Give an account of the salient features of Psilotum, emphasising its primitive features.

Ans. Salient Features: Psilotum, aside from pendant epiphyte and erect lithophyte, is normally of terrestrial habit growing in hilly regions of India. It represents the most simple and primitive sporophyte (length 20-100 cm) of modern era, having neither roots nor foliose leaves.

The sporophyte as a whole is divisible into 2 parts namely under ground rhizome and aerial shoot. The rhizome being cylindrical and dichotomously branched, is densely covered with rhizoids. The aerial shoot is denoted by dichotomously-forked photosynthetic stems bearing bifid scaly leaves as lateral appendages. Branches are lateral in P. nudum.

Anatomically the rhizome bears a haplostelic protostele. The cortex is neither mechanical nor chlorophyllous, rather the outer cortex harbours mycorrhizic fungus while middle one being storage.

The angular transitional stem devoid of scaly leaves is anatomically similar to aerial stem in all fundamental structures except the vascular strand which is actinostelic protostele.

The angular, ribbed, triquetrous or laterally flattened (e.g. P. flaccidum) aerial stem or its branches have an ectophloic siphonostele with stellate or spindle-shaped xylem strand. Heavily cuticularized epidermis is punctured with stomata. The broad cortex from periphery to centre is comprised of chlorenchyma, sclerenchyma and parenchyma.

The homosporous sporophyte reproduces vegetatively by multicellular gemmae and asexually by meiospores produced in tri-lobed trilocular sporangium / synangium borne in the axil of bifid scaly leaf. All the locules have their own multilayered jacket without tapetum. All the sporogenous cells do not transform into spore mother cells, rather a few of these disorganize to form plasmodial fluid of nutritive value. The mature sporangium exhibits loculicidal dehiscence. Its ontogeny is eusporangiate.

Minute subterranean saprophytic parenchymatous prothallus is cylindric and somewhat dichotomously branched. It bears unicellular rhizoids, harbours symbiotic fungus hyphae and carries a haplostelic protostele. Its ontogeny is exosporic.

The prothallus is monoecious-protandrous. The sexuality is oogamous. The spherical sessile antheridium is partly sunken. Its contains several spermatocytes. Its dehiscence is porous. Spirally-coiled spermatozoids are multiflagellate.

The projected archegonial neck, with 4 vertical rows and each row with 4-6 neck cells, contains 1-2 NCC.

Fertilization is internal in presence of water.

Protection to embryo is afforded by calyptrate sheath. The foot is derived from the hypobasal half while shoot primordium from the epibasal half of embryo. The formation of a primary root, a primary leaf and a suspensor has perfectly been suppressed. The embryonic sporophyte is parasite upon the prothallial gametophyte till the emergence of photosynthetic shoot.

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