Thallophyta, the lowest sub-kingdom of the vegetable world, comprising some plants in which sexual reproduction does not seem to have made its appearance, others in which propagation is direct and not cyclical, and others again in which the alternation of generations, though present, is rendered irregular by the intercalation of various asexually-produced generations or gonidia. In the first class, which comprises minute unicellular plants, reproduction consists merely in cell-division. In the second class, comprising many filamentous forms, the gametes conjugate and produce a cell, the zygospore or oospore, which develops directly into a gametophyte, so that there is no sporophyte or alternation. The name thallophyte is taken from the fact that there is very rarely in this sub-kingdom anything like a differentiation of stem and leaf, the whole plant being a thallus (q.v.). The thallus may be unicellular, coenocytic, or multicellular; but the cell-walls, though sometimes thickened or mucilaginous, are never lignified, nor is there any marked differentiation of tissues. Nevertheless, between the simplest Schizomycete and the higher Algae and Fungi there is an enormous difference in complexity. The most satisfactory subdivision of the sub-kingdom is into the two great groups or classes, the Algae (q.v.), and the Fungi (q.v.), since the presence of chlorophyll in the former and its absence in the latter is correlated with other characters.