Pteropsida

In this document salient features ,reproduction , classification and their origin from bryophytes is briefly explained.

1. PTEROPSIDA BY:- RANA ARSALAN JAVAID PMAS ARID AGRICULTURE UNIVERSITY R AWALPINDI

2. PTEROPSIDA A large group of vascular plants characterized by having parenchymatous leaf gaps in the stele and by having leaves which are thought to have originated in the distant past as branched stem systems. Some botanists regard the Pteropsida as a natural group which they recognize as a class, subdivision, or division. Others regard it as an artificial assemblage of plants that have undergone certain similar changes from a rhyniophyte ancestry. The various components of the Pteropsida are here treated as three separate divisions under the names Magnoliophyta , Pinophyta , and Polypodiophyta .

3. GENERAL CHARACTERISTICS OF PTEROPSIDA Occurrence: Most pteridophytes are terrestrial and grow in moist and shady places while some flourish well in open, dry places especially in xeric conditions. Some pteridophytes are aquatic and some are epiphytes. The sporophyte is the conspicuous and familiar plant body. It develops from the zygote, a diploid cell which results from the fertilization of the egg and antherozoid . The sporophytic plant body remains differentiated into true roots, stem and leaves. Some primitive members lack true roots and well developed leaves (e.g., in Psilophytales and Psilotales )

4. Reproduction: The sporophytes reproduce by spores which are produced within sporangia. In some pteridophytes the sporangia develop on stems (i.e., cauline in origin) while in other they are borne either on the leaves (foliar) or in the axils of the leaves. The leaves that bear sporangia are known as sporophylls . The sporophylls may be widely scattered on a plant (e.g., ferns) or may be clustered in definite areas and structures called cones or strobili ( Selaginella , Equisetum).

5. The gametophyte: The spores on germination give rise to the haploid gametophytes or prothalli which are small and inconspicuous. The gametophytes in some pteridophytes are subterranean and in others they are retained within the resistant wall of the spore. The antheridia: The antheridia may be embedded in the gametophyte or they may project from it. The embedded antheridia are commonly found in eusporangiate pteridophytes while the projecting ones are usually found in the leptosporangiate ferns.

6. The archegonia: The archegonia are flask-shaped. Each archegonium consists of a basal swollen, embedded portion the venter and a short neck. The venter encloses the egg and ventral canal cell. At maturity the apical cells of archegonium separate, the neck canal cells disintegrate forming a passage for antherozoids to reach the egg. Fertilization: In all cases the fertilization is accomplished by the agency of water. With the result of the fusion of male gamete and female egg a diploid zygote (2n) is formed.

7. The embryo (The young sporophyte ): 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.

8. CLASSIFICATION OF PTEROPSIDA Pteropsida is further subdivided into following categories:- The characteristic features of these divisions are as follows-

9. 1. Lycophyta : The sporophyte is differentiated into stem, roots, and leaves. The leaves are microphyllous , and with a single vein. The vascular strands or steles may be protostelic , siphonostelic , or polystelic . The leaf-gaps are always absent; sporophylls produce a single sporangium on the adaxial side near its base. The sporophylls are borne in strobili . They are homosporous or heterosporous . The antherozoids are biflagellate or multiciliate .

10. 2 . Psilophyta : The sporophyte is differentiated into a rhizoid bearing subterranean rhizome and an aerial portion. The aerial portion is branched. The vascular system is of protostelic type. Leaf gaps are absent from vascular cylinder. The terminal sporangia are borne singly at the tips of short or long branches. The gametophyte is subterranean and colourless . They are homosporous . Antherozoids are multiciliate .

11. 3. Arthrophyta : The sporophyte is differentiated into stem, roots and leaves. The stem possesses distinct ridges and furrows. The foliage leaves are borne in transverse whorls upon stems and their branches. The vascular cylinder is protostelic or siphonostelic . The leaf-gaps are absent. The sporangia are produced upon a specialized structure, the sporangiophores present at the apex of the stem. The antherozoids are multiciliate . They are homosporous .

12. 4 . Filicophyta : The sporophyte is differentiated into stem, leaves and roots. With the exception of protostelic forms, the other siphonostelic forms possess leaf-gaps in their vascular cylinders. The leaves are macrophyllous . The leaf bears many sporangia on either the margin or the abaxial face of the leaf lamina. They are homosporous . The antherozoids are multiciliate . The sex organs are found on the ventral surface of the heart-shaped prothallus (gametophyte).

13. How did Pteridophytes Plants Originated? The pteridophytes occupy the intermediate position in between the bryophytes on one hand and the phanerogams on the other. They possess certain combinations of major characters which are not found in bryophytes or in phanerogams . The most characteristic feature of the pteridophytes is the presence of independent gametophytes and sporophytes at maturity. The pteridophytes are also known as ‘vascular cryptogams’. The sporophytes of these plants possess a well developed system of conduction tissue which consists of xylem, phloem and other mechanical elements.

14. Origin Of Bryophytes from P teridophytes : According to other school of thought the bryophytes have been originated (descended) from pteridophytes by means of reduction. This view could not get general support yet several workers postulated the evidences in support of this view. According to Lang (1917), Kidston and Lang (1917), Scott (1923), Halle (1936), Haskell (1949) and Christensen (1954) the bryophytes have been descended by the process of reduction from pteridophytes . Kashyap (1919) also supported the view, because of common resemblances of the two groups.

15. Similarities between sporangia of some members of Psilophytales ( Rhynia , Horneophyton and Sporogonites ) with capsules of Anthocerotales , Sphagnum and Andreaea led to conclude this hypothesis. The Psilophytales are the oldest pteridophytes in which the sporophytes were rootless, leafless and dichotomously branched with terminal sporangia. Such sporophytes resemble the bryophytes, especially the members of Anthocerotales and are thought to have evolved by progressive reduction. -------------------------------------------------------------------------- THE END --------------------------------------------------------------------------