In this article we will discuss about:- 1. Occurrence and Distribution of Equisetum 2. Sporophyte of Equisetum 3. Gametophyte 4. Phylogeny.
Occurrence and Distribution of Equisetum:
Equisetum is popularly known as horsetail because of its bushy habit. There are about 25 species distributed in all parts of the world except perhaps in Australia and New Zealand. They are found mostly in North temperate and tropical zones.
In India the genus is represented by the following species:
E. arvense, E. ramosissimum, E. debile etc. Bir and Bhusri (1985) have reported three species of Equisetum from Shimla Hills in Himachal Pradesh. Equisetum flourishes very well in a variety of habitats. E. debile grows in sandy soil along the banks of rivers, and E. arvense in grass lands with open porous soil. E. palustre commonly called the marsh horsetail grows in ponds or marshes.
Sporophyte of Equisetum:
Morphology of the Plant:
The plant body of Equisetum consists of an underground prostrate rhizome from which spring up a number of aerial shoots (Fig.100). The aerial shoots may be perennial or deciduous. There is a great variety in the height of the aerial shoots.
They range from a few centimetres (E. scirpoides) to several metres (E. giganteum). In some instances aerial shoots as long as thirteen metres have been found. In spite of this length, the diameter is very less. Maximum diameter of the stem reaching ten centimetres, is reported in E. schallneri.
The prostrate rhizome grows 0.3 – 0.6 meters below the soil and branches profusely. It is differentiated into nodes and internodes. At each node are found a whorl of minute scale leaves. The leaves are connate at the base and form a sheath surrounding the rhizome. Alternating with the leaves are found the buds or the branch primordia. These buds develop either into a branch of the rhizome or the aerial shoot.
The aerial shoots arise from the nodal regions of the rhizome. All the serial shoots may be of the same type being chlorophyllous and much branched (E. ramosissimum) or as in E. arvense the aerial shoots are of two types, chlorophyllous and achlorophyllous. The achlorophyllous shoots have less branching in comparison with the chlorophyllous branches.
Further, achlorophyllous branches are fertile bearing the reproductive structure ‘Cone’, at their tips. The aerial shoots produce secondary branches in whorls at the nodal regions. These may be 5-6 internodes in length. The secondary branches arise from the branch primordia. Usually there will be many of them resulting in a whorl of branches.
The aerial shoots have the same general plan like the rhizome. They are also differentiated into nodes and internodes with the intermodal regions having longitudinal ribs. These ribs alternate with the grooves at successive internodes. On the aerial shoot there is deposition of silica which makes the plant body rough, hence the name scouring rushes. At the nodal regions are found the leaves.
The leaves are small microphyllous and scaly. They exhibit a whorled phyllotaxy. As in rhizome, the leaves are connate at the base. The number of leaves per whorl ranges from 3-40. The leaves are in line with the longitudinal ribs below and alternate with the grooves. The leaves are extremely reduced and of little value in photosynthesis. Much of the photosynthetic activity is carried out by the aerial shoots.
The reproductive structures viz., the cones are borne at the tips of the aerial shoots. There is great variety in the fertile branches. In some species (E. debile, E. myriochaetum) there is no distinction between fertile and sterile branches. All the shoots are chlorophyllous and at the same time produce cones at their tips. In E. arvense and E. telmateia, the fertile and sterile shoots are distinct.
The former are un-branched and achlorophyllous and drop down after the liberation of spores from the cone. In species like E. sylvaticum and E. pratense, the fertile shoots become chlorophyllous, develop branches and function like sterile shoots after the liberation of spores from the cone.
In E. palustrae three types of aerial shoots are found viz.:
(a) Sterile shoots
(b) Fertile shoots and
(c) Intermediate shoots which are at first non-green and reproductive, but subsequently function like typical green sterile shoots.
The rhizome is anchored to the soil with the help of adventitious roots that are produced at the base of the branch primordia in the nodal region. Some of the lower nodes of the aerial shoots also may produce roots. Even the roots are whorled in arrangement. They may or may not be branched.
In stem, apical growth results due to the activity of a pyramidal apical cell with three cutting faces. In root, the growth is brought about by an apical call with four cutting faces.
