Q.17. Describe the angiospermic characters found in Ephedra. (2012) Related Questions -
Q. What are the affinities of Ephedra.
Ans. Ephedra and other genera of order of Gnetales possesses resemblances with the true Gymnosperms on one hand and the angiosperms on the other. These resemblances may be as follows:
(I) Resemblance with Gymnosperms - The origin of Gnetales is very obscure there being sure records of gnetales as fossils. Whatever evidence available indicate that they are relatively modern among gymnosperms. Ephedra is more closely related to gymnosperms that a Gnetum and Welwistchia. Ephedra is evidently related to coniferales and Gnetum and Welwistchia are related to Ephedra. Therefore it appears probably that Gnetales represent a somewhat modern offshoot from coniferales.
The more important point of the resemblance what Gymnosperms are as follows:
1. Ovary being absent, the ovule remain naked.
2. Styles, stigma absent.
3. Pollen grains enter through the micropyle and get directly lodged on the surface of the nucleus.
4. The pollen grains consists of prothalial cell stalk cell and tube nucleus.
5. Tracheids with bordered pits in the, xylem and sieve cells in the phloem are present.
6. Pollination is of anemophilous type.
7. Occurance of polyembryony.
8. No fruit formation.
(II) Resemblances with Angiosperms.
The main points of resemblances with Angiosperms are as follows :
1. Presence of true vessels in the secondary wood.
2. perianth like envclops in additions to the usual single integument in the male and female flowers.
3. Presence of dicotyledonous embryo.
4. In genera Gnetum absence of archegonia.
5. In genera Gnetum presence of broad leaves with reticulate venation.
6. The development of the female gametophytes in Gnetum is tetrasporic.
Q.18. Describe the structure of male and female cones of Ephedra. (2002)
Related Questions -
Q. Describe the female flower of Ephedra. (2006, 07, 11, 12)
Q. Describe the male flower of Ephedra. (2006, 13)
Ans. The cones in Ephedra are called strobili. Most of the species dioecious but there are occasional reports of monoecious plants. The sex organs are borne on unisexual compound strobili which occur as small spikes at the nodes.
Male Flower or Strobilus of Ephedra: -
The male or microsporangia strobili develop in the axils of leaves. They arise in whorls of 2, 3 or 4 from nodes of branches. Each strobilus possesses a central axis on which are born 2 to 8 pairs of opposite decussate bracts. The lower one or two bract may be sterile, rest fertile. In the axile of fertile bract arise male flower. Normally one male flower arise in the axile of bract.
Each male flower is axillary and consists of an axis and a pair of thin opposite scales of branches, which are also interpreted as perianth. The axis at its apex bears 1 to 8 anthers (microsporangia). These microsporangia possesses 2 or 3 loculi. Each locules opens through a terminal slit to release the microspores.
Female Strobilus or Flower of Ephedra: -
The female or megasporangiate strobili arises in whorls of 3 or 4 in the axis of the leaves at each node. Each female strobilus is an elongated structure pointed at the apex. It consists of a central axis with 2 or 4 or more pairs of opposite decussate bracts. The bract mostly are sterile except the apical one or two which bears a single female flowers in the axil.
Each female flower possesses a short stalk, which at its apex bears an ovule of a megasporangium. Each flower arises in the axil of a fertile bract.
The megasporangium or ovule has two integuments. The outer integument consists of 4 basally coherent bracts, sometimes collectively reffered to as the perianths. The inner integument consists of two bracts also fused at base. The outer integument has four vascular bundles. The inner integument becomes very hard at the time of pollination and froms a very long micropyle tube at the top. The nucleus have a mass of sterile tissue over lying embryosac. The pollen chamber in Ephedra is very deep and due to this reason a remarkable funnel shaped pollen chamber is developed which extends to the embryosac and exposes the neck of the archegonia, hence the pollen grains come in contact with archegonium during pollination.
Q.19. Describe briefly gametophytic structure, pollination and fertilization in Ephedra.
Related Questions -
Q. Describe the structure of a mature male gametophyte of Ephedra. (2008)
Q. Describe the double fertilization in Ephedra. (2004, 06, 08, 11)
Ans. Male Gametophyte: -
Each microspore is a boat-shaped and unwinged structure possessing an outer thick wall, the exine and the inner thin wall, the initine.
