamites) reached a height of 25 m, secondary xylem was found in their trunks. However, in the Permian, their extinction begins, and first of all, woody forms die out, so that only herbaceous horsetails are known from the Mesozoic. To date, only the genus Horsetail (Equisetum) has survived from this large group.
From all known plants, both extinct and modern horsetails differ in shoots made up of separate segments. The name "Articular" comes from this specificity of the structure of their shoots, which are divided into clearly defined nodes and internodes, easily breaking up into segments. Articulation is due to the whorled leaf arrangement and the presence of an intercalary meristem in the lower parts of the internodes, along which breaking into segments occurs.
Modern horsetails are characterized by peculiar leaves - their leaf blades are greatly reduced to small dark, sometimes green or colorless teeth, and well-defined sheaths have grown together into a common membranous sheath. The reproductive organs of articulates are characterized by the presence of strobili in the form of spikelets, and only in some Paleozoic species - spore-bearing zones. In modern horsetails, sporangiophores have a shield shape, while in ancient extinct horsetails they had the most diverse shape, except for leaf-shaped ones. The vast majority of horsetails are isosporous plants, and only a few extinct species were heterosporous.
Classification
The subdivision articulated, or horsetail, includes three classes, the evolution of which apparently proceeded along independent, parallel paths - two extinct classes: Sphenophyllops (Sphenophyllopsida), Cladoxylea (Cladoxylopsida) and the current class Horsetails (Equisetopsida).
Department of horsetail. About 20-30 modern horsetail species are known. These plants grow on our planet everywhere in areas with different climatic conditions - from Antarctica to Africa. But in North America and Eurasia, in the temperate and arctic zones, the greatest species diversity of horsetails is noted. The name "horsetail" this plant received for its resemblance to the tails of animals, in particular, horses. Horsetails grow in forests, meadows, swampy areas, near water bodies.
All modern species of horsetail are perennial herbaceous plants with dense stems impregnated with silica, reaching, on average, a height of 1 to 12 m. The largest species is giant horsetail, found in forests of the tropics and subtropics with high humidity. Its length can reach 10-12 m. A characteristic feature of all types of horsetail is the presence of stem metamerism, that is, the correct alternation of nodes and internodes. Horsetails have small, scaly leaves arranged in whorls at the nodes. Lateral branches also form at the nodes of the stem. Photosynthesis takes place in the tissue of green stems and branches, the surface of which is further increased by ribbing. The sporangia are found on hexagonal corymbose sporangiophores connected to form apical strobili.
The underground part of horsetails is a powerful branched rhizome, in the nodes of which adventitious roots are formed. Some plant species of this group, for example, horsetail, have rhizomes modified into tubers, which serve both for deposition of reserve nutrients and for vegetative propagation.
Horsetails reproduce mainly by rhizomes, producing many twin daughter plants, especially in places with disturbed vegetation, such as after fires. Extensive, hard-to-eradicate thickets of these weeds are formed on pastures and fields. For the growth of horsetails, acidic soil is favorable, on which they grow especially quickly and serve as a kind of indicator of soil acidity.
Horsetails are also characterized by reproduction by spores, which are enclosed in sporangia on sporangiophores. On the stem there are spore-bearing zones of sporangiophores or they are packed into strobili at the top. The gametophyte, or sprout, looks like a small short-lived unisexual or bisexual green plant. Also, the gametophyte may look like a brownish-pink non-branching shoot that dies off after sporulation. On these growths are located antheridia with spermatozoa and archegonia with eggs. Water is required for fertilization. The resulting zygote gives rise to the sporophyte.
Horsetails are widely used in folk medicine, in particular, horsetail preparations. These drugs are effective for hypertension, gout, as wound healing agents. Horsetail dressing is used to supplement the nutrition of cows and goats in order to increase milk yield. Many wild animals eat horsetails. Some species of these plants, such as marsh horsetail, are poisonous to farm animals.
Horsetail shoots have a jointed structure, consist of nodes and internodes. The leaves are collected in whorls.
