Ephedra: Distribution, Features and Reproduction

In this article we will discuss about:- 1. Distribution of Ephedra 2. Morphological Features of Ephedra 3. Internal Structures 4. Reproduction 5. Economic Importance.

Distribution of Ephedra:

Ephedra (commonly known as joint pine, joint fir, Mormon tea or Brigham tea) is the only genus in family Ephedraceae and order Ephedrales. It is represented by 50 species.

These species grow in dry climate over wide areas of the Northern hemisphere including North America,Europe, North Africa, and South west and central Asia. Eight species of Ephedra are known from India. Some of the common Indian species are E. intermedia, E. gerardiana, E. sexatilis, E.foliata etc. These species are distributed in dry parts of Punjab, Haryana, Rajasthan and parts of Kashmir to Sikkim.

Morphological Features of Ephedra:

The plant body is sporophytic and shows xerophytic characters. Mostly the plants are woody shrubs (Fig. 1 A), a very few species are lianas and some species grow into a small tree. E. compacta reaches 30 cm in height E. triandra is a tree. Its height is several meters. Plant body can be differentiated into three parts – root, stem and leaves.

1. Root:

There is a prominent underground tap root system. Later on the adventitious roots develop. Many root hairs are present but there is no mycorrhiza.

2. Stem:

Like Equisetum, the stem is green, ribbed, branched, fluted and differentiated into nodes and internodes (Fig.1B). It is distinctly jointed fir) (therefore, commonly known as jointed fir). It performs the function of photosynthesis and may be called as phylloclade. The branches arise from the axillary buds and are, therefore, in pairs of threes or fours according to the number of the scaly leaves at the nodes in different species.

The branches are also green and differentiated into nodes and internodes. The branching starts early at the cotyledonary stage. The apical meristem is having well marked tunica layer but the growth of internode is independent due to the presence of the meristemetic zone at its base. This zone dries up at the end of each growing season. It results in the brittleness and shedding of the branches. These branches are again replaced in next season by new axillary branches.

3. Leaves:

Leaves are small scaly, present in pairs at the nodes and are arranged in opposite decussate manner. (Fig. 1 C, D). These leaves unite at the base to form a basal sheath. Each leaf contains two unbranched, parallel veins. They are so minute that they are of no use i. e., unable to perform photosynthesis. The function of photosynthesis is carried by green stem. In the axil of each leaf is present a bud for the branch. True foliage leaves are absent.

Internal Structures of Ephedra:  

1. Stem:

The stem is ribbed; so, in tranverse section stem shows ridges and grooves (Fig. 2A).

A T.S. of stem at node shows the following structures (Fig. 2A, B):

a. Epidermis:

It is the outermost layer of thick walled cells, covered with a thick layer of cuticle. Sunken stomata are present on the slopes of the ridges in the circular pits.

b. Hypodermis:

It is present just below the ridges. (Fig 2B). It is made up of sclerenchymatous cells and provides mechanical strength to the plant.

c. Cortex:

In is present between the thick walled sclerenchyma and vascular cylinder. It can be differentiated into outer and inner cortex. The outer cortex contains 2-3 layers of radially elongated palisade tissue and inner cortex consists of 2-3 layers of spongy parenchyma.

The cells of outer and inner cortex are loosely arranged with large intercellular spaces and are provided with chlorophyll to perform the function of photosynthesis. A few patches of scleranchymatous cells may also occur in the cortex to provide mechanical support to the young axis.

d. Endodermis:

It is the innermost layer of cortex. It is not easily distinguishable from the cortical cells.

e. Pericycle:

It is present below the endodermis. It is single layered and ill defined.

f. Vascular Cylinder:

It is endarch, siphonostele and consists of many vascular bundles arranged in a ring. Vascular bundles are conjointed, collateral, open and endarch. The number of primary vascular strands is generally eight, out of which four small represent the foliar traces while the other large four are stem bundles.

Foliar traces run upto the node. Xylem consists of tracheids, vessels and xylem parenchyma. Due to the presence of the vessels the Ephedra resembles angiosperms. The phloem consists of sieve cells, phloem parenchyma and albuminous cells. Phloem and xylem are separated by a narrow strip of cambium.

g. Medullary rays:

Broad, parenchymatous medullary rays are present in between the vascular bundles. Medullary rays connect the pith with cortex.

It is present in the centre. It is made up off thin walled parenchymatous cells. Near the node its cells become strongly lignified forming a peridermal diaphragm which accounts for the rapid separation of the branches in the region (Fig. 3).

