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Horsetail Plant Case Study Guide – A Road To (Equisetum)

Horsetail Plant – Classification, Characteristic Features, Morphology, Anatomy, Equisetum Thermale, Adaptations | Field Horsetail  (Equisetum Arvense) – Sporangium, Antheridium, Archegonium, Embryo Development, Life Cycle | Economic Importance

Horsetail (Equisetum) comprises of about 25 species. It belongs to order Equisetales. They are worldwide in distribution; however, no species have been recorded from Australia and New Zealand. They grow in a variety of habitats. Majority of the species are found in north temperate zone. On the other hand, many species are tropical and found in South America and West Indies. The common Indian species are. E. debile, E. arvense and E. ramosissimum. Certain species grow in ponds and marshy places, e.g., E. palustre; some species are shade loving and grow in damp shady places, e.g., E. pratense whereas some other species grow in exposed conditions. E. arvense is very widely distributed; E. debile is found along the banks of the rivers, canals and pools in the Indian plains. Equisetum are commonly known as Horsetail Plant.

Classification of Equisetum

The classification of plant Equisetum is given below along with the systematic position.

  • Systematic Position: Pteridophyta
  • Division: Arthrophyta.
  • Class: Calamopsida
  • Order: Equisetales
  • Family: Equisetaceae
  • Genus: Equisetum

Characteristic Features of Order Equisetales

The sporophytes possess distinctly articulated stems. The ridges and furrows present on the internodes of the stems. The stems may or may not by branches at the nodes. if branched, the branches are present in the whorls alternating to the leaves. The leaves are scale-like. They form a toothed sheath around each node of the stem by their lateral fusion. The stele is siphonostelic. It consists of a number of collateral vascular bundles in internode which, however, join to each other forming a continuous ring at each node. The sporangia are borne on the underside of peltate sporangiophores. The sporangiophores are arranged quite close to each other in terminal strobili or cone-like structures. The antherozoids are multiciliate.

Morphology of Equisetum

Habit

About all the species of Equisetum (Horsetail) are perennial. They bear much branched, horizontal, creeping and subterranean rhizomes which sometimes penetrate 3 or 4 feet deep in the soil. The rhizomes bear the erect and aerial branches. Some of the aerial branches are sterile and some are fertile. The plants vary in their height from species to species. E. scirpoides is only few inches high, whereas, on the other extreme a species of American tropics, i.e., E giganteum reaches up to 12 meters or 40 feet in height. However, its diameter is less than an inch. The plant of E. debile grows along the banks of the river, may attain the height of 10 to 15 feet when grows in dampy and shady places. Its diameter is about one and half centimeters.

Equisetum Stem

The rhizome of Equisetum (Horsetail) is horizontal, perennial, much-branched, subterranean and penetrates in the soil up to 3 to 4 feet and in many species, it is spread over in the area of 10 to 15 feet. It consists of well differentiated nodes and internodes. At each node there is a whorl of small, scale-like leaves united laterally to each other forming a brown sheath.

Equisetum-Arvense-External-Features
Equisetum arvense external features, the sporophyte showing rhizome, roots, tubers sterile and fertile branches

Alternating with each leaf of sheath at each node of the rhizome, there is a branch primordium or bud. These primordia may develop into aerial on subterranean branches immediately, or they remain dormant for a considerable long period and develop into new branches on the approach of favorable conditions. Sometimes in certain species, e.g., Equisetum telmateia and Equisetum arvense the primordia develop into short rounded branches composed of only one internode. These rounded branches are known as tubers. After being separated from the parent rhizome tubers develop into new plants. This is a method of vegetative propagation of the sporophyte generation. The whorls of the much-branched adventitious roots have been given from the nodes of the rhizome.

Usually the rhizomes bear two kinds of aerial branches-sterile and fertile, but there still other species which bear either sterile or fertile branches.

The sterile branches are of green color and bear a whorl of lateral branches at each node. These branches are assimilatory in function and practically synthesize all the food required by the plant. The lateral branches found on the primary branch also bear the whorl of branches which are comparatively smaller in size. Each whorl bears an equal number of branches to that of leaves, and alternate with the leaves. The sterile branches of a few species are unbranched, e.g., Equisetum hyemale.