Horsetails include both herbaceous plants (from living and extinct) with a stem from a few centimeters to several meters, and tree-like (only extinct), reaching 15 m and more than 0.5 m in diameter.
The conducting system of the horsetail stem is the actinostele or artrostele. Most horsetails are isosporous plants, and only a few fossil forms were heterosporous.
The horsetail department combines two classes: cuneiform (Sphenophyllopsida) and horsetail (Equisetopsida).
Previously assigned class hyenae with representatives protogyenia (Protohyenia), hyenia (Hyenia, rice. 25 ) and Calamophyton (Colamophyton) are currently considered by paleobotanists as the oldest cladoxyl ferns. In Kalamophyton, the nodes of the segmented shoot described earlier turned out to be simply transverse cracks in the rock. The anatomical structure of the hyena is still unknown, and the spore-bearing organs of both species rarely differ from the sporangiophores of Devonian horsetails (Meyen: Elenevsky et al. 2000).
CLASS Horsetail (EQUISETOPSIDA)
The class horsetail contains the order horsetail (Equisetales), families Calamitaceae (Calamitaceae) and horsetail (Equisetaceae).
Extinct representatives are united in the Calamitaceae family. Species of this family were widespread in the Carboniferous, and then, together with lepidodendrons, sigillaria, ferns and cordaites, they formed forests that gave rise to deposits of coal.
In appearance and structure, calamites resembled modern horsetails, but differed from them - they were trees, reaching a height of 8-10 m and even up to 20 m. Among them were both equally spore and heterosporous species.
The horsetail family includes one genus horsetail (Equisetum) and 25 kinds. In the Republic of Belarus, 8 species of horsetail grow. They are found in swamps (E. palustre, E. fluviatile), forests (E. sylvaticum), bushes (E. hyemale), meadows, fields (E. pratense, E. arvense), etc.
Modern horsetails are small herbaceous plants 80-100 cm tall, 2-5 mm thick. Tropical South American E. giganteum reaches 10-12 m in length and is a creeper.
Horsetail consists of a rhizome horizontally located in the soil, from the nodes of which thin roots depart and above-ground shoots rise upwards.
The stem of the horsetail is jointed, ribbed, consists of nodes and internodes. The internodes are hollow in the middle, the nodes are filled with parenchymal tissue.
Horsetail leaves are scaly, brown, brown, devoid of chlorophyll, fused in the lower part into a tubular sheath attached to the node. Due to the reduction of leaves, the function of assimilation is performed by green shoots and stem. The branches are arranged in whorls, punching through the sheath of fused leaves.
In cross section, the stem has the following structure. Outside, the stem is uneven, there are elevated areas (ribs), alternating with hollows. Outside, the stem is covered with a single-layered epidermis impregnated with silica, which gives it strength. Inside the epidermis is a cortex and a ring of small, isolated vascular bundles of a collateral type with carinal (from Latin carina - keel, crest) canals. In the center of the stem there is a cavity at the site of the destruction of the core. Under the ribs there are areas of mechanical tissue, and under the hollows - assimilation tissue and vallecular (from Latin vallis - valley, hollow) cavities. Under the mechanical tissue (under the ribs) there are vascular bundles of the collateral type, closed, without cambium. In the epidermis, above the assimilation tissue, there are stomata.
Spore-bearing spikelets in horsetails appear one at a time at the top of the main shoot, sometimes lateral branches. In most species, the spore-bearing shoot is green. Above-ground shoots in some species can combine two functions - spore-bearing and vegetative. Yes, at marsh horsetail (E. palustre) and riverine, or baked (E. fluviatila), vegetative and spore-bearing shoots appear simultaneously and do not differ morphologically from each other. Only in the middle of summer, strobiles form on some green shoots. In other species, a separation of shoot functions is observed. Yes, at horsetail (E. silvaticum) and meadow horsetail (E. pratense) in spring, along with vegetative shoots, non-branching, colorless or pinkish spore-bearing shoots develop. But after sporulation, they turn green, branch and do not differ from vegetative shoots. In some species, shoot dimorphism appears very clearly.
