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Pteridophytes General Characters

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Pteridophytes General Characters PTERIDOPHYTES GENERAL CHARACTERS Dr. S.SALAMMA NSPR GOVT. DEGREE COLLEGE (W), HINDUPUR • Pteridophytes are spore-bearing vascular plants. • They are commonly called as ‘vascular cryptogams’ or ‘botanical snakes’. • Pteridophytes constitute the most primitive group of land plants with a vascular system. • The origin of Pteridophytes dates back to Silurian period of Palaeozoic era. GENERAL CHARACTERS OF PTERIDOPHYTES Occurrence • Pteridophytes grow in a variety of habitats. Most of them are terrestrial (grow on land) and found luxurious in habitats of abundant moisture. Some of them grow as xerophytes Ex. Selaginella bryopteris • A few Pteridophytes are aquatic (Azolla, Salvinia, Isoetes, Marsilia) and some others are epiphytic (Drynaria). The plant body • In Pteridophyte life cycle, there are two generations, sporophyte and gametophyte. • The sporophyte is the recognised plant body of all pteridophytes and is diploid in nature. The sporophyte develops from the zygote which results from the fertilisation of the egg by a male gamete. • The sporophytic plant body is differentiated into true roots, stem and leaves. But some primitive Pteridophytes are without true roots and well developed leaves (Psilotum) • Branching in many Pteridophytes is monopodial. In some members, the branching is of dichotomous type (Dicranopteris). • The sporophytic plant presents a great range in form. Two main categories may be distinguished. • One category comprises megaphyllous types, in which the leaves are large in relation to the stem, and is represented by the ferns. • Second category consists of microphyllous type, in which the leaves are small in relation to the stem and is represented by the lycopods and the horse-tails. • All the vegetative organs of the sporophyte possess vascular supply. The vascular system comprises complex tissues, xylem and phloem. • Phloem is with sieve tubes but lacks companion cells. Xylem is made up of tracheids. In some members of Selaginella and Equisetum, primitive vessels are present. • The vascular system in pteridophytes may be a simple as protostele (without pith), siphonostele (with pith), or a dictyostele (advanced type of siphonostele). • Secondary growth is absent in majority of the living pteridophytes except Isoetes. Reproduction • The sporophytic plant reproduces asexually by means of spores which are produced in small capsules called sporangia. • The position of the sporangia differs among the groups, but they are always found on the sporophyte. In some pteridophytes, the sporangia are borne on stems. In some others, the sporangia are borne either on the leaves (foliar) or in their axils (between the leaves and the stem). • In some pteridophytes, the sporangia are produced within the specialised structures called the sporocarps (e.g., Marsilea). • Leaves bearing the sporangia are called sporophylls. The sporophylls may be widely scattered on a plant or may be clustered in definite areas and structures called cones or strobili (Selaginella and Equisetum). • According to the nature and mode of development, the sporangia are of two types: • A large sporangium developing from several initial cells producing many spores is called as eusporangium and the development is called as eusporangiate. • Small specialized sporangia developing from a single initial cell producing a small, definite number of spores is called as leptosporangium and the development is called as leptosporangiate. • Within the sporangia are developed the diploid spore mother cells or sporocytes. These spore mother cells undergo meiosis or reduction division and produce haploid spores. • If all the spores are of the same size, the plant is said to be homosporous (Lycopodium) and if the plant is producing two kinds of spores then it is called as heterosporous (8 genera in pteridophytes are heterosporous (Selaginella). • In the heterosporous type, the two different types of spores are produced in separate sporangia. The smaller spores produced in larger numbers in microsporangia are termed as microspores or male spores and the larger spores which are produced in smaller numbers in megasporangia are termed as megaspores. The Gametophyte • The haploid spores, on germination give rise to the haploid gametophytes or prothalli which are usually small and insignificant structures. The gametophytes are inconspicuous as compared to the sporophytes. • Hence the sporophyte has become the dominant part of the life-cycle of pteridophytes, while the gametophyte has been much