v] NATURE OF PLANT-ANIMALS 155 



less phases are known to occur in the life histories 

 of other micro-organisms, in diatoms, in various species 

 of Chlamydomonas and in Flagellates (Euglena), and 

 it is stated generally that they may be induced by 

 increasing the amount of soluble carbohydrate in 

 the culture medium. But in the cases of the algal- 

 infecting organisms of our plant-animals, the rapid 

 development of the chlorophyllous pigment appears 

 to be associated with the increase in the amount of 

 available nitrogen. So that, if this is the case, the 

 colourless phase would appear to be brought about, 

 not by excess of food-material, but by lack of nitrogen. 

 It may well prove to be that the colourless sapro- 

 phytic phases exhibited by such organisms as those 

 just mentioned diatoms, etc. are each a symptom 

 of nitrogen-hunger. For, failing proper supplies of 

 nitrogen compounds, no amount of carbohydrate photo- 

 synthesis will keep the organism from starvation. 

 Indeed, the more the carbohydrate photosynthesis, in- 

 volving as it must the wearing out and reconstruction 

 of the nitrogen-containing chlorophyll machinery, the 

 acuter will be the nitrogen-hunger ; whereas, on the 

 contrary, a shutting down of the photosynthetic process 

 will effect economies in the use of organic nitrogen 

 compounds and thus postpone the evil day of nitrogen- 

 starvation. Though the facts are not yet available 

 for a confident statement, the hypothesis may be 

 proposed that saprophytism generally depends for 



