No abstract

IN THE COURSE OF a study of prothalli of several species of ferns growing in sterile culture, it was observed that under these conditions there are frequent departures from the characteristic morphology. The occurrence of abnormal developments of fern prothalli in sterile culture has previously been reported on and extensively described by Hurel-Py (1950), Morel and Wetmore (1951), and others. Of all the species examined in this study, the bracken, Pteridium aquilinuni (1.) Kuhn, showed the greatest variety and frequency of occurrence of abnormal developments. It was further found that certain of these abnormalities could be isolated and grown in separate culture where they manifested varying degrees of permanency.In view of the increasing interest in the nature of changes in the morphology and physiology of cells, as particularly evidenced by current work on normal differentiation and on tumor formation, it was thought possible that a study of these abnorrnalities might have significant results. On general grounds it would seem that the fern prothallus is an extremely favorable organism in which to study the factors affecting cellular expression. ProthaIii of many species are easily grown on nutrient media under the controlled conditions of sterile culture (Hurel-Py, 1950); and are quick to respond to changes in cultural conditions. More important, however, is the structural simplicity of the fern pro· thallial generation, which makes it possible to study the range of morphological expressions of cells which, under normal circumstances, seem to possess limited potentialities. The thallus-like body of the mature prothallus consists of cells which are essentially similar and which are arranged in a single. layered sheet, except in the central portion where a cushion or pad of several layers is formed. The only specialized cells present in the prothalli of many species are those composing the reproductive organs, the archegonia and the antheridia, and the elongate rhizoids which attach the prothallus to the 1

The development of the fern gametophyte is affected by a number of rather diverse treatments. In particular the spectral quality of light has a strong influence on the change in form of the gametophyte. It was reconfirmed that in the developing fern gametophyte red light promotes elongation whereas in blue light the magnitude of elongation is greatly reduced. A correlation between rate of cell division and two-dimensional form was found. A high rate of cell division promotes the formation of a gametophyte that is increasing both in length and in width while a low rate of division promotes the formation of a gametophyte that is increasing primarily in length. On the basis of this observation the change in the form of the gametophyte is interpreted to be directly related to the rate of cell division, which in turn influences the plane of division. This interpretation is also applied to the transition from unidimensional to two-dimensional growth in the developing gametophyte.

A callus of the common garden peony, Paeonia suffruticosa, was subcultured on solid and liquid media and analyzed intensively for a period of 153 days in order to test the effects of subculturing and the physical conditions of culture on the mitotic cycle kinetics of a population of cells, particularly in relation to the degree of heteroploidy. The parameters investigated in the kinetic studies included mitotic index values, cell generation time, and the time required for the cell population to double. The mitotic index of Paeonia cells cultured in liquid medium was found to be about two and a half times higher than for those cultured on solid; successive subculturing did not affect the mitotic index on either type of medium. The most significant results of the study came from the chromosome count data, in which diploid and tetraploid cells fluctuated in predominance in successive subcultures, and the apparent earlier manifestation of polyploidy on liquid medium. Mitotic index, cell generation, and population doubling times remained constant throughout the study.

The development of the gametophyte of Pteridium aquilinum was examined in the presence of several biochemical agents. These compounds, by virtue of their influence on the processes of cell division and cell elongation, brought about a change in the gross morphology of the gametophyte. It was thus reconfirmed that a change in the form of the gametophyte may be explained in terms of differential cell behavior. This interpretation was extended to include the response of the gametophytes to several amino acid and nucleic acid analogues. It was also demonstrated that there is no quantitative increase in protein concentration associated with the development of a 2-dimensional structure and it was suggested that an examination of gametophyte morphology in terms of an interaction of cell division and cell elongation may provide a better insight into the problem of gametophyte morphogenesis.