Q.5. Write the xerophytio characters found in needle of Pinus. (2012, 13) Ans. Xerophytic Characters: -
The plant usually grow on slopes where water is not propertly available to the plants. The cold winster, i.e. low temperature retards the root absorption causing a physiological draught. High altitude strong winds and low pressure are the causes for increase in transpiration. The Pinus leaves, therefore, exhibit typical zerophytic characters which are as follows:
1. Needle shaped leaves there by reducing the transpiration surface.
2. Presence of suken stomata.
3. Thick walled epidermis.
4. Well developed hypodermis.
5. Simple vascular tissue.
6. Presence of tansfusion tissue.
Q.6. Describe the development of male and female gametophyte of Pinus. (1999)
Related Questions -
Q. Describe the development of female gametophyte of Pinus with labelled diagrams. (2004, 09, 11, 13)
Q. Describe with diagram the development from fertilization till the maturity of seed in the ovule of Pinus. (2008)
Q. Explain the polyembryony in Pinus. (2002)
Q. Describe the post fertilization changes in the ovule of Pinus till seed formation. (2001)
Ans. The gametophyte: -
The plant is heterosporous. It bears both microspores (pollen grains) and megaspores (embryo sacs) which develop in microsporangium and megasporangium respectively.
Male gametophyte: -
The microspore (pollen grain) is the basic unit of male gametophyte. It is a uninucleate structure with two assymetrically placed air-bladders. The air bladder is also known as wing or saccus and arises on the lateral sides of microspore. Each pollen grain has cuticularised outer layer called exine and an inner thin layer called intine. The outer wall of the wing has a reticulate sculpture. Male gametophyte develops partially in the microsporangium and partially in ovule.
The exine of the 4 celled male gametophyte now ruptures and intine grows out from the begining of the pollen tube into which the tube nucleus passes. The pollen tube penetrates into the nuclear tissue and grows slowly throughout the ensuring summer. In the second year the generative cell divides into a stalk cell and a body cell. Now the body cell migrates through the tube during its migrating the nucleus of body cell divides to form two male gametophyte or cells each consisting of its nucleus surrounded by a twin cytoplasmic sheath. The male gametes are non flagellate. Th pollen tubes this time pierces the nucleus and reaches the archegonia which have developed by now on the female Prothallus.
Female Gametophyte : -
The magaspore is the basic unit of female gametophyte one lower most megaspore from the linear tetrad of megaspore remain functional and rest three megaspores disintegrate. The functional megaspore increase in size and its nucleus undergoes a series of free nuclear division and a vacuole appear. Now the wall formation begins from the periphery and proceeds towards the centre. Thus a completely cellular female gametophyte, without a central vacuole is formed. The gametophyte is also known as endosperm.
The archegonia are formed towards the micropylar side. The number of archegonia is highly variable and it is usually 2 to 5 in Pinus. Any cell from the female prothallus or endosperm tissue function as archegonium initial cell. The initial cell enlarge considerably and then by transverse division from a 8 celled neck, the cells of which are arranged in two tiers of four cells each. The central cell enlarges in size and then divides into very small ventral canal cell or sometimes even a ventral canal nucleus and a large egg cell. There are no neck canal cells. Thus the archegonium of Pinus is a very simple structure consisting of a large venter and a short neck. Around the archegonium is found a nutritive jacket.
Polliation: -
In Pinus the pollen grains (microspores) is affected by wind (anemophilous). There are large number of microsporangia in each cone bearing in numerable micro spore.They form yellow cloudy appearance. yellow cloudy appearance. In 4 - celled stage the pollen is shed. A drop of mucilage substance secreted by the upper part of the nucellus at the apex of the ovule and the pollen is caught in this pollination drop. After pollination the drop of mucilage dries up is drawn in together with the microspores to the apex of the nucellus where they are held by the dried mucilage in a position of suitable for germination. After pollination the scales of the female one bend downwards so that the cone hangs vertically and this position remains during the remainder of its existence. The fertilization normally brought about the following year at about the same time and in the third year the cone mature. During the internal various changes in the pollen grains and ovule takes place. The male cones after pollination dry up and fall.
Fertilization: -
The pollen tube becomes active in April of the 2nd year and after piercing through the nucellus reaches the neck of archegonium. The tip of the pollen tube now contains 2 male gametes and tube and stalk nuclei. The pollen tube comes in branches close contact with the archegonium and the rupture, discharging its contents into the egg. One of the male gamete approches the egg nucleus and unites forming the zoospore or zygote. The 2nd male gametes along with the stall and five nuclei disintegrate. Fertilization takes place in June or july. More than one pollen tube may develop in association with a single ovule and more than one archegonium may be fertilized, but as the food material is sufficient for one to maturity the others die off after a short period by development.
Post Fertilization Changes : -
Or Embryogeny: -
The zygotic nucleus moves toward the base and then divides to form four nuclei. The nuclei organise into four quadrately arranged cells with open upper end. The four cells divide similtanousing thrice to form four tiers of four cells each. These tiers are desginated from top down-wards as open tier, rosette tier, suspensor tier and apical tier.
Since only a part of the oospore is involved in the formation of the embryo, the development is said to be meroblastic.
As there is no cell wall on the microylar end the cells of open tier provide nutrition to the remain tiers of proembryo. The cells of this tier do not divide. The cells of rosette tier divide in various planes. They simply conduct the food obtained by the cells of open tier. The cells of suspensor tier elongate pushing the embryonal cells, into the endosperm. The four suspensor cells due to considerable elongation may become coiled. These cells may divide transversely to form secondary suspensor or embryonal tubes. By another transverse division, two whorls of emrbyonal tubes, designated as first and second series of embryonal tubes, are formed. The cells of embryonal tier develop into embryos.
The cells of embryonal tier separate from each other along with the embryonal tubes forming four embryos and the phenomenon is called as polyembryony. Since it is caused by splitting of the embryo, the phenomenon is called as cleavage polyembryony. Polyembryony may also be caused by budding from the cells of rosette tier amounting to rosette polyembryony. The rosette embryos do not develop extensively. They abort after forming 10-12 cells. Sometimes more than one egg may be fertilized in an ovule but only one embryo reaches maturity in a seed. The embryo soon gets differentiated into plunile, hypocotyl and cotyledons. The number of cotyledons is always more than two.
Seed: -
Fertilized ovule is called seed and it consists of seed coat, perisperm, endosperm and embryo. Pinus seed consists of a wing like structure which helps in dispersal.
Seed Germination: -
Seed germination is epigeal in Pinus. During germination of seed the radicle protudes out through the micropyle and enters the soil forming the primary root. The plumule comes out and along with cotyledons is pushed in airdue to elongation of hypocotyle.
