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Zoospores of Pythium aphanidermatum in distilled water suspension responded to intact pea roots by massing in large numbers at the sites of elongation behind the root tips. They showed preferential attraction to wounds made by puncturing the epidermis, and to the stele exposed at the cut ends of roots. Massing of zoospores was followed, almost immediately, by encystment and early germination, the germ tubes being directed towards the source of stimulation. The phenomenon was non-specific over a wide range of plants examined. Root exudate and extract, diffusing from the ends of capillary tubes containing these materials in agar, caused identical zoospore responses. Analysis of zoospore movement under stimulation revealed a sequence of responses—initial disruption of the typical course of movement, attraction, trapping, and early cessation of movement by encystment. Attraction was equated with chemotaxis, whilst trapping showed features of kineses.

A method was devised to produce abundant zoospores in distilled water suspension from pure cultures of Pizytophthora nzegaspernza var. sojae. Sporangia were predominantly non-papillate, but occasionally inconspicuously or conspicuously papillate, germinating with the forination of a delicate, evanescent vesicle. Proliferation of sporangia was observed.Flagella action of freely swimming zoospores was investigated photographically. Both flagella were proved to undulate. Zoospores remained motile longest a t 15 "C. Motility was marlredly reduced a t extreme temperatures (5 and 36 " C ) , a t extreme pIH values (below 5.2 or above 9.25). or by mechanical disturbance, dilution, and frequent contact with solid surfaces. The fate of flagella during encystment was followed. Encysted zoospores germinated by germ tubes, or by secondary zoospores, or in rare cases, the germ tube was terminated by a miniature sporangium. Repeated emergence of zoospores was favored a t 15 "C whereas total cysr germination and germ tube production was best a t 25 "C and in the presence of nutrients.

Studies have been made of the ultrastructure and cytochemistry of zoospores, cysts, and germinating cysts of Phytophthora palmivora. The bulk of the zoospore cytoplasm is occupied by lipid bodies, crystalline vesicles containing lipid plus other unknown material(s), and granular vesicles containing protein. During a 6-h motile period after zoospore release no noticeable changes occur in crystalline and granular vesicles or in lipid bodies. Granular vesicles are retained if encystment is caused by centrifugation or agitation, but few remain when zoospores encyst under undisturbed conditions. Cysts begin to germinate 15 min after encystment is complete. Granular vesicles begin to disappear 30 min after germination starts and the process is complete 60 min later. Crystalline vesicles and lipid bodies also begin to disintegrate 30 min after germination begins, resulting in osmiophilic network-like breakdown products. The breakdown of these products increases with time but is not complete 90 min after germination.