The fertile branches are also found above the ground, they bear strobili at their apical ends. These branches arise first and shed their spores even before the appearance of the sterile branches. In E. arvense the fertile shoots are unbranched and devoid of chlorophyll, and they die as soon as the spores are shed. In certain other species, e.g., E. cylvaticum and E. pratense the strobili are thrown off after spore dissemination, and the fertile branch now acts as the sterile branch by developing lateral branches on it. Simultaneously, the chlorophyll is also developed in these branches. E. palustre bears three kinds of branches – (a) the sterile branches which are deep green and much branched, (b) the fertile shoots which are short lived and devoid of chlorophyll, (c) the intermediate type of branches which are fertile, devoid of chlorophyll and unbranched at first, but later on the strobili are thrown off and the branch becomes green, branched and persistent. This is a good example of division of labor in (Horsetail) Equisetum plant.

As regards function, the rhizome acts as store house of the food material and helps in anchoring plant to the soil. The aerial sterile branches are green, and therefore, they synthesize all the food required by the plant. The fertile branches are without chlorophyll and bear the strobili at their apical ends.

The internodes of both the rhizome and aerial branches are ribbed longitudinally. These ribs alternate with the leaves in the subtending node. The ribs also alternate to the ribs of the successive node, in their position. The general structure of the stem is same both in the rhizome and the aerial shoot, but it may be more clearly understood in the aerial shoots.

Equisetum Leaves

The leaves are arranged in whorls at each node of the aerial branches. They are simple, scaly, slender, uninured, brownish and more or less fused at their bases laterally to form a sheath around the base of the internode. Their apices are, however, free and pointed. The number of the leaves in each whorl ranges 3 to 40 from species to species. At first, the leaves are green, but later on they become brownish, dry and scale-like. The mature leaves are protective in function. In certain other species they become dead, dry and scaly.

Equisetum Roots

In Horsetail all the roots are adventitious except the primary root of the young sporophyte. The roots arise in whorls from each node of the creeping, sub-terranean rhizome. The roots may be many years together, but do not increase in their length. The roots do not arise directly, but from the lateral branch primordia situated on the nodes of the rhizomes and main aerial shoot.

Anatomy of Equisetum

Stem (Anatomy)

To study the internal structure of the stem, the transverse and longitudinal sections passing through the nodes and internodes are required. This way the internodal and nodal anatomy of the aerial stem may be studied separately. It is as follows:

To study the anatomy of the node, one has to cut the aerial stem surface of the stem looks wavy, because of the presence of ridges and furrows.

Scouring Rush Definition: The stem is covered over by a single layered epidermis interrupted by stomata situated in the grooves of furrows. The epidermis is always impregnated with a thick layer of silica. The deposition of the silica on the epidermal layer gives the rough appearance- to the stem, and therefore, the Equisetum plants are also known as ‘scouring rush or scouring rushes’.

Equisetum-Older-Stoma-Views
Equisetum older stoma views: A, vertical section of an older stoma; B, inner surface of an older stoma ahowing silicious bars on the inner walls of guard cells (A, after Strasburger, B, after Campbell)

The stomata are found in grooves of the aerial shoots. The development of the stomata is somewhat peculiar in the sense that the initial divides twice by successive longitudinal divisions and this way the two innermost cells develop into the guard cells, whereas, the two outermost cells develop into accessory cells on being mature of the stomata, the two accessory cells completely overarch the guard cells and the stoma. In majority of the species, e.g., E. hymeale, E. ramosissimum etc., the stomata are sunken, whereas in some other species, e.g., E. palustre, E. pratense etc., they are situated on the surface of the epidermis. The silica is deposited in the wall of guard cells in transverse radial bands.

Just underneath the epidermis, there is broad cortex. The cortex consists of mechanical and assimilatory tissue. In the outer cortex, just beneath each of the ridges, there is a strand of sclerenchyma. Usually the sclerenchyma is restricted to the periphery but in Equisetum giganteum it extends inward to the endodermis. These columns of sclerenchyma are chief mechanical elements of the shoot. In addition to the above mentioned sclerenchymatous columns, the sclerenchyma strands are also found in the furrows. Here, each strand is situated in between the curved strands of chlorenchyma. The chlorenchyma possess well developed intercellular spaces. These spaces are very much distinct below the stomata. These chlorenchymatous bands are found beneath the sclerenchymatous strands situated underneath the ridges. The ends of the chlorenchymatous bands touch the epidermis in the grooves. Since the leaves are small, scaly and have a smaller number of chloroplasts the chloroenchyma of the cortex of the shoot is the major photosynthetic tissue. The inner cortex is composed of thin walled parenchyma with well-developed intercellular spaces. In this region of the cortex the air spaces known as vallecular canals are also present Usually vallecular canals are found opposite the furrows in the deeper tissue of the cortex. They are alternate to the vascular bundles. These canals are filled up with air.

