INFLUENCE OF GRAVITY AND LIGHT ON THE DEVELOPMENTAL POLARITY OF CERATOPTERIS-RICHARDII FERN SPORES

The polarity of germinating single-celled spores of the fern Ceratopte ris richardii Brogn. is influenced by gravity during a time period pri or to the first cellular division designated a ''polarity-determinatio n window''. After this window closes, control of polarity is seen in t he downward (with respect to gravity) migration of the nucleus along t he proximal face of the spore and the subsequent downward growth of th e primary rhizoid. When spores are germinated on a clinostat, the dire ction of nuclear migration and subsequent primary rhizoid growth is ra ndom. However, in each case the direction of nuclear migration predict s the direction of rhizoid elongation. Although it is the most obvious movement, the downward migration is not the first movement of the nuc leus. During the polarity-determination window, the nucleus moves rand omly within a region centered behind the trilete marking. While the po larity of many fern spores has been reported to be controlled by light , spores of C. richardii are the first documented to have their polari ty influenced by gravity. Directional white light also affects the pol arity of these spores, but this influence is slight and is secondary t o that of gravity.