Cellular Mechanics, Taxonomy and Evolution in the Uredinales and Ustilaginales

SUMMARY

This discussion does not cover the whole field of cellular mechanics, but deals with the following topics: basidiospore formation, teliospore evolution in Puccinia, effect of pressure upon teliospore shape, spore wall ornamentation, aeciospore discharge, spore form in Tolyposporium, pedicel structure in Phragmidium teliospores, constriction at teliospore septa, and the supposed homology between the ascus hook and clamp connection. It is shown that convergent evolution has been overwhelmingly important in evolution of the teliospores of Puccinia and Uromycesand in the development of the various types of spore wall ornamentation. Its occurrence elsewhere in the fungi is indicated and examples are given from other groups of organisms to illustrate the circumstances under which it may be expected to operate. The rearrangement of nuclei after division of a dikaryon is of such importance and is capable of so few satisfactory solutions that the situation is extremely favorable to convergent evolution; and it is considered that acceptance of the ascus hook and clamp connection as homologous is not warranted.