The endodermis is the last layer of the cortex. In E. arvense, E. telmateia, E. palustre and certain other species, the endodermis surrounds the entire stele. In E. giganteum, E. limosum and some other species, each vascular bundle is surrounded by separate endodermal layer. In other cases, e.g., E. hyemale, E. ramosissimum etc., there is a common internal endodermis inside the ring of bundles delimiting the pith and common outer endodermis found Outside ring of bundles. Just beneath the endodermis a single layered pericycle is found. The pericycle is the outermost layer of the stele.

Equisetum-Arvense-Anatomy-of-Stem
Equisetum arvense anatomy of stem. A. T.S. of stem (diagrammatic structure); B. T.S. of stem (detailed structure)

The vascular skeleton, i.e., stele of Equisetum is siphonostelic. The separated vascular bundles are arranged in a ring. The vascular bundles are situated opposite the ridges and alternate to the vallecular canals. The number of bundles varies from species to species. In each internode, as many vascular bundles are found as there are leaves at the node. The vascular bundles alternate to each other in their position in the successive nodes. The internodal portions are longitudinally perforate, and each perforation runs the length of an internode. The perforations in the internode are situated above the traces. These perforations are not branch gaps. The vascular bundles are collateral type and resemble to some extent to that of monocotyledons. The vascular bundles contain both metaxylem and protoxylem. A carinal canal is developed in each bundle, because of the disintegration of the early-formed tracheids of the protoxylem during elongation of the surrounding cells of the internode. The remaining protoxylem elements are composed of few tracheids. These protoxylic elements are found arranged to the margin of carinal canal. The metaxylem elements are found in two groups. The two groups are found arranged on the margin of the carinal canal towards outside. The protoxylem lies in between the two groups of metaxylem. The metaxylem elements are composed of reticulate, scalariform or pitted tracheids. Sometimes spiral and annular tracheids are also found. The protoxylem is endarch and as supported by Eames (1909) and Miss Barrat (1920) the development of xylem is centrifugal. The phloem is composed of sieve tubes and phloem parenchyma. The sieve plates may also be seen. The companion cells are not found. The secondary growth is altogether absent. The central region of the stem is occupied by a hollow pith. The carinal canals are filled up with water.

Nodal Anatomy

The alternate vascular bundles of the successive internodes are connected to each other by short branches and this way a continuous ring of the vascular cylinder is found in the node. Eames (1909) reported that the bundles at the nodes do not have carinal canals. Here, the protoxylem elements are intact and completely occupy the lacuna or carinal canal. At the node, the pith is not hollow and it forms a diaphragm separating the two successive internodes.

Rhizome

The anatomy of the rhizome is quite identical to that of the aerial shoot. However, the assimilatory tissue and the stomata are not found in the rhizome. The mechanical tissue. i.e., sclerenchyma is poorly developed as compared to that of the aerial shoot. In E. arvense, the pith is solid, whereas in certain other species, the pith and the vallecular canals are very much reduced.

Leaf (Anatomy)

Equisetum-Horsetail-Plant-Stem-and-Rhizome
Equisetum horsetail plant stem and rhizome: A. Vascular structure of the stem; B. Rhizome split longitudinally through nodal region

The anatomy of the leaf is quite simple. The leaves are uninerved, this means that each leaf contains a single vascular bundle. The vascular bundles of the leaf sheath are simple and collateral. The carinal canals are not found. Individual bundles are surrounded by separate endodermal layers (endodermis). The outer tissues of the leaf sheath are sclerenchymatous bands. These bands of sclerenchyma pass up the leaf ridges and alternate the strips of chlorophyllous tissue associated with stomata.

Root (Anatomy)

The adventitious roots are borne at the nodes of the rhizome and aerial shoot. The anatomy of adventitious roots is quite simple. To study the anatomy, the cross section of the root is required. The outermost layer is known as piliferous layer which bears unicellular hairs upon it. Just beneath the piliferous layer there is a multilayered cortex. In small root, the cortex is divided into two zones. The outer zone is composed of three to four layered lignified exodermis. The inner zone of the cortex consists of thin walled parenchyma with well intercellular spaces.

Equisetum-Arvense-Anatomy-of-Root
Equisetum arvense anatomy of root. A. T.S.of root (diagrammatic structure); B. T.S. (detailed structure)

The endodermis is two celled in thickness. The inner layer of the endodermis acts as pericycle. However, the pericycle is absent. The lateral roots originate from the inner layer of the endodermis. In between the two layers of the endodermis, intercellular spaces are found. The Casparian are strictly found in the outer layer of the endodermis.