reduced. The gametophytes are of two types • Gametophytes that develop from homospores grow upon the soil and form independent plants-these are known as exosporic gametophytes. • Gametophytes, that develop from heterospores are for the most part, retained within the original spore case and dependent on sporophyte are called endosporic gametophytes • Exosporic gametophyte is typically a delicate, thin thallus and is commonly called the prothallus. In most of the pteridophytes, they attached to ground by numerous rhizoids. They live independently. • In some pteridophytes, the exosporic gametophytes are devoid of chlorophyll and are subterranean in habitant are saprophytic. In such cases, they obtain their food by symbiosis through the agency of mycorrhiza which occurs within the tissue of the prothallus or gametophyte (Psilotum) • Endosporic gametophytes that develop from heterospores are greatly reduced structures. They develop largely or entirely within the spore wall and live on food deposited in the spores. • The gametophyte or prothallus bears the sex-organs, the antheridia and archegonia. Typically, the gametophytes formed from the homospores are monoecious, that is both antheridia and archegonia are born in large numbers on the same gametophyte or prothallus. • The gametophytes formed from the heterospores are dioecious, the antheridia and archegonia developing on separate male and female gametophytes. Prothallus • The sex organs are called as antheridia and archegonia • The antheridia may be embedded either wholly (Lycopodium) or in part in the tissue of gametophyte (some ferns) or they may project from it. • At maturity, each antheridium is a globular structure. It consists of an outer sterile wall inside which are found a large number of androcytes. Each androcyte gives rise to a single motile antherozoid or sperm. • Each archegonium is a flask-shaped structure, consisting of a basal swollen, embedded portion, the venter and a short neck. The wall of the venter develops from the tissue of the prothallus. The venter encloses the egg and ventral canal cell. Inside the neck are found the neck canal cells. • At maturity, the apical cells separate, the neck canal cells disintegrate forming a passage for the antherozoids to reach the egg cell. Fertilization • Fertilization in all cases is accomplished by the agency of water. One of the male gamete fuses with the egg of archegonium and form zygote. The fusion of a male gamete and an egg restores the diploid nature chromosomes number and results in the formation of the zygote. The embryo • The zygote undergoes repeated divisions to form a new sporophyte. The young sporophyte remains attached to the gametophyte by means of a foot and draws nourishment from the prothallus until it develops its own stem, roots and leaves. The sporophyte is dependent on the gametophyte only during its early stages. Life cycle of Pteridophytes • The life cycle of a pteridophyte comprises of two distinct phases or generations, sporophyte and gametophyte. The first generation, the sporophyte, is the diploid and dominant part of the life-cycle and is organised into stem, leaves and roots. • The sporophyte plant develops sporangia within which are diploid cells are produced called spore mother cells. Each spore mother cell divides by meiotic or reduction division and produce four spores which are haploid. • In some Pteridophytes, all spores produced by sporangia are of one type; such a plant is said to be homosporous. In these plants gametophytes are of same kind are bisexual. In heterosporous Pteridophytes, the spores are of two kinds-the smaller ones are termed as microspores or male spores which developed in microsporangia, while the larger spores are called megaspores or female spores which formed in megasporangia. • The microspores on germination produce the male gametophyte. • The megaspores produce the female gametophytes. • Gametophytic generation is always small, inconspicuous and bears male and female gametes. The male gametes are produced in large number within the antheridium. The female gamete is generally borne singly within the archegonium • Fertilization takes place in the presence of moisture when a sperm cell fuses with an egg to produce a diploid zygote. The zygote germinates again into a new sporophyte. • Thus the life cycle of a pteridophyte exhibit alternation of generations comprising of an alternate succession of sporophytic and gametophytic generations. There is a distinct difference in case of sporangia and spores and nature of gametophyte in homosporous and heterosporous pteridophytes. LIFE CYCLE OF A FERN .
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