Anatomically the stem shows an epidermis, cortex and stele. The outline of the section is wavy due to the presence of ridges and grooves. The different parts of the stem (node, internodes etc.) vary slightly in their anatomical details.
Internode of an aerial sterile shoot:
A transverse section shows a wavy epidermis, a multilayered cortex and a ring of vascular bundles surrounding a central cavity (Fig.102).
Epidermis is single layered being made up of elongated cells with thick walls. The cell walls have a heavy deposition of silica. The continuity of the epidermis is interrupted by the presence of stomata. These however are restricted to the grooves.
The stomatal apparatus is composed of an inner pair of guard cells and a pair of subsidiary cell’s which cover the guard cells. Another characteristic feature of the stomata is the presence of rib like silica thickenings present between the subsidiary cells and guard cells.
Hauke (1963), Pant and Mehra (1964) and Pant and Kidwai (1968) have worked on the stomatal development in Equisetum. The stomatal structure is used as a criterion to classify Equisetum into two sub-genera.
In the sub-genus Euequisetum (E. arvense, E. svlvaticum etc.), the stomata are on level with the epidermal cells, and or either scattered or arranged in broad bands. In the sub-genus Hippochaete (E. giganteum, E. hyenale etc.), the stomata are sunken and are arranged in narrow bands.
Above the epidermis there is the deposition of a cuticle. The cortex is broad and highly differentiated. Below the ridges are found patches of sclerenchyma (Fig. 103) offering mechanical support to the stem. Below the grooves or sometimes forming a continuous band is found the chlorenchyma, which is the chief photosynthetic tissue of the plant.
Internal to the chlorenchyma is found the parenchyma constituting the innermost region of the cortex. This region consists of air cavities called Vallecular Canals, arranged in a ring alternating with the vascular bundles. The vallecular canals lie directly below the grooves.
The vascular bundles are arranged in a ring alternating with the vallecular canals. They surround a central cavity (pith cavity). The nature of the endodermis varies in different species. All the vascular bundles may have a common outer endodermis (E. arvense) or each vascular bundle may have a separate endodermal covering (E. giqanteum).
In some species (E. svlvaticum) in addition to an outer endodermis there will be an inner endodermis also lining the pith cavity. The vascular bundles are collateral and endarch with poorly developed xylem. Some of the protoxylem elements disintegrate to form a cavity known as the Carinal canal. Usually there will be a thin layer of pericycle lining the endodermis.
Node of aerial shoot:
It is similar to the internode except for the following. In the central region is a parenchymatous pith. Vallecular canals are absent. Since the alternate bundles of successive nodes are joined by short branches at the nodes, there will be a continuous ring of vasculature.
Sometimes carinal canals are also absent. Sharma and Bohra (1979) have studied the vascular structure of the nodes of rhizome and aerial shoots in Equisetum ramosissimum sub spp. ramosissimum.
According to these workers the cavities and canals disappear at the nodes with the vascular bundles arranged in two rings. The vascular bundles of the outer ring represent leaf traces, while the inner cauline bundles have xylem tracheids with well-developed reticulate thickenings.
There are no stomata in the epidermis; so also there is no chlorenchyma in the cortex (Fig. 104). The development of the mechanical tissue is less when compared with the aerial shoot. The pith is solid (E. arvense)
There is considerable difference between vegetative branches and the cone axis. In the latter, vallecular and carinal canals are absent. In the vascular bundles the protoxylem strands form a continuous system. The metaxylem is separated from the protoxylem by a band of parenchymatous cells, Sporangiophoric traces depart from the vascular cylinder and enter the successive whorls of sporangiophores.
Nature of the stele:
The nature of the stele as to whether it is protostelic or siphonostelic is debatable. According to Barratt (1920), the vascular cylinder at the base of the first shoot of the sporeling is protostelic, subsequently it becomes siphonostelic. Barratt (1920), believes, the intermodal system (of having separate bundles) has been derived from a protostele through the intermediate stages of siphonostele and solenostele.
According to Jeffrey (1894) the inter-fascicular gaps of the internode are branch gaps, but according to Barratt (1920), these are actually areas of xylem in which tracheids have been replaced by parenchyma. Since these gaps are not present above the branch trace they are not related to either the branch or the leaves.