The stele of the root varies in the nature from species to species. It is triarchy to hexarch. There is a big central metaxylem element surrounded by three to six narrow points of protoxylem. Each protoxylem point is represented by single tracheid. The tracheids of xylem elements are spirally thickened. The angles between the protoxylem points are completely filled with phloem. The phloem is composed of phloem parenchyma and sieve tubes.

The Apical Growth

The apical growth of the main aerial shoot and its laterals takes place by means of a single pyramid like apical cell. This apical cell cuts the segments at its three faces. The segments are cut off regularly. Each segment divides anticlinally forming two upper and lower segments. Both these segments give rise to two tier of cells by successive divisions. The upper tier of the cells develops into the node and the lower into the internode.

In the roots, a young root primordium is differentiated into a group of initials which develop into a root cap and a pyramid-like apical cell, which develops into the root proper. Here, the behavior of apical cell differs from that of the stem that it cuts off segments from all its four faces, whereas. in the stem only three faces of the tetrahedral apical cell cut off the segments. The lateral segments form the tissues of the root proper, and the terminal segment forms the root-cap.

Horsetail Land Adaptations

According to the studies of Alan Channing (Geo-biologist of Cardiff University, Wales), the fossilized remains of plant species in southern Patagonia can provide the evidence about the age of these ancient plant group and also tell us about their adaptations, living habits, ecology, physiology and types of environments they were adapted to.

They amazing fact was that how their aspects can might closely relate to Equisetum, especially those who are living in same environmental conditions.

Discovery

The interesting finds was the equisetum species abundant fossilized remains well preserved in Chert Blocks, which includes:

  • Dense stands of aerial stem
  • Variety of organs like Strobili
  • Apices
  • Branches
  • Roots
  • Rhizomes
  • Leaf sheaths
  • Reproductive parts

Location

The location where they made discovery was Deseado Massif (Santa Cruz Province, Argentina). In the area of ancient volcanic rocks at San Agustin Farm, a near-intact and extremely well exposed fossil hot spring deposit of Jurassic age.

Note: Do you know that hot spring deposits are extremely rare in fossil records and specially those which are older than the Miocene period.

Equisetum Thermal

Through the study of the fossil, it was also discovered that its anatomy and morphology is indistinguishable from those of species living today i.e. Equisetum and Hippochaete. As an example:

  • It grew in an upright single straight stem with a double endodermis
  • And it was evergreen.

The fossil justified the rise of a new species named as Equisetum thermal. This specie of Equisetum grew in geothermally effected habitats. By the living habits of the equisetum, Alan and his colleagues were able to conclude many aspects, such as:

  • The stresses it endured
  • Potential mechanism if stress tolerance

Adaptations

The study also revealed that the anatomy of E. thermale is adapted for both dry and wetland settings. For example, in stems and rhizomes of E. thermale there was an extensive air spaces network. The same stems and rhizomes were responsible for providing aeration for its water-flooded rooting system.

Alan also pointed out the fact that E. thermale also shows a number of other adaptations that reduce water loss. It has a thick outer walls, well-developed cuticle and silica deposits. Its stomata were protected by silica deposits and cover cells. These stomata were situated well beneath the surface of stem.

The E. thermale silica deposits hinted towards the physiological mechanism of stress tolerance. Alan said, “As silicon uptake has been demonstrated to ameliorate salt, heat, and heavy metal stresses in living crop plants.”

Equisetum Arvense (Field Horsetail) – Sporangium, Antheridium, Archegonium, Embryo Development, Life Cycle and Economic Importance

Equisetum-Arvense-Developmental-Stages-and-Life-Cycle-of-Equisetum
Equisetum Arvense (Field Horsetail) – Developmental Stages and Life Cycle of Equisetum

Strobilus of Equisetum

Equisetum arvense (Field Horsetail) consists of strobilus, each strobilus has a thick axis, having several whorls of densely crowded peltate appendages situated on the sporangiophores. The sporangiophores are arranged in whorls. Usually, each such whorl is composed of twenty sporangiophores or so. In many cases, immediately below the sporangiophores, the central axis of the strobilus bears a small ring-like outgrowth, known as the annulus.

