iv] GREEN CELLS OF CONVOLUTA 127 



The existence of paired and single colourless 

 resting ceils formed by the infecting organism in 

 its free state completes the evidence as to the 

 identity of this alga with the green cells of C. ros- 

 coffensis. For, as we have seen, though the alga 

 may be ingested in its green, flagellated stage, 

 the more usual mode of infection is by means of 

 a colourless cell surrounded by a thick wall. This 

 cell, lying in the central vacuole, undergoes division 

 into daughter cells, which escape subsequently from 

 the mother wall and are sown about the body of 

 C. roscoffensis. The cell originally taken into the 

 body may be paired or single. In the former 

 case it gives rise to eight, in the latter case, 

 to four daughter cells. We conclude, therefore, 

 that the alga which is the infecting organism of 

 C. roscoffensis, lives a double life. At times, it has 

 the form of a green cell, at others, of a colourless 

 cell. As a green cell it is holophytic, that is it manu- 

 factures photosynthetically its food materials. As a 

 colourless cell it is a saprophyte, feeding like an 

 animal on ready-made organic material. In its 

 active stage, it is green and seeks the light : in its 

 passive stage, it may be colourless and may live in 

 darkness. Beneath the sand, therefore, where C. 

 roscoffensis is born, abound vast numbers of the 

 colourless infecting organism. On its emergence 

 from the egg-membrane, the larva encounters them 



