Reproduction of Psilotum

                                       Reproduction

Vegetative reproduction:

Vegetative reproduction in Psilotum takes place by formation of gemmae, develop freely in large number on the rhizome. These gemmae are small, oval bodies, one cell in thickness. Gemmae germinate either before or after detachment from the sporophyte and form new plants.

 Asexual reproduction:

Plant bears large and conspicuous sporangia at the distal ends of the dichotomously branched shoots. In Psilotum the sporangia are borne in triads on the adaxial side of the appendage or leaf at the point of dichotomy and they are slightly raised on broad but short stalks. As the sporangia are fused with one another so the group is often referred to as synangium. Each mature sporangium or sporangial complex is 3 lobed and each lobe  contains a spore sac with numerous spores of one kind (homosporous). The sporangial wall is multi layered.  The mature sporangium dehisces by a vertical slit.

Structure of the gametophyte: Spore is the first cell of the gametophyte. Each spore is bilaterally symmetrical with outer delicate and thin reticulate wall. A spore germinates after about 4 months and develop into a gametophytic plant. The gametophytic plant body is small, brown in colour, sub terranean and saprophytic in nature, the mature gametophyte is irregularly or dichotomously branched, elongated and covered with brownish hair like rhizoids. Cells of the gametophyte filled with mycorrhizal fungus.

Sexual reproduction:

Gametophytes are homothallic ie. monoecious. Antheridia and archegonia develop in large number on throughout the gametophyte. .

Antheridia: Antheridia begin to develop on the gametophyte earlier than archegonia, each antheridium is a projected spherical body with a jacket of single layer of cells. Within the jacket of the antheridium, there is numerous spiral and multiflagellate sperms.

Archegonia: The archegonia are sunken with short projecting neck which breaks away at maturity. The neck of the archegonium is 4-5 cells in height and carries 2 neck canal cells. The venter contains 1 ventral canal cell and an egg.

Fertilization: As the archegonia mature there is continuous disintegration of neck canal cells and ventral canal cell which leaves a passageway for entrance of antherozoid into the venter of the archegonium. Finally, fertilization occur inside the venter.

After fertilization, the diploid zygote enlarges and starts to divide to form the young embryo, the embryo continues its growth with elongation of covering calyptra (gametophytic origin). Finally,  calyptra rupture and embryo grow to form the young sporophyte of Psilotum (at the same time gametophyte gradually degenerates).

Sporophyte structure of Psilotum

                                           

                              Psilotum

Psilotum belongs to the family Psilotaceae, class Psilophytinae, division Psilophyta of pteridophyte (Smith, 1957).

Habitat: Psilotum with two species ie. Psilotum nudum and Psilotum flaccidium is widely distributed in tropical and subtropical regions. The plant is xerophytic in nature but can grow in various habitat. Psilotum nudum is the only member found in India.

Habit: Psilotum is a slender often densely tufted shrubby plant with a height of  20-100 cm. Both the species are generally epiphytic in habit and grow upon tree ferns and palms, they may also grow terrestrially in the soil or in crevices of rocks.

                           Structure of the sporophyte

Morphology or external structure:

The sporophyte ie. plant body is differentiated into a slender root less subterranean rhizome and an aerial shoot. The rhizome contains a mycorrhizal fungus and it is covered with hair like absorbing structures called rhizoids. The rhizome branches dichotomously. On the rhizome erect or aerial green (photosynthetic) perennial branches are developed. Aerial branches are either pendent (epiphytic) or erect (terrestrial). Aerial shoots are also dichotomously branched. The basal parts of the shoots are cylindrical but the distal parts are ribbed or flattened.

The aerial branches on the upper part bear small scale like appendages are known as leaves. These leaves are arranged irregularly or more or less definitely in two or three rows.

Anatomy or internal structure:

                                  Rhizome and aerial branch:

Epidermis: Epidermis of the rhizome is indistinct and thin walled (without cuticle), stomata absent in epidermis.

Epidermis of the aerial branches is one celled thick with heavily cutinized outer walls. Stomata present (mainly at the grooves).

Cortex: In the rhizome, cortex is composed of only thin walled parenchymatous cells containing mycorrhizal fungus, the cells of the cortex are without any intercellular spaces.

Cortex of the aerial branches is massive and differential into three regions ie. a) hypodermal outer cortex is 2-5 layered, composed of chlorenchymatous cells with intercellular spaces, b) middle cortex lies internally next to outer cortex and is composed of  4-5 layers of elongated sclerenchymatous cells with little or no intercellular spaces. c) Inner cortex lying internal to middle cortex, this zone is composed of thin walled parenchymatous cells without intercellular spaces but containing abundant starch grains. Inner cortex is internally delimited by an endodermis with distinct casparian strips.

Stele: Stele of aerial branches is actinostelic protostele with exarch xylem. Phloem lies between endodermis and xylem. The stele of rhizome is haplostelic or actinostelic protostele.

Leaf: The leaf has single layered epidermis without stomata. The interior of leaf is composed of parenchyma cells without a vein.