Each sporangiophore is composed of a single slender, stalk of which the terminal end is expanded into a flattened peltate disc situated at right angles to the stalk. The peltate disc is usually hexagonal in outline in its surface view. This hexagonal appearance of the discs is attained due to the mutual pressure of the discs of the crowded sporangiophores situated on the strobilus. Each peltate disc bears a ring of 5 to 10 isolated sporangia on its underside. Each sporangium is elongated and sac-like. On their maturity, the sporangia open by longitudinal slits for dispersal of the spores.

Development of Sporangium

The development of the sporangia of Equisetum is eusporangiate type. This means, that they are not entirely derived from a single sporangial initial. According to Bower (1894) all the sporogenous tissue in a sporangium (special enclosure structure which is responsible for the production as well as storage of spores) are developed from a single superficial cell of the young sporangiophore.

Equisetum-Arvense-Logitudinal-Section-of-Strobilus
(Horsetail) Equisetum Arvense Logitudinal Section of Strobilus: A, longitudinal section of immature strobilus; B-D, development of sporangiophore and inversion of orientation of developing sporangium.

The development of the sporangia begins as the strobilus ceases to grow in length. The young sporangiophores are found in the form of convex outgrowths in acropetal succession on the conical axis of the strobilus. In the beginning of the development, the sporangiophore is hemispherial, but later on this soon becomes constricted at its base developing a short stalk and sporangia bearing peltate disc at its apical end.

The development of the sporangium begins from a single superficial cell known as sporangial initial situated in the peripheral region of the young hemispherical sporangiophore. The centre of the tip of young sporangiophore increases in size actively and the sporangial initials are shifted to its lateral sides.

Sooner or later, because of further development, the outer edge of the sporangiophore becomes inverted and faces the axis, this way the sporangial initials have also changed their position, and now facing the axis. At first, the sporangial initial divides periclinally forming the two daughter cells.

Equisetum-Arvense-Structure-of-Strobilus
(Field Horsetail) Equisetum arvense Structure of Strobilus: A, a strobilus; B, single sporangiophore with ten sporangia on the underside of the peltate disc; C, L.S. of strobilus showing sporangia; D, T.S. of strobilus showing sporangia

The daughter cell gives rise to the sporogenous tissue by repeated division, whereas the outer cell gives rise to both the sporogenous tissue and the portion of the jacket of the sporangium radial to the sporogenous tissue. The remaining portion of the jacket layer is developed from the cells laterally placed to the original initial. Ultimately the jacket layer becomes multilayered. The innermost layer is the tapetum and supplies nourishment to the developing spores. During the development of the spores, the tapetum and all but two layers of the jacket become disintegrated. This way, the mature sporangium is surrounded by a two layered jacket. The cells of the outermost layer of the jacket are spirally thickened.

During the development the cells of the sporogenous tissue become separated and act as spore-mother cells. About one third of the mother cells disintegrate. The disintegration of the mother cells, tapetum and the other jacket layers, together form a plasmodial fluid in which the remaining two third spore mother cells float about. Ultimately the spore mother cells undergo usual meiotic division developing the spore tetrads. Each tetrad is composed of four haploid spores.

Structure of Sporangium

Equisetum-arvense-Successive-Stages-in-Development-of-Sporangium
Equisetum arvense. A-E, successive stages in the development of sporangium; F, sporangiophore with peltate disc and sporangia in longitudinal section; G, spore with uncoiled elaters; H, spore with coiled

Each mature sporangium is elongated, sac-like and rounded at its apex. on maturity, it consists of single layered intact layer. By now, rest of the jacket layers are aborted. The sporangium contains within it the mature spore all alike. The Equisetum is homosporous. The sporangia are attached on the underside of the hexagonal peltate disc in a ring. Each sporangiophore bears 5-10 sporangia.

Dehiscence of Sporangium

On the maturation of the spores within the sporangium, it bursts by a longitudinal slit. Prior to the opening of the sporangia, the internodes of the strobilus elongate separating the sporangiophores quite apart from each other. The walls of the cells of the jacket layer are spirally thickened they shrink and lose water and the longitudinal slit develops on the sporangium for the release of the spores.

The Spores & Work of Campbell

According to Beer (1909) the spore (spores are defined as agents of sexual/asexual reproduction which are adapted in later stages of life cycle for dispersal or survival especially in plants, bacteria, fungi and algae) secretes a wall around it, composed of four concentric layers. The outermost layer is known as exosporium and the innermost endosporium. Outside the exosporium there is delicate cuticle called the middle layer, On the extreme outside of the spore there lies epispore which later on develops into four strips which get separated from each other from rest of the wall but or one common point of attachment. These strips are elaters. The strips or elaters remain coiled around the spore unless and until it is not disseminated from the sporangium. The elaters are hygroscopic. They remain coiled and uncoiled around the spore due to the changed conditions of the moisture of the air. When the air is moist, they remain coiled around the spore and as soon as the air is dry the elaters are uncoiled. The elaters of Equisetum do not resemble in any respect to elaters of Bryophyta. The ends of the spiral elaters are spoon-like. Each spore contains a centrally located nucleus and many small chloroplasts. The spores remain viable only for a few days after they are released from the sporangium.

