In psilopsida plants have rootless sporophytes. The stem is differentiated into an underground rhizome and an aerial part. Both are dichotomously branched. The rhizome bears rhizoids, both perform the function of root. The aerial branches are green, leafless and bear small veinless out growths and carry out photosynthesis.
The reproductive organs of sporophyte are sporangia which develop at the tips of long or short branches, or on lateral sides of branches (fig. 9.11). internal structure of stem is simple. Vascular tissue is narrow, central and solid without pith, with a broad cortex.
Psilopsida is considered to be the earliest group of vascular plants. Most of the representatives of this group have become extinct, for example, Horneophyton, psilophyton, cooksonia (Fig. 9.11) etc.
Fig. 9.11 Psilotum
(a) Dichotomously leafless branches.
(b) The erect branches of another species, showing brown sporangia.
There are only two living genera Psilotun, and Tmesipeteris.
The gametophyte is thalloid. It is colorless and underground. Its cells contain a fungus which provides food to the gametophyte and in return gets protection from it. Such beneficial symbiotic relationship among the two members (fungus and plant) is said to be symbiosis; or mycorrhizal association. Examples are psilotum, and tmesipeteris.