106 PLANT-ANIMALS [CH. 



consists of colourless, nucleated protoplasm contain- 

 ing chloroplasts, so, on Haberlandt's hypothesis, the 

 green cell of C. roscoffensis consists of a colourless, 

 animal part containing a green chloroplast. 



Pursuing this hypothesis to its natural conclusion, 

 it is easy to imagine, with Haberlandt, that, in some 

 remote past, algal cells came to exist in symbiosis 

 with colourless C. roscoffensis ; that the animal offered 

 such a congenial lodging as to induce the algse to give 

 up going out altogether. They abandoned their cell- 

 wall as an enclosing apparatus no longer of service 

 to them. In return for security and all the comforts 

 of a home the green cells prepared the food both for 

 themselves and for their host. Submitting itself to 

 the guidance of the animal, the green cell aban- 

 doned its nucleus and became reduced to a naked 

 chloroplast. 



So it might have come about that the only powers 

 retained by this relic of a once complete and free 

 algal cell are those possessed by the chloroplast of 

 a green plant, namely, the powers of photosynthesis 

 and of division to form new chloroplasts. Moreover, 

 just as the chloroplasts contained in the egg-cells of 

 plants lose their green pigment, and become colourless 

 leucoplasts, which, dividing as the cells of the plant- 

 embryo divide, give rise to the chloroplasts of the 

 next generation, so, on this hypothesis, it would follow 

 that the green chloroplasts of C. roscoffensis might 