Function of Elaters

The function of elaters is not very certain, but it is assumed their expansion may help in the dehiscence of sporangium. It is also thought that their hygroscopic movements may assist in the dispersal of the spores. According to Buchtien due to the presence of elaters, the spores are entangled together and thus dispersed in groups so the Prothalli are found in close vicinity. According to Goebel the elaters help in fastening the spore to the Substratum.

Germination of Spore and Development of Prothallus (Gametophyte)

Usually the spores germinate in the clayey soil and mud along the banks of the rivers and streams. As already mentioned, the spores contain chloroplasts and therefore. they can survive only for few days after their liberation from the sporangium. Prior to the germination the spore increases to some extent in its size. Within 10 to 12 hours, the first cell division in the development of the gametophyte (Gametophyte definition: the sexual phase in the alternation of generations of plants and certain algae) takes place, after the spore is sown. way, the two daughter cells are resulted one small and the other large in size. The small cell develops into the first rhizoid and the large one gives rise to the remainder of the thallus (Campbell, 1918; Kashyap, 1914 and Walker, 1931). The large cell divides irregularly several times giving rise to one to several celled thick cushion-like tissue. This cushion-like structure bears numerous rhizoids on its towel surface. This cushioned is known as gametophyte or prothallus. Upon this cushion-like massive structure, the green lobes or branched are found.

Equisetum-arvense-Successive-Stages-in-Spore-Germination
(Field Horsetail) Equisetum arvense A-F, successive stages of the spore germination and formation of the gametophyte; G, mature prothallus with antheridia and archegonia

The prothallus is elongated and lobed. The cells of the lower portion of the cushion are colorless whereas those of portion bear chloroplasts and are green. The entire margin of the prothallus is meristematic. When the growth of the entire margin is uniform the prothallus becomes quite smooth and circular, whereas on the other hand when growth is more active on one side or the other, the irregularly lobed prothallus is resulted. The upright green lobes arising from the upper side of the prothallus are densely crowded and almost cover the whole upper surface of the gametophyte. In certain species the gametophytes are quite big in size, e.g., in E. debile Rox, the gametophyte is of 3 centimeters in diameter (Kashyap, 1914), and two to three millimeters in thickness. In majority of the cases the gametophytes are less than a centimeter in size. The gametophytes are long lived. According to Walker (1921), it has been reported on the basis of culture experiments that they can survive for more than two years. The large gametophytes survive for a long period, whereas small prothalli die soon.

Nature of Prothalli

There are two types of prothalli-the small male prothalli bear only-antheridia whereas the large hermaphrodite prothalli bear both the antheridia and archegonia. whether Equisetum is monoecious or dioecious is a matter of controversy.

Kashyap (1917): According to Kashyap’s research, in E. debile when spores are sown thickly, the prothalli, remain small in size and bear only one type of sex organs. On the other hand, if the spores are shown quite separate from each other the prothalli become large in size and bear both types of sex organs, i.e., antheridia and archegonia. In such cases, however, the archegonia develop first and thereafter the antheridia.

Walker (1921): According to Walker (1921), E. laevigatum is a monoecious species. In Equisetum arvense, under unfavorable conditions the prothalli remain small in size and only antheridia are developed, upon them. In favorable conditions, half of the prothalli are developed as males and the remaining half as females. According to Walker (1931) the normal prothalli are monoecious, which develop archegonia first and the antheridia later on. He also stated that the prothalli developing in crowded conditions remain small and dioecious. In Equisetum arvense, the normal gametophytes produce archegonia at first and thereafter antheridia. In conclusion, the Equisetum arvense is monoecious. This view has been supported by several eminent bryologists.

Joyet Lavergne (1931): According to Joyet Lavergne (1931), morphologically the spores are alike, but physiologically they are different, and in the same lot some of the spores ate and some are female, so according to him, there is internal heterospory.