Secondary growth:

The secondary growth takes place by the activity of intrafascicular cambium and interfascicular cambium. After forming a complete ring of cambium, the cambial cells cut of secondary phloem on the outer side and secondary xylem towards the inner side. (Fig. 4)

Due to formation of the secondary tissues, primary phloem is crushed and the primary xylem is pushed towards the inner side at the base of the secondary xylem. In addition to vascular tissue cambium also forms medullary rays (secondary). These rays are long, broad (multiseriate) and traverse between secondary xylem and secondary phloem.

Radial Longitudinal Section (R.L.S.):

In R.L.S. xylem tracheids, vessels and medullary rays are clearly visible. Medullary rays are cut lengthwise and their length and height are revealed (Fig. 5A). Each medullary ray is composed of irregularly dispersed ray cells and ray tracheids. Tracheids possess bordered pits on their radial and tangential walls. In vessels, the bordered pits are also arranged in the same way as tracheids (Fig. 5 B, C).

Tangential Longitudinal Section (T.L.S.) of wood:

Like R.L.S. in T.L.S. also, the xylem, tracheids, vessels and medullary rays are clearly visible but they are cut transversely here. (Fig. 6). Bordered pits and simple pits are seen on the radial and tangential walls. The medullary rays are elongated and on their tangential walls are present simple pits.

2. Leaf:

The transverse section of scaly leaf is oval in shape and can be differentiated into epidermis, mesophyll tissue and vascular tissue.

a. Epidermis:

It is outer most single layer of thick walled elongated cells. The cells are covered with thick cuticle. Sunken stomata are present (Fig. 7).

b. Mesophyll tissue:

Two or three layers of palisade tissue are present inner to epidermis. The cells are filled with chloroplast and large intercellular spaces are present between them. In the centre of the leaf parenchymatous tissue is present.

c. Vascular tissue:

Two vascular bundles are embedded in the parenchymatous tissue. The vascular bundles are collateral and closed. Xylem is present towards the upper side.

3. Root:

The transverse section of root shows single layer epiblema, outer cortex (composed of collenchymatous cells), inner cortex (composed of parenchymatous cells) endodermis and pericycle. Vascular bundles are radial and exarch. The root may be diarch or triarch.

Reproduction in Ephedra:

Ephedra is heterosporous (produces two types of spores: microspores and macrospores) and dioecious (both these types of spores are produced on two different plants of the same species. E. fuliata is monoecious. Microspores are formed in male flowers while megaspores are formed in female flowers.

These flowers are present in the form of cone like compound strobili. Male flowers are present in the form of male strobilus while female flowers are present in the form of female strobilus. Both male and female strobili are compound i. e.,the cone axis bears pairs of bracts which subtend either microsproangiate or ovulate shoots.

Male Strobilus (Staminate Strobilus):

Male strobili arise in clusters from the nodes of the branches. Each strobilus is rounded, ovoid or spherical in shape and arises in the axis of a scale leaf. Their number at the node depends upon the number of scale leaves.

Each strobilus has a central axis which bears 2-12 pairs decussately arranged simple, broad and cupped bracts. Lower most 1-2 pairs of bracts are sterile. In the axil of each fertile bract arises a male flower or staminate flower (Fig. 8 A-C). A male strobilus with several male flowers can be compared with an inflorescence.

Male flowers:

Each male flower has two lipped thin bractioles (perianth) which encloses a stamen. Bracteoles are united at the base. The flower has a short stalk known as microsporangiophore and two, eight to twelve microsporangia at its tip (Fig. 8 D).

Microsporangia are sessile and dehisce terminally. Male flower is also called simple strobilus. A compound male strobilus, therefore, consists of many such strobili.

Structure of microsporangium:

Each microsporangium has 2-3 loculi and is often called as synangium. Its wall is two layered followed by a prominent tapetal layer enclosing a sporangial sac having many pollen grains or microspores (Fig. 8E).

Development of microsporangium:

The development of microsporangium is eusporangiate. Microspangia arise at the tip of microsporangiophore. The microsporangiophore arises as small protuberance in the axil of the fertile bract of male strobilus. The apex of microsporangiophore becomes lobed after growing for some time.

Each lobe represents a sporangium. Few hypodermal cells in each lobe enlarge in size. These cells consist large nuclei, denser cytoplasts and are known as archesporial cells. These cells divide periclinally into outer primary wall cells or primary parietal cells and primary sporogenous cells (Fig. 9A).

Primary sporogenous cells further divide by two periclinal divisions to differentiate middle wall layer, inner tapetal layer and sporogenous cells. The primary wall cells function directly as the outer wall of the sporangium.