Soon after the microspore is formed within the microsporangium its nucleus divides and cuts off the first prothallial cell. Subsequent division cuts off the second prothallial nucleus without wall formation between this and the other nucleus, the antheridial initial. The generative nucleus now gets surounded by a cytoplastic sheath and forms a naked cell that divides to form a stalk (sterile) cell nucleus and a body (spermatogenous) nucleus. Both the stalk cell nucleus and the body cell nucleus remain in a common sheath of cytoplasm. The microspores are shed from the microsporangium at this 5-celled stage.
On reaching the pollen chamber microspore germinates. Its exine ruptures and the initine elongates to form a pollen tube. Body nucleus divides to form two male gametes (nuclei). By this time the prothallial cells degenerate and the pollen tube, containing a tube nucleus, a stalk nucleus and two male cells, grows through the tissue of the long neck.
Female Gametophyte: -
During the formation of female gametophytes, the megaspore undergoes free nuclear division, forming 225 to 503 nuclei. After this free nuclear division the cell wall formation starts as a result of which the gametophyte become a cellular structure. The female gametophyte get differentiated in upper reproductive region, middle storage region and basal haustorial region.
The archegonium develops from a single superficial cell at the micropyler end of the female gametophyte. Usually two archegonia are present. Each archegonia is elongated consisting of a long extend to the funnel-shaped pollen chamber. There is no wall between egg and ventral canal nucleus. There egg deeply sunken in the tissue of prothallus.
Pollination: -
Polinaton is anemophilous. The 5-nucleate microspores (pollen grains) are carried no by wind in early spring when a parking pollination drop is excuded by the long tubular tip of the inner layer of the integument. Ultimatly the microspores are sucked in by the drying liquid into the pollen chamber. Microspores come in direct contact with the archegonia by developing pollen tubes. Germination of the microspores takes place only a few hours after they have entered the pollen chamber.
Fertilization: -
The pollen tube forces its way through the neck of the archegonium and ultimately brusts apically. The tube nucleus, stalk cell and to male, nuclei are then releas into the cytoplasm of the mature egg. Land W.J.C. (1917) reported that fertilization occurs in 10 hours after pollination in Ephedra trifurca (Torr). The egg at the time of fertilization is surrounded by a cytoplasmic sheath. Normally a male nucleus fuses with the egg nucleus and a oospore or zygote is formed which is diploid (2X) in nature.
Khan R. (1943) have described the double fertilization in Ephedra and reported that the second male nucleus and ventral canal cell fuses. He concluded that the type of “double fertilization” as seen in Ephedra may have no phylogenetic significance at all and two nuclei outcome of a tendency towards fusion between any two nuclei of opposite sexual potencies that happen to live free in a common chamber.
Embryo: -
In Ephedra, the nucleus of the oospore undergoes free nuclear division resulting in the formation of 8 diploid nuclei which get surrounded by wall around them. This eight celled stage is known as the “proembryonal stage”. Out of these 8 proembryonal cell 3 to 5 proembryonal cell on development forms embryo. Hence, in Ephedra there is the occurrence of polyembryony.
During the development of embryo from proembryonal cell, the proembryonal cells develop a tubular protuberance called ‘suspensor tube’. The nuclei of these tubes divides into, two one of the which migrates to the tip of the tube and other remaining in the middle, a transverse portion wall later divides the embryonal initial form the elongated suspensor cell. Secondary suspensors have also been observed in Ephedra tifurea. The apical embryonal initial cell undergoes several divisions forming the embryo. The two cotyledons and the shoot apex develop from the lower end of the embryo. Khan R. (1943) observed 18 to 19 embryo in a single ovule. According to him this is due to cleavage plolyembryony. Out of the several embryos developing in a seed, only mature while others perish.
Seed : The seed of Ephedra is ovoid or triangular in shape. The testa is dark brown, the seed of foliata consists of two large cotyledons which remain embedded within the tissue of the female gametophyte. There are two distinct seed coats derived from two layers of envelope. With the development of the seed an additional thick and fleshy layer also develops round the seed, this is formed by the fusion of the subtending bracts of the trobilus.
Germination of seed: -
The seed has an obligatory resulting period and usually germinates immediately. The two cotyledons steadily grow, having two parallel bundles which units basipetally and pass into a diarch root. The germination is epigeal. On germination, The two linear cotyledons act as absorbing organs, and later form the first green leaves.