122 PLANT-ANIMALS [CH. 



the animals were green. The synthesis of the plant- 

 animal had been effected. As the result of introducing 

 an undoubted green alga to a colourless, larval C. 

 roscoffensis, a green plant-animal was formed. 



Since a similar last stage has not been reached in 

 the case of C. paradoxa though to reach that stage 

 is but a matter of time and experiment we will 

 devote ourselves not to a description of the incomplete 

 evidence of its infection by a yellow-brown algal 

 cell (see Keeble, 1908), but to a continuation of 

 the study of the life-history of the infecting organism 

 of C. roscoffensis. The securing of material for this 

 purpose is comparatively easy once the art of culti- 

 vating the organism on the egg-capsules has been 

 learned. It is facilitated also by the fact that the 

 motile, green cells are, like C. roscoffensis, positively 

 phototropic and assume, in a vessel of sea-water 

 exposed to unilateral light, a position identical with 

 that taken up by C. roscoffensis. Thus, though each 

 algal cell is far too small to be visible to the unaided 

 eye, the green, motile cells aggregated together at 

 the surface of the water on the side toward the light 

 become collectively visible as a green scum at or just 

 above the water-line. The fact that they react to 

 light by a vertically upward movement as well as 

 by a movement toward the source of light suggests 

 that the pyrenoid (Fig. 21), a dense mass of protein 

 surrounded by protoplasm, may perform for the green 