However, according to some workers, the primary wall cells divide periclinally to form three layered thick wall. The sporogenous cells divide further to form large number of microspore mother cells. Each microspore mother cell divides by meiosis to form four haploid microspores arranged in a linear tetrad.

Structure of pollen grain:

Pollen grain is the first cell of the male gametophyte. Each pollen grain is elliptical, uninucleate and has two wall layers. The outer wall layer is thick and is called exine while the inner male layer is then and is called intine (Fig. 10A, B).

Female Strobilus (Ovulate Strobilus) or Female Cone:

They usually arise in pairs at each node in the axil of scale leaves. A female strobilus appears to be an elliptical structure with a pointed apex (Fig. 11 A, B). It retains the same compound structure as the male strobilus. It consists of a short axis to which are attached three or four pairs of decussate bracts.

In E. Americana these bracts are swollen and juicy (Fig. 11E). All the pairs of bracts are sterile except the uppermost one which bears a pair of ovules in its axil (Fig. 11C, D) and may be variously coloured. Out of the pairs of the ovules only one survives and it takes up a false terminal position.

Female flower:

The female flower has short stalk and an ovule (megasporangium)

Structure of ovule (megasporangium):

Longitudinal section of an ovule shows that it consists of a mass of parenchymatous cells in the centre. It is called nucellus. The nucellus is surrounded by a two-layered envelope. These are usually designated as outer and inner integuments. The outer envelope is formed by four segments and receives four bundles while the inner one is formed of two segments and receives two bundles.

The lower half of the inner envelope is fused to the nucellus but upper half is free and prolongs into a long micropyle tube. By the time of pollination just below the micropyle pollen chamber develops. Pollen chamber in Ephedra is the deepest known among the Gymnosperms. The floor of the pollen chamber is formed by female gametophytic tissue and not by the nucellus as in other gymnosperms. (Fig. 12).

Development of Ovule:

Development of the ovule takes place in the form of a small cellular protuberance. This protuberance increases in size and becomes the nucellus. Soon neighbouring cells of the base forms inner and outer integuments. Inner integument surrounds the nucellus except the top where it form a small opening called micropyle.

A hypodermal archesporial cell differentiates in the nucellus. It divides periclinally into outer parietal cell and inner megaspore mother cell. The latter is pushed quite deep into the nucellar tissue.

The megaspore mother cell divides meiotically to form four hapliod megaspores. Generally the lowermost megaspore (towards the chalazal end) remains functional. It enlarges and gives rise to female gametophyte (first cell of the female gametophyte) and the remaining upper three megaspores degenerate.

Gametophytic Phase:

The sporogenesis results in the formation of micro- and megaspores representing the gametophytic stage. They undergo gametogenesis to form the male and female gametophytes respectively.

Development of male gametophyte before pollination:

It takes place in microsporangium. After the reduction division spores tetrads are formed. The four cells of the tetrad separate and develop into microspores. The microspore divides by a transverse wall to form a small prothallial cell and a large outer cell is (Fig. 13 A). The outer cell again divides by a transverse wall and forms a second prothallial cell and an antheridial cell. (Fig 13 B).

The antheridial cell divides to form a small generative cell and a large tube cell (Fig. 13 C, D). The generative cell soon divides into the nuclei of stalk cell and body cell. The nuclei of stalk cell and body cell remain surrounded by a common mass of cytoplasm (Fig. 13 E, F). Pollens are shed at this five celled stage.

Development of female gametophyte:

As mentioned earlier, the functional megaspore is the first cell of the female gametophyte. It enlarges and its nucleus divides into two. These nuclei move towards the opposite pole and are separated by a large central vacuole.

Later these two nuclei divide by free nuclear division to form as many as 256 nuclei. These nuclei are arranged in a peripheral layer around the central vacuole. Later the central vacuole disappears and free nuclei are evenly distributed throughout.

Centripetal wall formation (from periphery towards the centre) starts and thus a mass of cellular tissue is formed. It is called female gametophyte or endosperm. Gradually the female gametophyte is differentiated into two regions.

Micropylar region and antipodal region. Micropylar region consists of loosely arranged thin walled cells, which later on give rise to archegonia. Antipodal region is further differentiated into lower storage zone and basal haustorial zone. Storage zone comprises of bulk of endosperm. This zone consists of compactly arranged cells which are full of starch and other food. The cells of the haustorial zone absorb the food material from the nucellus.