Development of Archegonium in Equisetum Arvense

If the favorable conditions prevail, the first sex organs appear when the normal separated gametophytes are 30 to 40 days old. The sex organs appear only when the is differentiated into two green lobed region and the lower massive tissue. Normally archegonia first on the archegonia develop in the meristematic margin of the prothallus. They are situated among the upright green lobes.

Equisetum-Successive-Stages-in-Development-of-Archegonium
Equisetum sp. A-F, successive stages in development of archegonium; G, mature

The archegonial initial is a superficial cell situated in the meristematic margin of the prothallus. The archegonial initial divides first by a periclinal wall giving rise to two daughter cells. The outer daughter cell is known as the primary cover cell and the inner is known as the mother cell of the central cell. The primary cover cell divides further producing four primary neck cells. The four primary neck cells. The four primary neck cells divide transversely producing an archegonial neck three or four cells in height. The central cell divides by a transverse wall producing primary neck canal cell and a primary venter cell. In some species, the primary neck cell rarely divides, whereas in other species it divides regularly producing two boot-shaped neck canal cells. In almost all the species, the primary venter canal cell divides by an asymmetrical transverse division producing a venter cell and an egg. Lastly, all the axial cells situated the archegonium are disintegrated but the egg.

Structure of Archegonium

An archegonium (the female reproductive organ in plants) consists of a sunken base in the prothallus and a projecting neck. The archegonial neck is composed of four vertical rows of cells. Each row is two to four cells in height. The terminal cells of the archegonial neck are quite long, which become separated and curved outward on their maturity. The axial row of the cells situated within the archegonium consists of three or four cells-the egg, the venter canal cell and or two neck canal cells. When there are two neck canal cells, they are boot shaped, e.g., E. debile. In the last stage of the development of archegonium the axial cells are disintegrated but the egg.

Development of Antheridium in Equisetum Arvense

Equisetum-Successive-Stages-in-Development-of-Antheridium
Equisetum sp. A-D, successive stages in the development of antheridium; E, antherozoid

In Equisetum arvense (Field Horsetail) as soon as the formation of archegonia is over the antheridia are formed on the same prothalli. At the time of the development of antheridia the growth of the upright lobes on the prothallus is stopped, and the prothallus turns and expands upwards. The antheridia develop in these expanded and turned portions of the prothalli. On the other hand, in starved, crowded prothalli, only antheridia are developed.

These prothalli may also called the male prothalli. Usually, the antheridia are of two types – (a) the embedded type and (b) the projecting type. The former type of usually develop on the massive tissue of normal gametophytes and the later type of antheridia are developed in the meristematic margin of the starved crowded and dwarfed male prothalli. development of the antheridium (the male reproductive organ in plants) is as above right corner.

Equisetum-Successive-Stages-in-Development-of-Sporophyte
(Field Horsetail) Equisetum sp. A-D. successive stages in the development of the sporophyte (embryo); E, young embryo (sporophyte)

A superficial cell of the meristematic margin of the prothallus acts as an antheridial initial. The first division of the initial is periclinal. This way, two daughter cells are formed. The outer daughter cell is known as the jacket initial and the cell is known as the primary androgonial cell. The jacket initial divides rise to single layered jacket. Sometimes there lies a triangular opercular cell at the apex jacket layer. The primary androgonial cell divides repeatedly giving rise to a group of cells. Ultimately, the androcyte mother cells are formed. Near the nucleus of each mother cell two blepharoplasts appear. Now, each androcyte mother cell divides and give rise to two each androcyte each possess a blepharoplasty.

Each androcyte metamorophoses into a spirally coiled multi-flagellate antherozoid. In the beginning, the rounded nucleus and the blepharoplast of the androcyte become elongated. The blepharoplast is composed of many granules. Later on, all the granules become united. The nucleus elongates continuously and ultimately becomes the crescent shaped. In the same way, the blepharoplasty also becomes spirally elongated. Later on, the posterior end of the nucleus becomes expansive and flattened. A large vacuole is developed within androcyte. The cytoplasm of the becomes foamy and coarse.

Equisetum-Arvense-LS-of-Young-Sporophyte
Equisetum sp. A, L.S. of a young sporophyte showing the stem apex, the two first leaf sheaths, the foot and the base of root; B, as in A, but the bud for the secondary shoot is upon the primary root. (After Campbell).

Structure of Antherozoid

The antherozoids (they are defined as male gametes found in a haploid structure called antheridium) are quite large in size. Each antherozoid is spirally coiled. The posterior part of the body of the antherozoids is thick and flat. The anterior part is conical and bears numerous flagella. The body is almost entirely derived from the nucleus of the androcyte. The numerous flagella at the anterior end of the antherozoid originate from the cytoplasm of the mother cell. The flagella are attached to the blepharoplast, which also lies at the anterior end.

