iv] GREEN CELLS OF CONVOLUTA 107 



give rise to colourless leucoplasts which pass into 

 the egg and provide the rudiments from which the 

 chloroplasts of the larval animal are developed. 



Yet again, if this were indeed the course of events 

 in C. roscoffensis, if, from their free, complete con- 

 dition, the original green cells which gained access 

 to the body of our plant-animal have become reduced 

 to mere chloroplasts, might not this animal provide an 

 illustration of the mode of origin of the higher green 

 plants themselves ? In a remote past, a symbiosis 

 was struck up between a colourless organism and 

 a green alga such a communal mode of life, for 

 example, as that presented by lichens at the present 

 day. Convenient models these to show us the 

 relation between colourless organism and undoubted 

 algal cells. So happy is the hypothetical partnership 

 between alga and colourless organism that fresh 

 developments ensue. A new and composite form of 

 life comes into existence. The colourless tissues 

 burrow in the earth and supply, along well-defined 

 conduits, the water and minerals required by the 

 green cells. They form tall trunks and spreading 

 branches to lift the chloroplasts the representatives 

 of the algal cells nearer to the sun. The green plant 

 is in being. 



Of this alluring picture, evoked by syren-voiced 

 hypothesis, we are bound to ask the simple, sober 

 question, is it true? To this question we can give 