A transverse section of the root shows an outermost single layered epidermis (piliferous layer), middle cortex and central stele (Fig. 105). Here and there, root hairs arise from the epidermis. The cortex has two zones. The outer zone is sclerenchymatous while the inner zone consists of thin walled parenchyma with air spaces. Internal to the cortex is the endodermis.
The endodermis is apparently two layered thick because the so called inner layer of endodermis lacks the casparian thickenings. In the absence of the thickenings it seems best to regard the inner layer as the pericycle, because the initials of the lateral root also originate from this layer. The stele is a protostele with di, tri, or tetrarch xylem. In smaller roots, in the centre there is a large metaxylem element. In larger roots there will be more than one metaxylem element.
The leaf has a single vein which is collateral and is surrounded by an endodermis. Xylem is poorly developed. Surrounding the vein are found bands of sclerenchyma alternating with parenchymatous bands. Encircling this are found thin walled parenchyma cells. Stomata are found in the lower epidermis.
Sporophyte reproduces vegetatively as well as by spore production. Vegetative propagation takes place by tubers which are produced on the rhizome. The tubers represent short rounded branches consisting of a single internode. At maturity a tuber detaches itself from the rhizome and develops into a new individual.
Spore Producing Organs:
At the apices of the aerial shoots are produced the cones or the strobili. Each cone has a stout central axis (Fig 106) bearing whorls of peltate appendages called the sporangiophores. In each whorl there are 10-20 sporangiophores. In some species (E. cryptophora) below the lowermost whorl of sporangiophores there is a ring like outgrowth called the annulus (Fig.106a).
Each sporangiophore has a short stalk, with which it is attached to the cone axis and a peltate disc. The ventral surface of the peltate disc (hexagonal) bears 5-6 sporangia in the form of a ring. In a young cone the flattened tips of the peltate discs are in close contact with each other protecting the sporangia.
Development and Structure of the Sporangium:
The sporangiophores arise, first on the cone axis in the form of hemispherical swellings. A constriction appears in the emerging sporangiophore delimiting the basal stalk and the terminal peltate appendage. The sporangial initials first arise on the dorsal surface of the peltate disc.
But centrifugal growth taking place in the centre of the disc pushes the developing sporangia first towards the margin of the disc and subsequently towards the ventral surface, where they lie ultimately. A single superficial cell is differentiated as the sporangial initial, but the surrounding cells also take part in the further growth of the sporangium.
Hence the development is of the eusporangiate type. A periclinal division in the sporangial initial results in forming an outer cell and an inner cell (Fig. 107). The outer daughter cell divides periclinally as well as anticlinally to form several layers of cells.
Of these, some of the inner layers contribute to a part of the sporogenous tissue while the remaining form the jacket of the sporangium. Meanwhile the inner cell divides in all the planes to produce a mass of sporogenous tissue. In Equisetum, the sporogenous tissue is derived partly from the inner cell and partly from the outer cell.
As the sporogenous cells are about to transform themselves into spore mother cells, the innermost wall layer differentiates itself into a tapetum. The tapetal cells become glandular in appearance. The cells of the sporogenous tissue round off and form the spore mother cells.
Of these, about one third disintegrate and form a piasmodial mass of cytoplasm. At this stage the tapetal cells also break down and add upto the piasmodial mass. The spore mother cells are bathed in this piasmodial mass from which they derive the nutrition. The nuclei present in the Plasmodium increase their number by amitotic divisions.
The spore mother cells undergo reduction division and produce tetrads of haploid spores. The developing spores absorb the Plasmodium. Sometimes, even the inner layers of the sporangial wall are absorbed so that the mature sporangium has a single layered wall. A mature sporangium is sac like and elongate. It is covered by a single layered jacket and contains spores of only one type (homosporous).
Dehiscence of the Sporangium:
The wall of the sporangium has spiral and annular thickenings present in the form of bands. These help in the dehiscence. When sporangia are mature the cone axis between the whorls of sporangiophores extends separating the contact between the peltate discs.
