The below mentioned article provides an overview on Gnetopsida. After reading this article you will learn about: 1. Introduction to Gnetopsida 2. Systematic Position of Genus 3. Habit of Genus 4. Growth 5. Anatomy of the Stem 6. Reproduction 7. Economic Importance.
Introduction to Gnetopsida:
Class-Gnetopsida; Order-Gnetales; Genus Ephedra:
The Gnetales are a small group of plants, comprising of three genera-Gnetum, Welwitschia and Ephedra. The importance of this group is because of its forming a connected link between gymnosperms and angiosperms. They possess extraordinary morphology and diverse habit.
They are gymnosperm-like in that their ovules are naked and borne on cones, whereas angiosperm-like in that their ovules and microsporangia are borne on somewhat flower-like fertile shoot. The ovules are, however, enclosed within coverings additional to the true integument, which are sometimes considered to be the equivalent of ovary walls.
Another angiosperm-like character is the presence of vessels in the xylem.
A common character of the group is the prolongation of the apex of the integument into a long narrow tube with a flattened orifice, by means of which the pollen is collected. They are so advanced in structure that in several respects they resemble the angiosperms.
However, there is no direct relationship, and it has been established that the Gnetales were not the ancestors of the angiosperms. They are a parallel line of evolution.
Three genera, i.e., Gnetum, Welwitschia and Ephedra, were long considered to form a single family Gnetaceae of the order Gnetales, but on the basis of morphological evidences the order has been divided into three families, i.e., Gnetaceae, Welwitschiaceae and Ephedraceae. Now the three orders have been established known as Ephedrales, Welwitschiales and Gnetales.
Whatever the system of classification may be, there are three genera:
2. Welwitschia and
The genus Ephedra, consists of shrubby plants with minute leaves. There are about 35 species distributed in the New and the Old Worlds. The species have mostly a restricted range of distribution.
The genus Welwitschia, comprises the remarkable living plants. There is only one species, W. mirabilis, particularly confined to the desert region of South West Africa. The stem resembles an enormous woody carrot, the top of which is concave and may reach 4 feet in diameter. It is entirely buried in the sand and bears only two long strap-shaped leaves, which persist throughout life.
The genus Gnetum includes trees, shrubs or woody climbers. The climbers are predominating. There are about 4 species distributed in the tropical regions of the world, especially West Africa to the Amazon region. They are invariably the jungle plants.
Their vegetative appearance is entirely angiospermic, and their large net-veined leaves cannot be distinguished from those of dicotyledons.
Here the detailed account of the genus Ephedra has been given.
Systematic Position of Genus:
There are about 35 species of Ephedra growing in the arid regions about the Mediterranean, and in tropical to temperate Asia, North America and South America. The species have mostly a restricted range and none of the species is common to both sides of the Atlantic.
A few species are found in our country. They are Ephedra intermedia, E. gerardiana, E. foliata and E. nebrodensis. E. intermedia occurs in the natural state in the Kashmir and Northwest Himalayas at an altitude of about 2500 to 4200 metres above sea level. E. foliata occurs in the wild state in the Punjab and certain parts of Rajasthan.
This species is commonly met within the forests near Lahore in Pakistan. E. gerardiana and different varieties of this species are found along the length of the Himalayas at an altitude of about 3050 metres above sea level.
Habit of Genus:
Usually the plants are low straggling shrubs reaching the height of one metre or so. The green, long jointed, slender branches bear minute leaves. The scaly leaves are opposite and connate in a two-toothed sheath. The whole aspect of the plant is xeromorphic. True foliage leaves are generally lacking and the whole habit is suggestive of a shrubby Equisetum. The leaves also occur in threes and rarely in fours.
Growth of Genus:
The growth of the internodes is independent and takes place by a pale-coloured meristematic zone at the base of each node. The meristem in some species becomes hard and turns into an abscission layer in the end of the growing season, with the result the branches drop off, which are to be replaced by new axillary shoots in the next growing season.
The species may be called deciduous and they are supposed to be analogous to the trees which drop their leaves in winter.
Anatomy of the Stem of Genus:
The surface of the stem is ribbed and therefore, the outline of the stem in transverse section is not circular. The epidermis is quite thick and just beneath the ribs there are groups of fibre cells. The stomata occur in furrows between the ribs and each stoma is situated in a circular pit.
The cortex is differentiated into a loose outer palisade layer and an inner spongy tissue. Both of these contain chlorophyll. The innermost layer of the cortex is known as endodermis.
The endodermis surrounds the ring of collateral and end-arch vascular bundles. The large and small bundles are grouped into pairs of threes. Each small bundle is represented by a leaf trace. Each vascular bundle runs through two internodes and thereafter passes out in pairs into the leaves. This way, there is a double leaf trace.
