96 A Pht/toj)hf/iora Rot of Pears and Apples 



length of the sporangium the vesicle suddenly becomes ruptured and 

 disappears from view, the enclosed zoospores dispersing immediately. 

 The rest of the zoospores come out singly in rapid succession and Rave 

 to squeeze their way through the opening, becoming hour-glass shaped 

 when half-way through. If one should fail to get out with the main 

 stream it swims about frantically inside the now almost empty spor- 

 angium; one that was so left behind escaped eventually but took two 

 or three minutes to force itself through the mouth of the sporangium. 

 On reaching the exterior the zoospores are at first somewhat irregular 

 in shape for they appear to undergo temporary deformity during the 

 passage through the narrow aperture, and often make a slight pause, 

 just outside the mouth of the sporangium, in order to resume their 

 normal form before swimming away. 



The size of the zoospores was usually not easy to determine owing 

 to the rapid movements but occasionally approximate measurements 

 were possible and the dimensions found to be about 12 x 9^. The 

 zoospores are pyriform to reniform with two cilia inserted within the 

 sinus. 



When in motion the zoospore revolves round its longitudinal axis. 

 The posterior cihum projects backwards like a flexible bristle, becoming 

 inclined with each change of direction of the body of the zoospore. 

 The anterior cilium is very rarely seen but a glimpse of it may be ob- 

 tained when the zoospore pauses for a moment. Even then its appear- 

 ance is only momentary and at first it was thought possible that it was 

 the posterior cilium brought round anteriorily. Later, however, both 

 were occasionally seen at the same time, one directed forward, the other 

 backward. It would seem that the anterior cilium is the organ of loco- 

 motion for its invisibility when the organism is in motion doubtless 

 indicates that it is in rapid vibration. 



In those cases where the zoospores emerge slowly from the spor- 

 angium cilia can be seen while they are still inside the sporangium. It 

 is probably the anterior cilia which are visible in this case for they are 

 pointed towards the mouth of the sporangium; again when a zoospore 

 is about half-way through the aperture its anterior cilium may again 

 be seen turned over to one side, then as the zoospore becomes free the 

 other cilium is seen trailing behind through the opening (Fig, 2). 



►Sometimes a zoospore was seen having a posterior attenuated " tail "' 

 of protoplasm. On one occasion two zoospores were found attached to 

 each other by a protoplasmic strand; in such a case co-ordinated move- 

 ment seemed to be absent for they swayed from side to side without 



