The effects of cellular and environmental variables involved in the population assay of zoospore germination have been examined. The following results were obtained: 1. 1. Within the standard conditions employed, only growth density and, to a lesser extent, temperature, seriously affect either the synchrony or the rate of zoospore conversion to round cells. 2. 2. Synchrony of round cell formation varies as a function of growth density in DM-2, but not PYG, medium. 3. 3. Synchrony, and probably rate, of round cell formation are profoundly influenced by temperature; neither a temperature threshold nor a temperature optimum was found within the viable (growth) range. 4. 4. Neither synchrony nor rate of round cell formation is affected by various manipulations of the zoospores prior to germination (centrifugation, deflagellation, prechilling, extended periods of swimming) or by zoospore concentration (within the standard range). At high zoospore concentrations, T50 is increased. 5. 5. Synchrony and rate of round cell formation are indistinguishable in germination solution, an inorganic salts medium, and DM-2, a defined growth medium. Round cell formation is considerably more asynchronous in the previously standard, undefined growth medium (PYG). 6. 6. Of the constituents of germination solution, only KCl is necessary for moderately synchronous round cell formation, and the degree of synchrony is markedly dependent on KCl concentration. On the basis of these and other (Soll et al., 1969) results, the following conclusions bearing on the germination process and on its control have been made: 1. 1. The moment of initiation of zoospore germination is precisely defined. 2. 2. KCl (or chloride salts of certain other monovalent cations) can initiate the changes associated with zoospore germination. 3. 3. Neither initial presence of a flagellum, flagellar retraction nor later presence of an internal flagellar axoneme are required for the successful completion of germination. 4. 4. At standard zoospore concentrations, cells do not interact to permit semisynchronous germination; at higher concentrations, cells do interact to inhibit germination. 5. 5. Zoospores can successfully complete germination at ice temperature. 6. 6. Heterogeneity in zoospore populations with respect to time of germination can be predictably varied by controlling the density of growth cultures from which the zoospores are derived. © 1969.
