146 PLANT-ANIMALS [CH. 



of combined nitrogen, it begins to divide and soon 

 forms a colony of numerous green cells. 



Now, as we have indicated previously, C. ros- 

 coffensis and C. paradoxa are remarkable among the 

 Turbellarian worms in possessing no excretory sys- 

 tem. Unlike their allies, they possess no apparatus 

 for the systematic discharge of the waste products of 

 their metabolism. Hence such products, compounds 

 of nitrogen of a kind useless to the animal, are stored 

 in the tissues of the body. But such substances, 

 though useless for the nutrition of the animal, serve 

 well for plants. Even a terrestrial green plant is 

 very catholic with respect to the compounds of nitro- 

 gen which it takes up and utilises for the synthesis 

 of proteins. Thus, experiment has shown that the 

 root-system of a green flowering plant is capable 

 of absorbing, not only nitrates and, in many cases, 

 ammonium salts, but also such organic, nitrogen- 

 containing substances as urea, uric acid, asparagine 

 and many others. Now the infecting organism of C. 

 roscoffensis occurs, as we know, in a colourless as well 

 as in a green stage, and, in the colourless form, it can 

 obtain its food materials only after the manner of an 

 animal, that is, in combined organic form. So that 

 its powers of taking up and utilising organic nitrogen 

 compounds are likely to be even more marked than 

 those of a self-supporting green plant. This con- 