At the same time the sporangiophore stalk also elongates. This facilitates the easy dispersal of the spores. Due to the differential hygroscopic response of the wall cells, the sporangium splits open along a longitudinal line. Spores are wind disseminated.
Gametophyte of Equisetum:
Structure and Germination of the Spores:
The spores are minute in size and are spherical in shape. The spore wall is four layered (Beer 1909). The outermost is the epispore (serispore). Internal to this is a cuticular middle layer.
Next to the middle layer internally is the exine. The innermost wall layer is the cellulosic intine. According to Beer (1909), the middle layer and the epispore are secreted by the tapetal cytoplasm while the exine and the inline are derived from the spore protoplast.
Kedeves (1979) has studied the spore morphology in 14 species of Selaginella using scanning electron microscope. Biochemical studies of Gullvag (1969) on the storage materials in the spores have shown the presence of lipid granules and proteinaceous substances besides large chloroplasts.
The epispore splits spirally and forms two ribbon like bands which are attached to the spore at their central points. Due to this, the bands look like four distinct appendages (Fig. 108). The free ends of these appendages are spoon shaped. When the spores are in the sporangium, the bands remain spirally coiled around the spore.
In the absence of moisture they open out. This can be easily seen by placing a few spores in a drop of water on a slide. When observed under microscope the spiral bands gracefully uncoil as the water dries up. The spiral bands are given the name Elaters as they are believed to help in spore dispersal.
Various functions are attributed to the elaters. These are:
(1) Expanding elaters may help in dehiscence of the sporangium,
(2) Elaters help in spore dispersal;
(3) Elaters entangle several spores (they are shed in a mass), so that several prothalli grow together;
(4) Elaters act as parachutes;
(5) When spores fall on a substratum, elaters may help in temporary anchorage.
The spores of Equisetum are chlorophyllous. The spore germination takes place immediately on liberation from the sporangium. The first division results in the formation of a smaller lenticular cell and a larger cell (Fig. 108). The small cell elongates and forms the first rhizoid. The remainder of the gametophyte is derived from the larger cell.
The development of the gametophyte is exosporic. Further divisions in the young gametophyte apparently do not follow any pattern. The first few divisions in the larger cell may be transverse resulting in a filamentous gametophyte or vertical and oblique producing a cushion like pro-thallus.
Structure of the Mature Gametophyte:
The gametophyte has two regions i.z., the basal compact cushion like region and an upper photosynthetic region made up of a number of chlorophyllous lobes (Fig. 109). Some of the prothalli when growing under crowded conditions may not show these well marked regions. The gametophytes are wholly parenchymatous. There is no indication of any mycorrhizal association. The nutrition is autotrophic.
The meristematic tissue is situated at the basal cushion in the form of a rim. Archegonia are formed first, in between the photosynthetic filaments, later the meristematic edge turns up and forms the antheridia. With the formation of the antheridia the growth comes to a stop.
There is a controversy as to whether Equisetum is monoecious or dioecious. According to Kashyap (1917) crowded prothali are dioecious. Walker (1921) believes that prothalli are monoecious.
The controversy seems to be the result of protogynous nature of the prothalli which form archegonia first and only very lately, the antheridia. Recent studies of Duckett and Pang (1984) indicate that Equisetum giganteum exhibits certain characters which point in the direction of heterospory.
In this context a mention may be made of ‘Law of cytoplasmic sexualization’, proposed by Joyet Lavergne (1931). According to him, there is physiological heterospory in Equisetum. The females spores have a lower oxidising power than the male spaces and possess a fat which reduces osmic acid. This property is not seen in the male spores.
Irrespective of the fact whether the gametophytes are monoecious or dioecious the sex organs develop after the gametophytes are about a month old.
Development and Structure of the Antheridia:
Generally the antheridia are formed later than the archegonia. The meristematic rim of the basal cushion turns up and it is in this upturned part that the antheridial initials differentiate. In some of the dwarfed and starved prothalli, soon after the formation of antheridia the pro-thallus dies.
The antheridia are of two types, viz., the embedded type and the projecting type. While the former develops on the basal cushion, the latter develops on the margin or apex of the photosynthetic lobes. The development is essentially similar in both the types of antheridia except for the first one or two divisions.
