v] NATURE OF PLANT-ANIMALS 147 



jecture is confirmed by experiment. Comparative 



cultures of the free stage of the infecting organism 



have demonstrated that the alga flourishes better when 



supplied with nitrogen in the form of uric acid than 



when it is supplied with a nitrate (potassium nitrate). 



Thus our argument brings us to the following 



position : We have evidence that the infecting 



organism increases rapidly as soon as it gains access 



to the body of the plant-animal. We know that it 



is able to utilise organic nitrogen compounds such 



as uric acid for the construction of its proteins. We 



know, further, that no apparatus for the removal of 



waste nitrogen compounds, uric acid, urea, etc., occurs 



in the bodies of C. roscoffensis or C. paradoxa. The 



conclusion forces itself upon us that the green and 



yellow-brown cells in the bodies of their respective 



hosts obtain access to and utilise the stores of waste 



nitrogen-compounds accumulated therein. Or, to 



put the same idea in another way, green cells and 



yellow-brown cells constitute the excretory organs 



of C. roscoffensis and of C. paradoxa respectively. 



The plants flourish in the bodies of these animals 



because there they discover large accumulations of 



waste nitrogen compounds: the animals, looking to 



the algse to come and take charge of the work of 



getting rid of these waste substances, have ceased 



to construct any excretory apparatus whatever. 



Hence it is not surprising that, when the algse fail 



102 



