The Hormonal Regulation of Protonema Development in Mosses

Bud formation, a fundamental step in the morphogenesis of the protonema of Funaria hygrometrica and other mosses, is, as is well known, regulated by external cytokinins like kinetin, zeatin etc. The first visible effect of kinetin (10 μM l⁻¹) is an arrest in the lengthwise growth of the side branches of caulonema. This effect is observed in less than three hours after hormone application. Thereafter, growth in the cell tip takes place in the widthwise dimension. Branches longer than 80 μm continue the filamentous growth. Within a few hours after removal of kinetin, tip growth is restored. From these observations it can be concluded that the effect of cytokinins is to change the growth form in the tip region of the side branches. Furthermore, the extent of this reaction depends on the caulonema cells themselves. For example, in isolated single cells it is always observed that the sixth cell of a filament produces the highest number of buds upon treatment with cytokinin.