While a superficial cell directly functions as the antheridial initial in the embedded type, a superficial cell on the photosynthetic lobe after cutting off the three peripheral cells (Fig.110d) functions as the antheridial initial in the projecting type. Further development is similar in both the types of antheridia.
The antheridial initial divides periclinally and produces two superposed cells. Of these, the outer by undergoing several anticlinal divisions builds up the single layered jacket of the antheridium (Fig.110a-110e).
The inner cell divides in all the planes to form a mass of androgonial cells. The last generations of these cells i.e., androcytes metamorphose into antherozoids. A mature antherozoid is spirally coiled and multi-flagellate (Fig.110f).
The embedded type of antheridium is sunk in the gametophytic tissue, while the projecting type, as the name indicates protrudes a little above the surface of the gametophyte. About 256 antherozoids are formed in each antheridium. A mature antheridium dehisces by the splitting open of one of its jacket cells. This is due to the absorption of moisture. The antherozoids come out in a mass.
Development and Structure of the Archegonia:
Archegonia normally appear in the meristematic region in between the photosynthetic lobes. Any superficial cell near the meristem can function as the archegonial initial. As usual the first division is periclinal and an upper primary cover cell and a lower primary central cell (Fig. 110g) are formed.
The former divides by quadrant walls to form four neck cells which by further transverse divisions form a neck, 2-4 cells in height (Fig. 110i, 110j). The primary central cell divides transversely to form an upper neck canal initial and a lower central cell. Usually the neck canal initial divides to form two boot shaped neck canal cells (Fig.110i). The central cell divides to form an egg and a venter canal cell.
In a mature archegonium, the neck projects out a little while the venter region is sunk in the prothallial tissue. It has an egg, a venter canal cell and two juxtaposed (Fig. 110i) or superposed (Fig. 110k) neck canal cells.
This is brought about by the antherozoids swimming down the canal of the archcgonial neck. Several enter the archegonial neck, but only one fuses with the egg forming a diploid zygote. Many archegonia are fertilized on the same pro-thallus resulting in the development of several sporophytes on a pro-thallus.
The first division of the zygote is usually at right angles to the long axis of the archegonial neck resulting in the formation of an epibasal cell and a hypo basal cell. No suspensor is formed. Both the epibasal and hypo basal halves contribute to the embryo.
The walls of the next division are laid perpendicular to the plane of the first division, resulting in the formation of a quadrant. But the four quadrants do not give rise respectively to stem, root, foot and cotyledons. In E. debile the ensure hypo basal half gives rise to the foot, while in E. arvense, hypo basal half gives rise to both foot and root.
In the epibasal half, a tetrahedral apical cell is formed by three intersecting walls. The cells lateral to the apical cell develop into the leaf sheath. These leaves are minute and of little value in photosynthesis for the young sporophyte. The first root is differentiated either from the epibasal half or from the hypo basal half depending upon the species.
This root penetrates the pro-thallus and fastens the young plant to the substratum. Growth and elongation of the stem and root parts of the embryo are rapid. As soon as the root establishes itself on the soil, the stem tip pierces through the archegonial neck region and grows up. Differentiation of nodes and internodes in the stem (Fig. 111) are noticeable at this stage.
The growth of the first shoot is limited. It ceases to grow after it has become 10-15 internodes long. A secondary shoot develops by a bud at the base of the primary shoot. This is more vigorous than the primary shoot.
In E. debile, the secondary shoot arises from the tissues of the primary root. The secondary shoot also is short lived and a tertiary shoot is formed. Eventually a shoot grows into the ground, forms the rhizome from which aerial shoots are given off.
Morphology of the Spore Bearing Organs:
The nature of the cone is interpreted in two ways. According to Browne, the cone is a condensed vegetative axis consisting of nodes and internodes. The cone consists of sporangiophores at the nodal regions instead of leaves and buds. It seems highly likely that the sporangiophores are the modifications of buds which would have otherwise developed into branches.
Barratt (1920) disagrees with Browne (1915) and holds that anatomical evidence does not favour the existence of nodes and internodes in the strobilus. The arrangement of xylem and the origin of sporangiophore traces, support Barratt’s views.
