Life Cycle of Psilotum | Pteridophyta | Botany

Psilotum has two species – P. nudum (= P. triquetrum), which is erect and P. flaccidium, which is lax and pending, and grows epiphytically. The common Indian species of Psilotum is P. triquetrum.

Psilotum is a rootless, slender plant, 20-100 cm. in length. The rhizome is covered with hair-like absorbing organs and fre­quently contains a mycorrhizal fungus. During the course of undergoing repeated dichotomous branching, the tip of any dicho­tomy comes out of the soil and develops into a sub-aerial shoot. These shoots are flattened or ridged, green in colour and are pro­vided with minute, awl-shaped, scaly appendages in the upper portions only.  

Internally, the sub-­aerial shoot of Psilotum consists of an epidermis, interrupted frequently by stomata, below which there are a few layers of chlorenchyma cells. The cortex is differentiated into an outer sclerenchymatous and an inner parenchymatous zone; the cells in the later region are without any intercellular space.

A typical endodermis separates the stele from the extrastelar tissues. The stele is an actinostele with 5-6 rays; the xylem is exarch. Typical sieve cell has not been demons­trated.  

Though fertile and sterile regions cannot be strictly demarcated out, yet generally the sporan­gia are borne in triads on minute stalks in the axils of bifid scales. There is a good deal of controversy as regards the morphological nature of the sporangium; some regards it as a synangium (a group of sporangia -fused together), while others regard it as a single septate sporangium.

A large number of sporogenous cells are produced from a single archesporial cell. Some of these sporogenous cells develop into spore mother cells, which finally undergoing reduction divisions give rise to spore tetrads.

In some cases Psilotum may reproduce vegetatively by the formation of gemmae developed on the rhizoids.  

The Gametophyte of Psilotum:

A spore on germination develops into the gametophyte, which is minute, brown in colour and perfectly subterranean and saprophytic in habit. It is irregularly cylindrical, somewhat dichotomously branching and thoroughly covered with unicellular rhizoids. The cells of this prothallus are filled with a symbiotic fungus. The gametophyte is monoecious and often it develops vascular tissue.

The antheridial usually develop earlier than the archegonia. These are spherical in outline with an antheridial jacket, which is single layer thick. Mature antheridia project above the gameto­phyte and each one bears numerous spiral, multiflagellate sperms.

The archegonia are also projecting and for­med from the superficial cells of the prothallus. The neck canal cells are probably two in number and within the venter there are ventral canal cell and egg cell.  

The New Sporophyte of Psilotum:

The development of the new sporophyte starts with the first division of the zygote which is transverse, (i.e., a plane at right angles to the long axis of the archegonium) or very nearly so. From the outer part develops the axis, while the inner one gives rise to the foot of the embryo.

This foot is very clearly distinguishable during the early stages of development. It enlarges downwards and sends out finger-like haustorial branches, which penetrate into the body of the adjoining gametophyte. Ultimately, when the young sporo­phyte becomes independent of the gametophyte for its food supply, the foot gets dissociated from the sporophyte and attaches itself to the gametophyte.