DYNAMICS OF INDIRECT GERMINATION IN PHYTOPHTHORA-CACTORUM SPORANGIA

Indirect germination of the sporangia of the facultative parasite P. cactorum (Leb. and Cohn) Schroet. (Oomycetes) is described in detail. The significance of the germination vesicle, the cover of the papilla and the opening mechanism of the sporangium are discussed in particular. The entire process of germination is characterized in a mathematical physical way. The opening of the sporangium and the release of the zoospores are understood as being passive responses to a change of osmotic gradient and turgor pressure inside the sporangium. This release of the zoospores and the nature of the enlargement of the germination vesicle correspond qualitatively to a curve of pressure decrease and to a model of the stretching of plastic substances, respectively. The maximum size reached by the germination vesicle is approximately the size of the sporangium and has a surface area 25 times that of the original papilla. The sporangium is only emptied completely if the first 8 zoospores pass the pore with more than a certain critical velocity.