The sporangiophore itself has been interpreted in various ways. Eames (1936) has listed some of these views as follows:
1. It is a lateral branch bearing a whorl of sporangia.
2. The sporangiophore represents the fertile ventral lobe of a dorsiventrally divided sporophyll, the dorsal sterile lobe of which is lost during the course of evolution.,
3. It is the stalk of a raised and divided sporangium.
4. It is a modified sporophyll.
5. It is an organ sui generis.
Some of the teratological (abnormal) specimens observed by Tschudy (1939) indicate the possible sporophyll nature of a sporangiophore. In some abnormal strobili off. telemateia, there were sporangiophores ranging from normal peltate ones to expanded leaf like organs bearing one or two sporangia.
Nature of the annulus:
The morphological nature of the collar like structure present at the base of cones in some spices of Equisetum is in dispute. Some botanists regard it as the vestige of the last whorl of vegetative leaves before the transformation of the shoot into a cone.
But in some cases (E.cryptophora) it has been seen that even the annulus may be sporangiophoric in nature. Barratt (1920) believes that annulus represents the reduced whorl of sporangiophores. The presence of sterile bracts in the cones of fossil members like Calamostachvs and Palaeostachys have made many people to interpret that the annulus is a vestige of such bracts.
Classification of Genus:
The genus is divided into Hippochaete and Equisetum based on the nature of the stomatal apparatus (Hauke, 1963). In the former the stomata are sunken, while in the latter the stomata are on level with other epidermal cells.
Duckett (1979) has justified the division of the genus into two based on the gametophytic characters also. According to him, the sub genus Equisetum is characterized by plate lamellae and projecting antheridia, while Hippochaete has column lamella and sunken antheridia with two opercular cells.
Dubois Tyeski and Giererd (1986) have made a comparative study of the species of Equisetum included in the sub genus Hippochaete. Bandich and Anderson (1983) have studied the DNA sequences in many species of Equisetum and have used this to indicate interspecific relationships within the genus. Biochemical studies of Czeczuga (1985) have revealed that ‘horsetails are characterized by the occurrence of β carotene, β crypto xanthin, lutein epoxide and zeaxanthin.
The basic chromosome number is approximately n = 008. It seems that the sub-genus Hippochaete has larger chromosomes than the sub-genus Equisetum.
Phylogeny of Equisetum:
The members of sphenopsida are unique among pteridophytes for more than one reason. The jointed plant body with intercalary meristem, whorled leaves alternating with the branches, presence of vascular bundles, sporangia being borne on sporangiophores are some of the notable features.
Equisetum is remarkable for the curious intermingling of hydrophytic and xerophytic characters. While the reduction of vascular tissues (specially xylem) and presence of air cavities are indicative of its hydrophytic nature, reduced leaves photosynthetic stem, well developed mechanical tissues, cuticle and sunken stoma indicate the xerophytic characters.
Of the two sub-genera, the sub-genus Hippochaete seems to be more primitive than the sub-genus Equisetum. The presence of separate fertile shoots in Equisetum points out to this fact. E pratense belonging to Equisetum is an intermediate type. Irregularity of divisions in the early embryogeny in Hippocrates is another evidence of its primitiveness.
There has been a good deal of speculation about the relationship of Equisetum with other extant pteridophytes. According to Jeffrey (1925) and Scott (1928), Equisctineae and Lycopodineae show sufficient resemblance to place both of them under Lycopsida.
The reasons for this are the nature of gametophyte, stelar organization etc. Van tieghem (1886), compares the stele of Equisetum to that of Ophioglossum. Campbell (1939) believes that the stelar organisation in Equisetum is more akin to forms than to Lycopodineae. He states that the intermodal bundles represent fused leaf traces as in ferns. The fact that the vascular bundles are not as complicated as in ferns is due to the reduced nature of the leaves.
According to Campbell (1939), the nature of the gametophyte, multi-ciliate antherozoids (bi-ciliate in Lycopodium) all point out a fern relationship to Equisetum. Based on all these evidences it may be said that Equistitineae and filicineae may have arisen from a common stock close to the Devonian psilophytales.