194 PHYTOPHTHORA PARASITICA. 



Varying temperature, alternate light and darkness and moisture 

 are essential factors in the formation of sporangia. Cultures kept 

 for months in a dark cupboard at varying room temperature and in 

 incubators at the constant temperatures of 20° C. in summer and 

 35° C. in winter, produced luxuriant mycelium but remained sterile. 

 But when they were removed to an ordinary room in ordinary 

 atmospheric conditions, sporangia were soon formed. Inoculated 

 castor seedlings kept in a dry place in the laboratory produced no 

 sporangia but those kept in the laboratory compound on misty 

 nights and those kept in the laboratory but occasionally sprayed 

 with water, gave sporangia in forty-eight hours. 



An immature sporangium has a single vacuole about 10-12/x 

 in diameter. It glides from place to place in the sporangium 

 by very slow, almost imperceptible " amoeboid " movements. 

 As it contracts and expands the size of the sporangium has 

 been found to be decreasing and increasing. The formation of 

 zoospores is made faintly evident by the segmentation of the pro- 

 toplasm into so many units while the vacuole is still present. At 

 a later stage the vacuole disappears and the zoospore origins 

 become more prominent. At this stage they have no vacuoles. 

 They then become more coarsely granular. The sporangium looks 

 rather dark. A little later in each of the zoospores a vacuole is 

 seen appearing and disappearing. The papilla is now very highly 

 refractive. The contents of the sporangium have contracted a little 

 from the inner wall. A slight pulsating movement in it heralds the 

 approaching discharge of the zoospores. There is a slight spasmodic 

 rotation of the whole mass. Individual zoospores are restless, 

 they contract and swell. There is also a slight motion visible in 

 their granular protoplasm. The inner wall opposite the papilla is 

 occasionally pushed into it by a zoospore striking against it. The 

 discharge of the zoospores takes place in either of the following two 

 ways : — 



1. The papilla dissolves or bursts under pressure. The zoo- 

 spores are immediately discharged but sometimes a little granular 

 fluid flows out before them. Sometimes the papilla dissolves before 