Horsetail has two types of shoots. In spring, brown, spore-bearing shoots grow from the rhizome, bearing one spikelet. The spikelet of a horsetail consists of numerous sporangiophores collected in whorls on its axis. Sporangiophores consist of a stem and a corymbose hexagonal disk. On the underside of the disk, around the stem, there are 5-13 saccular sporangia. In sporangia, a large number of identical spores are formed (equisporous). The spore has three shells: endospore, exospore and the outer layer of the shell, which, when ripe, cracks with the formation of two hygroscopic ribbons around the spore, called hapter, which are attached to the spore in the center. In dry weather, they, like springs, unwind and help loosen the spores. In this case, the hapters of neighboring spores cling to each other. As a result, loose balls of spores spill out of the sporangia, easily carried by the wind. Horsetail growth has the appearance of a green plate, and in thickened crops or in water - a green thread. A single-layer plate, growing, turns into a multi-layer prostrate pillow with rhizoids on the underside. On the upper side of the pillow, vertical lamellar blades develop, on which the genitals are formed. The size of gametophytes in different species varies from 1 mm to 2-3 cm. Within a species, male gametophytes are smaller than female ones.
Some species of horsetail are physiologically heterosporous.
In the best conditions of moisture and lighting, larger outgrowths (female) develop from spores, in the worst conditions, small outgrowths (male).
Antheridia of horsetails are immersed in the tissue of the outgrowth. They develop up to 100 polyflagellated spermatozoa. Archegoniums rise above the growth with a neck. Fertilization occurs in wet weather. The pendant does not form an embryo and consists of a stalk, 2-3 leaflets and a root.
After the spores fall out of the spike-shaped strobe of the field horsetail, the spore-bearing shoot dies off. New ones grow from the rhizome - green, strongly branched summer shoots.
The practical value of horsetails is small. The stems contain silica and are therefore used for cleaning metal utensils and polishing wood. Nodules on the rhizome of horsetail are sometimes eaten (contains starch). Some of the horsetails (horsetail, meadow horsetail) are weeds. Some are poisonous (marsh horsetail).
Rare relict species are included in the Red Book of the Republic of Belarus - horsetail (Equisetum telmateia) and horsetail (E. variegatum).
What are the features of the structure and reproduction of horsetails (horsetails)?
Horsetails are perennial herbaceous plants with a jointed structure. Shoots are divided into nodes and internodes. The leaves are reduced to whole small plates. The function of photosynthesis is performed by the stem. Silica accumulates in horsetail cells, which makes the stem stiff. Horsetails reproduce by spores, which are formed in spore-bearing spikelets. Spore-bearing spikelets are formed either on special spore-bearing shoots (in horsetail), or on the tops of vegetative shoots (meadow horsetail, wintering horsetail). The spore-bearing spikelet consists of an axis on which sporangiophores are located. The sporangiophore consists of a stalk with a hexagonal plate. On the underside of the plate are sporangia. Spores are formed in sporangia. In dry weather, spores spill out. The gametophyte is formed from the spore. It has antheridia with spermatozoa and archegonia with eggs. After fertilization, a zygote is formed, and an embryo and a new plant develop from it.
Introduction
Department Horsetails ( sphenophyta, or Equisetophyta), in the past diverse not only at the species level, but also at the generic and family levels, now includes a single genus Equisetum. Only about 30 species are distinguished in it, the history of which can be traced from the beginning of the Cretaceous period. Some of them, such as from the tropics, reach 8 m in height and 4 cm in diameter. But most species are small - up to 30 cm in height and 0.5–2 cm in diameter. Horsetails are vascular plants capable of sexual reproduction through spores produced in sporangia at the ends of stems. The cell walls of horsetails contain silica granules, which they accumulate from the soil solution, which gives their stems rigidity and vertical stability.
Horsetail has long been used in folk medicine as a hemostatic and diuretic. Some species, such as wintering horsetail ( Equisetum hiemale), whose epidermis is particularly rich in silica, were used to polish walls.