Equisetum-Arvense-Embryo-Prothallus
Equisetum arvense. A, older embryo B, V.S. Of a lobe of prothallus with a young sporophyte (A, after Sadebeck; B, Hofmeister)

Fertilization

On the maturity of the archegonium the neck canal cells and the venter canal cell are disorganized. The antherozoids swim down the neck canal and approach the egg. Several antherozoids travel down the neck canal but only one of them penetrates the egg. In E. debile, Sethi (1928) had observed that the complete antherozoid enters the egg and fertilization is affected forming a diploid oopore (2x). Unlike other pteridophytes usually more than one archegonia fertilized and several sporophytes are developed on the same prothallus. Usually 8 to 10 develop on a single prothallus, e.g., E. debile. Kashyap (1914) has recorded as many as 15 sporophytes on a single gametophyte in E. debile.

Development of Embryo (Young Sporophyte)

Equisetum-Young-Sporophytes
(Field Horsetail) Equisetum sp. Young sporophytes. A-C, young sporophytes of Equisetum hiemale; D, large prothallus with six developing young sporophytes

As supported by Campbell (1918, 1928); Jeffery (1899) and Walker (1921), usually the first division of the oospore is transverse, giving to two cells-the upper one is epibasal and the lower one the hypobasal. Here in the case both the halves of the oospore take part in the development of the embryo proper and no suspensor is developed like that of Selaginella. The next division takes place at right angle to the first and the quadrant stage of the embryo is attained.

According to Sadebeck (1878) in Equisetum (Horsetail) the first division of the oospore is followed by two successive divisions at right angles to each other giving rise to eight cells. According to Campbell (1921), in E. debile the entire hypobasal half develops into a foot, whereas in E. arvense (Sadebeck, 1878) the hypobasal half of the embryo gives rise to both foot and root. Early in the development, in E. arvense the epibasal cell divides by three intersecting walls in such a way through that a tetrahedral apical cell of the stem is differentiated.

Graphical Life Cycle of Equisetum

The three cells, lateral to this apical cell are also differentiated, which develop in the three leaves of the first leaf sheath. Sometimes, there may develop two or four leaves instead of three. The so-developed leaves are quite small and scale-like. In Equisetum debile, where the entire hypobasal half develops into foot, a superficial cell of the epibasal half lying near the hypobasal half develops into the apical cell of the root. The portions of the stem and root of the embryo grow and elongate quite rapidly.

Equisetum-Graphical-Life-Cycle
Equisetum Graphical Life Cycle
Equisetum-Life-Cycle
Equisetum Life Cycle: A, sporophyte; B, strobilus; C, sporangiophore with sporangia; D, L.S. of Sporangiphore showing sporangia and peltate disc; E, spore mother cell; F, spore tetrad; G, spore with coiled elaters; H, spore with uncoiled elaters; I, young prothallus; J, mature prothallus with archegonia and antheridia; K, antheridium; L, archegonium; M, antherozoid; N, egg; O, oospore; P, young embryo; Q, young sporophyte

The stem portion of the embryo elongates and bursts through the neck of the archegonium, forming the calyptra and grows upright. On the other hand, the root grows downward of the gametophyte and reaches the soil. Soon after, the stem differentiates into nodes and internodes. Generally, a whorl of three leaves, or so is found at each node. The stem grows upward unless and until 10 to 15 nodes and internodes are developed. Thereafter a secondary branch arises from the base of the primary branch. This branch bears four or five leaves at each node. The growth of the secondary branch is also limited and reaches probably to the same height as that of primary branch. More vigorous branches arise successively from the base of the primary stem. The third or four branch turns downward into the soil and becomes rhizome of the (Horsetail) Equisetum plant.

According to Campbell (1928), the origin of secondary branch is endogenous and it arises from the of the primary root. The secondary branch possesses adventitious roots. Several secondary branches of Limited growth arise successively from the young sporophyte. All such branches are endogenous in their origin. From the point of view of its anatomy, the primary stem is solid and carinal canals which develop later on in the adult stem. However, the xylem is better developed in the primary stem. The base of the seedling bears protostele, but later on the stem bears siphonostele at the level of the first branch.

Economic Importance

Some of the species of equisetum (Horsetail) are used in the preparation of the indigenous medicines

  • E. arvense is used as diuretic
  • E. debile is used as a cooling medicine.

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