The cambium is present and sometimes the older stems become greatly thickened due to secondary growth. In the course of the formation of secondary wood the annual rings are formed. The young stems contain uniseriate medullary rays, whereas in older stems these rays become very broad and long, vertically with the result of their own growth as well as by the fusion of adjacent rays.
They are composed of lignified cells and therefore, the wood texture is very hard. The xylem elements, i.e., the tracheids possess single rows of scattered bordered pits arranged on the radial and tangential walls. Bars of Sanio and trabeculae are also present across the tracheids.
The most interesting feature is the presence of vessels, resembling those of angiosperms. The vessels are found in abundance. In the spring or growing season they are largest in size but they decrease in size and number in the autumn wood which completely lacks the vessels.
The pith cells of the nodal region become lignified resulting in the formation of transverse plates. The resin ducts are not found, but the cells with large stellate crystals of calcium oxalate are found in abundance.
The phellogen appears just outside the phloem in the third or fourth year and with the result the periderm is formed which displaces all the outer tissues.
Reproduction of Genus:
The Ephedra plant is normally dioecious, but monoecious plants and even hermaphrodite flowers are known. Both male and female reproductive organs are borne on small strobili. The scales of strobili are arranged in decussate pairs forming a short membranaceous strobilus. The strobili normally arise from the axils of the young shoot. They may develop simply or in small sessile groups.
As the rule the plants are dioecious and strictly male or strictly female flowers are formed. However, P.N. Mehra (1950) reported the occasional occurrence of hermaphrodite flowers in Ephedra inter-media. Here, the hermaphrodite flowers are borne on bisexual strobili.
According to Eames (1952), the Ephedra plants lack totally the hermaphrodite flowers, he regarded these hermaphrodite flowers mere monstrosities. Thibout and Takhtajan have also reported ‘bisexual cones’ in Ephedra. The bisporangiate strobili produce stamens at the base and ovules above.
The Male Strobilus:
Each male strobilus arises from the axil of a leaf It consists of a short axis which bears a number of thick and closely arranged bracts found in decussate pair. The pairs of bracts vary from two to twelve in number. A single male flower stands in the axil of each bract.
Each male flower consists of a perianth of two scales and a stalked stamen arises from between the two scales of the perianth and bears two to several anthers (sporangia) at its top. The simple stamen bears two sporangia and when it becomes branched it bears several sporangia. The anther is two or three lobed. Each lobe opens by an apical slit and the pollens are released. The pollen are elliptical in shape.
The Female Strobilus:
Each female strobilus is found in the axil of a leaf on the young shoot. The female strobilus though quite similar in structure to the male strobilus yet smaller in size.
Each strobilus bears two to four pairs of bracts. Each strobilus ends in a group of female flowers, one, two or three in number. Each flower consists of a nucellus with two envelopes. The inner envelope consists of two segments while the outer of four segments.
The inner envelope is a true ovular integument, and the outer envelope is perianth. The whole structure is found in the axil of one of the bracts of the uppermost pair.
The upper half of the nucellus is free from the inner integument, whereas the inner integument is prolonged out into a thin walled micropylar tube. Sometime, this is found spirally twisted and opening at its tip into a tongue-like extension.
The pollination drop extrudes from here by means of which the pollen is drawn into the micropylar tube. Later on the perianth becomes closely fit to the integument so that the structure becomes woody or fleshy developing into the outer coat of the mature seed.
The Male Gametophyte:
The nuclei of the pollen (microspore) of Ephedra divide in the similar plan as that of Pinus. This indicates the positive sign of linkage of Ephedra with conifers. After a short time of the rounding off of the spore, the nucleus divides resulting in the formation of first prothaliial cell. A second division gives rise to second prothallial cell and the antheridium initial, which are not separated from each other by any wall.
The second prothallial cell and its nucleus become flattened, whereas the nucleus of antheridium initial becomes very much enlarged, and divides producing the generative and tube nuclei. A zone of cytoplasm surrounds the generative nucleus. This zone may be differentiated from the general cytoplasm.
The second prothallial cell and tube nuclei are still embedded in the general cytoplasm with no wall of demarcation. The generative cell nucleus divides giving rise to the stalk and body nuclei. These nuclei remain invested in the cytoplasm of the generative cell, and the body cell becomes larger than the other.
The pollination occurs at this stage during the first two weeks of April. At this stage the spore contains five nuclei, i.e., first and second prothallial, stalk, body and tube nuclei.
After reaching the deep pollen chamber the exine of the spore is thrown off and the in-tine remains free with its contents. The two prothallial cells soon disappear and the body cell nucleus divides forming two equal male nuclei.
The tube nucleus moves about the cavity of microspore and in the end rests against the spore wall. Soon after a pollen tube comes out from this very point where the tube nucleus rests. The tube nucleus and the two male nuclei accompanied by the nucleus of the stalk cell pass into the pollen tube.