Horsetails are distributed almost throughout the globe - from the tropics to the polar latitudes. Their ecology is also diverse, from swamps flooded with water to dry sands and rocks (Fig. 1).
Rice. 1. Ecological extremes of horsetails: A - on a rocky substrate; B - in the swamp
External structure
All modern horsetails are herbaceous perennials. They have an upright aerial stem, a developed network of underground rhizomes. The stems and rhizomes are divided into nodes and internodes, in connection with this, horsetails are often called arthropods (Fig. 2). Outwardly, they vaguely resemble bamboo. Stem nodes are surrounded by reduced scaly leaves called microphylls and whorls of branches (Fig. 3). The leaves are not photosynthetic and are brown in color. But the cells of the stem and twigs are rich in chlorophyll. The stems have monopodial branching, they are hollow inside. At the top of the horsetail stem, spore-bearing organs, strobili, are formed (Fig. 4). Above-ground shoots of horsetails die off for the winter. However, among horsetails there are also evergreen species, for example, wintering horsetail - Equisetum hiemale).
Rice. 3. Knots of horsetail stems with whorls of twigs (A) and reduced leaves (B)
Rice. 5. Vegetative propagation of horsetail: A - appearance; B - view under a microscope
Like other plants, horsetails are capable of vegetative reproduction. It is carried out by means of young shoots formed in the nodes of rhizomes or in the lower nodes of the stem (Fig. 5).
In the internodes of the stems, rhizomes can also form if for some reason they come into contact with the ground.
Anatomy
At the top of the stem is the apical meristem, which carries out apical growth. Like other vascular plants, leaves and twigs in horsetails are formed from the apical meristem. In addition, the intercalary meristem provides intercalary growth at the nodes of the plant. Cells are formed from it, which grow the stem not only in height, but, radially (Fig. 6). Therefore, it never has a strictly cylindrical shape - the diameter of the nodes is always slightly less than the diameter of the internodes (Fig. 7).
A transverse section of an internode shows that it has a large central cavity surrounded by a cortex, into which numerous bundles of vessels are inscribed (Fig. 8). In the node, on the contrary, there is no central cavity, and its anatomical structure is similar to the structure of the stems of other vascular plants (Fig. 9). Sclerenchyma cells are rich in lignin. Chlorenchyma cells contain chlorophyll. The cells of the epidermis have a thick, cutinized membrane. Its surface is rough due to silica granules. They are arranged in regular rows, so that ridges and grooves are formed between them. Stomata are located in the grooves (Fig. 10). Their structure is similar to that of the stomata of other vascular plants.
Rice. 8. Anatomical structure of the internode
Rice. 10. Stomata on a cross section of the epidermis
Vessel bundles are located under silica ridges. They consist of phloem and xylem (Fig. 11, 12). A stele with such a structure is called an eustela.
generative organs
At the top of vegetative shoots in horsetails there are sporangia collected in strobili (Fig. 4).
Rice. 13. Mature horsetail strobilus: A - appearance; B - longitudinal section
Individual sporangia are relatively large and elongated. They are collected on corymbose sporangiophores, or sporophylls (Fig. 13). Each sporophyll contains 5–10 sporangia on the inner side - sacs with spores (Fig. 14). Since the spores of horsetails are of the same size and bisexual growths grow from them, they are classified as equally spore plants. Spores have a spherical shape and are surrounded by elaters along the equator - special hygroscopic threads (Fig. 15). On a hot summer day, with low air humidity, the spore elaters in the opened sporangia are in a twisted state. However, as soon as the air humidity rises a little (for example, after rain), the elaters straighten and catapult the spores out. Once on moist soil, spores germinate (Fig. 16).
The seedling growing from the spore is haploid. It has rhizoids and is capable of photosynthesis. Over time, female (archegonia) and male (antheridia) reproductive organs appear on the growth. After fertilization of the ovum located in the archegonium, a young sporophyte grows from the zygote, giving rise to a new arthropod plant.