Study Material, Biology, Plant Kingdom, PteridophytesStudy Material
* It The Pteridophytes include horsetails and ferns.
* Pteridophytes are used for medicinal purposes and as soil-binders. They are also frequently grown as ornamentals.
* Evolutionarily, they are the first terrestrial plants' to possess vascular tissues - xylem and phloem.
* The pteridophytes are found in cool, damp, shady places though some may flourish well in sandy soil conditions.
* In pteridophytes, the main plant body is a sporophyte which is differentiated into true root, stem and leaves. These organs possess well differentiated vascular tissues. The leaves in pteridophyta are small (microphylls) as in Selaginelia or large (macrophylls) as in ferns. The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls. In some cases sporophylls may 'form distinct compact structures called strobili or cones (Selaginella, Equisetum). The sporangia produce spores by meiosis in spore mother cells. The spores germinate to give rise to inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid gametophytes called prothallus. These gametophytes require cool, damp, shady places to grow. Because of this specific restricted requirement and the need for water for fertilisation, the spread of living pteridophytes is limited and restricted to narrow geographical regions.
* The gametophytes bear male and female sex organs called antheridia and archegonia, respectively. Water is required for transfer of antherozoids the male gametes released from the antheridia, to the mouth of archegonium. Fusion of male gamete with the egg present in the archegonium result in the formation of zygote.
Zygote thereafter produces a multicellular well differentiated sporophyte which is the dominant phase of the pteridophytes.
* In majority of the pteridophytes all the spores are of similar kinds; such plants are called homosporous. Genera like Selaginella and Salvinia which produce two kinds of spores, macro (large) and micro (small) spores, are known as heterosporous. The megaspores and micros pores germinate and give rise to female and male gametophytes, respectively. The female; gametophytes in these plants are retained on the parent sporophytes for variable periods. The development of the zygotes into young embryos take place within the female gametophytes. This event is a precursor to the seed habit considered an important step in evolution.
* The pteridophytes are further classified into four classes: Psilopsida (Psilotum); Lycopsida (Selagineila, Lycopodium), Sphenopsida (Equisetum) and Pteropsida (Dryopteris, Pteris, Adiantum).
* Selaginella is commonly called the little club moss or spike moss. Selaginella is mainly found in damp shaded places.
* The plant body is sporophytic (2n), which is an evergreen and delicate herb having adventitious roots. The plants show great variation in their morphology. Some species are prostrate growing upon the surface (e.g., S.kraussiana), some are suberect (e.g., S.trachyphylla) and others are climbers (e.g., S.allegans). The stem is covered with four rows of small leaves, out of these two rows are of smaller leaves and two of large leaves species with dimorphic leaves such as S.kraussiana, S.helvetica, S.lepidophylla, S.chrysocaulos etc.
* Reproduction takes place by vegetative and sexual (by spores) method.
(1) Vegetative reproduction: It is of rare occurrence and may takes place by following methods:
Fragmentation: It occurs during very' humid conditions. Some branches act as adventitious branches, which get separated from the plant and give rise to new Selaginella plants, e.g., in S.rupestris.
By resting buds: In some cases, terminal leaves get overlapped and become fleshy and form resting buds, which are means of vegetative reproduction, e.g., in S.chrysocaulos.
By tubers: In S.chrysorrhizos, some branches penetrate into substratum and at terminal ends swell to form tubers, which give rise to new plants.
By apogamy: In some cases, development of sporophyte occurs directly from gametophyte without intervention of sex organs, it is called apogamy and such plants are genetically haploid.
(2) Sexual reproduction: The reproductive structure in Selaginella is strobilus or spike. It is a sessile structure and develops at the terminal ends of the branches and its length varies from 1/4th of an inch to 2-3 inches in different species.
The sporangia are of two types:
A strobilus showing compactly arranged sporophylls (b) L.S. through strobilus
(1) Megasporangia: Borne on megasporophylls. Megasporangium is pale greenish and contains chalky white, yellow or orange megaspores.
The megasporangium is four-lobed structure with a 2-layered jacket, one layer of tapetum and a large number of microspore mother cell. However, only one megaspore mother cell is functional. After meiosis it produces 4 megaspores out of which 1-3 may degenerate. In S.rupestris, there is only a single megaspore.
(2) Microsporangia: Borne on microsporophylls having a large number of small spores. Thus Selaginella is heterosporous. Microsporangium is pale yellow, oval or spherical body, with 2-layered jacket, one layered tapetum and a number of microspore mother cells which undergo meiosis and form haploid microspores. The main body consists of a wall having two layers, inside which are present numerous small microspores (400-2000).