After about twelve hours of pollination, the fertilization takes place. The pollen tube penetrates between the neck cells of the archegonium and the two male nuclei pass directly into the oosphere. In the case of Ephedra the fertilization takes place only twelve hours or so after pollination, whereas there is prolonged delay in other Gymnosperms.
The Female Gametophyte:
The details of the female gemetophyte have been worked out by Land (1904) in Ephedra trifurca. In the beginning a row of four megaspores is formed, the lowest megaspore enlarges in size and becomes conspicuous.
This megaspore nucleus divides into two nuclei, thereafter these two nuclei divide again forming from nuclei situated at the periphery of the megaspore sac at equidistant positions. Successive simultaneous divisions follow rapidly until about 256 free nuclei are formed by the process of free nuclear division.
This process takes place through a period of about twenty days. The cell walls appear in the endosperm, and the latter differentiates into two distinct regions known as micropylar and antipodal regions. The micropylar region consists of loose cells and the antipodal region of compact cells. The antipodal region becomes differentiated into two regions known as storage and haustorial regions.
The storage region comprises of the large portion of the endosperm which is full of starch and other foods. The haustorial region consists of one or two outermost layer of endosperm ceils situated at the antipodal end. The micropylar region contains two elongated archegonia and consists of elongated thin-walled cells.
Development of Archegonia:
Usually the two archegonium initials begin to develop in the micropylar region. The initial divides transversely cutting off the primary neck cell. The latter divides periclinally and other periclinal divisions follow it. Thereafter the anticlinal divisions take place which divide the cell of each tier into 4 cells.
Eight tiers of such neck cells are formed. Later on irregular divisions may take place and the appearance of the neck becomes like the ordinary tissue. The minimum number of neck cells is probably thirty-two, but sometimes they may be more. The archegonium neck of Ephedra is longest among the gymnosperms and thus associated with the deepest pollen chamber.
Structure of Archegonium:
Each archegonium is elongated and possesses an ill-defined jacket layer. The neck is at first made up of three tiers of cells but later eight or more tiers may be formed. According to the findings of Narang in some species of Ephedra archegonium groups occur at the micropylar end and sometimes an archegonial complex is formed by the aggregation of 4 to 8 archegonia.
The oosphere (egg) of the archegonium contains a large nucleus, with a dense plasmatic sphere around it. A large ventral canal nucleus is present but ventral canal cell is absent.
Both the male nuclei are released from the pollen tube into the archegonium, but only the first male nucleus fuses with the nucleus of the egg. The second male nucleus and the ventral canal nucleus divide forming many nuclei for a number of small cells around the top of the egg whose fate is not clearly understood.
This tissue is soon absorbed by the embryo and, therefore, may be regarded as physiological endosperm. The oospore nucleus divides into eight nuclei of which 3 to 5 may give rise to embryos.
Development of Embryo:
Land (1907) has worked out the details of the formation of the embryo of Ephedra trifurca which is as follows:
The oospore nucleus gives rise to eight free nuclei, more or less unequal in size. Three to five free nuclei enter into the organization of walled cells and produce embryos. The proembryonal cells are isolated from the surrounding cytoplasm by the cleavage formation, and around each nucleus a more or less irregular sheath of cytoplasm is deposited.
In usual way, the lower cells successfully produce embryos but sometimes, the upper cells of micropylar region produce embryos. Each functioning proembryonal cell becomes rounded and its nucleus divides producing two daughter nuclei which are equal in the beginning but soon one becomes larger than the other. Due to the formation of the vacuoles, the two nuclei become shifted on one side of the cell.
The larger nucleus passes nearly to the end of the tube and a wall is formed which cuts it off in a terminal embryo forming cell. The outer larger nucleus enters the suspensor and functions as suspensor nucleus. The terminal embryo cell divides and becomes a proembryo. The remainder of the tube forms the suspensor, which elongates and pushes the young embryo down in the nutritive tissue at the base of the prothallus.
The embryo cell divides transversely, and this is followed by a succession of division in irregular planes producing an ovoid mass of cells. The basal cells of the pro-embryo elongate successively, and form a secondary suspensor, which is not clearly differentiated from the root cap of the embryo proper. Although several embryos may begin to develop in a single ovule but only one of them usually matures.
During the development of the embryo the apex of the prothallus becomes meristematic and gives rise to a protective layer which closes the base of the pollen chamber. This layer also checks the suspensor from being pushed out of the prothallus.
Germination of Seed:
The resting period for the embryo is very short. The seed may germinate immediately. The two linear cotyledons come out and grow steadily until they become several centimetres long. In Ephedra trifurca, the vivipary is quite evident, i.e., in some cases numerous seedlings are being emerged from the seeds while the strobili were attached to the stem. The whole period from the appearance of the seed is six months.
Economic Importance of Genus:
The valuable drug known as ‘ephedrine’ is obtained from E. sinica, E. gerardiana, E. nebrodensis, E. equisetica and E. intermedia although the percentage of the available ephedrine is variable.