126 DISEASE IN PLANTS. 



with. Moreover we know from experiments on 

 heliotropism, thermotropism, etc., that even in- 

 tangible stimuH such as rays of light, etc., imping- 

 ing unsymmetrically on these delicate cells cause 

 alterations in their behaviour e.g. arrest or 

 acceleration of growth. 



Perhaps the most remarkable class of stimula- 

 tions, however, is that due to the presence of the 

 entire protoplasmic body of one organism in the 

 cell of another, each living its own life for the time 

 being, but the protoplasm of the host cell showing 

 clearly, by its abnormal behaviour, that the 

 presence of the foreign protoplasm is affecting its 

 physiology. A simple example is afforded by 

 Zopfs' Pleotrachelus, the amoeboid protoplasmic 

 body of which lives in the hypha of Pilobolus, 

 causing it to swell up like an inflated bladder, in 

 which the parasite then forms its sporangia. The 

 Pleotrachelus does not kill the Pilobohis, but that 

 its protoplasm alters the metabolic physiology of 

 the latter is shown by the hypertrophy of the cells, 

 and by the curious fact that it stimulates the 

 Pilobolus to form its sexual conjugating cells, 

 otherwise rare, an indication of very far-reaching 

 interference with the life-actions of the host. 



An equally remarkable example is that of Plas- 

 modiopJiora, the amoeboid naked protoplasm of 

 which lives and creeps about in the protoplasm of 

 a cell of the root of a turnip, to which it gains 

 access through the root-hairs. It does not kill the 

 cell, but stimulates its protoplasm to increased 

 activity and growth and division, itself dividing also 