Structure and development of archegonium:

Archegonia arise in the micropylar region. The number of archegonia in Ephedra varies from 1-3 but they are generally two in number. Any superficial cell of female gametophyte towards micropylar region acts as archegonial initial (Fig. 14A). It divides by a transverse division to form outer primary neck cell or neck initial and inner central cell (Fig. 14B). The neck cell undergoes a number of divisions to form a long neck of 8 or more tiers (minimum of 32 cells). It encloses no neck canal.

The neck of archegonium of Ephedra is the longest in the gymnosperms. The central cell enlarges in size. Its nucleus divides into a ventral canal nucleus and an egg nucleus but no wall is laid down between the two.

As the archegonium reaches towards maturity, the cells of neck usually merge with surrounding gametophytic cells and become undistinguishable from the surrounding cells of female gametophyte. The cells adjacent to the central cell divide transversely to form a distinct jacket layer, which may be two or three layer thick.

A mature archegonium consists of a long neck and a central cell having a ventral canal nucleus and egg nucleus (Fig. 13, 14).

Pollination:

The pollination is anemophilous i.e. it takes place by wind. Pollen grains are carried by the wind on the female strobilus. The cells of the nucellus secrete pollination drop which comes out through the micropylar canal. Pollen grains to adhere to the pollination drop. Pollen grains are sucked inside and come to lie in a deep pollen chamber.

Development of male gametophyte after pollination:

Pollen grains germinate in the pollen chamber. The exine ruptures and intine comes out in the form of pollen tube. The nucleus of the body cell divides to form two male gametes (Fig. 16)

Fertilization:

It occurs 10 hours after pollination. The pollen tube along with its four nuclei (2 male nuclei, 1 stalk nucleus and 1 tube nucleus) gradually penetrates the neck cell of the archegonium and discharges all the four nuclei into the egg.

One male nucleus fuses with the egg nucleus forming the zygote (2x) or oopsore. Khan (1941) observed in E.foliata that second male gamete fuses with the ventral canal nucleus (double fertilization) but this diploid nucleus does not develop into embryo Oospore is the first cell of the sporophytic phase (Fig. 17).

Embryogeny:

More than one archegonium may be fertilized in an ovule, but only one oospore develops into embryo. The zygote nucleus divides by three free nuclear divisions to form eight nuclei. These nuclei are irregularly distributed in the cytoplasm of the archegonium.

Later wall-formation takes place and this structure is known as proembryo. Each cell of inproembryo is capable to develop into an independent embryo. Three to five of these nuclei individually enclose in somewhat irregular walls and become globular.

These are known as pro-embryonal cells, each of which produces an independent embryo. In Ephedra, this type of polyembrony without any cleavage, it unique among gymnosperms. Because the polyembryony occurs without any cleavage, it is known as embryo sac polyembryony. Each proembryo grows into tubular structure called the suspensor (Fig. 18A-C).

Tube nucleus of the proembryo divides into two. Both these nuclei move into the tube. A wall separates these two daughter nuclei and forms two cell(Fig. 18D). The cell towards the micropylar and disintegrates while the cell formed towards the chalazal end of the tube survives and is called embryonal initial.

The tube grows more and   carries the embryonal initial deep into theprothallus tissue. This embryonal initial divides into a proximal suspensor cell and a distalembryo cell. The embryo cell divides and develops into the embryo proper which contains two cotyledons (Fig. 18E-G).Although several embryos may develop in a single ovule but only one survives and reaches at maturity as seed.

Structure of Seed:

Longitudinal section of the seed shows that it consists of a dicotyledonous embryo in the centre. This embryo is situated at the tip of the elongated suspensor and remains embedded in the endosperm (Fig. 19). The nucellus is consumed during the development of embryo and persists as a nucellar cap at the micropylar end of the seed.

The seed is enclosed by the seed coat which consists of two separate layers derived from the two envelopes. At the time of maturity, the subtending bracts of the megasporangiate strobilus become thick and fleshy and form an additional covering around the seed e.g.,E. foliata.

Germination of the seed:

Seeds germinate without undergoing a period of rest if the atmospheric conditions are favourable. The seed germination is epigeal (Fig. 20A-G).

Economic Importance of Ephedra:

1. An alkaloid ephedrine is obtained from E. gerardiana, E. intermedia, E. nebrodensis etc. It is used in preparation of medicines that cure cough, bronchitis, asthma and hay fever.

2. Tincture of E. gerardiana is also used as a cardiac and circulatory stimulant.

3. Decoction of the stem and roots is used to cure rheumatism and syphilis e.g.,E. antisyphilitica.

4. The juice of berry is used to cure respiratory disorders.

5. Mormon tea is brewed from the species of Ephedra in south western United States.

6. Some species are grown as ornamentals.