SYNTHESIS OF PROTEIDS 115 



mosynthesis and Electrosynthesis are conceivable, though they 

 are not known to occur. 



3. SYNTHESIS OF PROTEIDS. 



In his study of the chemical composition of the photosyn- 

 thate, the student must have come into contact with references 

 to a very important class of plant substances, of which Proto- 

 plasm is largely a mixture, called proteids, whose most notable 

 characteristic is the possession of nitrogen, sulphur, and phos- 

 phorus, in addition to the carbon, hydrogen, and oxygen of the 

 photosynthate. It becomes important now to investigate the 

 mode of synthesis of these proteids, a subject of all the more 

 consequence since it involves economic interests of magnitude. 

 Unfortunately our knowledge of the subject is very defective, 

 and its experimental study is correspondingly difficult, but the 

 student must follow it even if only through the literature. 



We consider first the nitrogen of the proteids, and the first 

 question naturally relates to the source of supply. Both general 

 probabilities and the analogy of photosynthesis would lead us 

 to expect to find the source of the plant's nitrogen in the abun- 

 dant store of the atmosphere. Yet as ample nitrogen in the form 

 of nitrates is present in the soil, that possible source cannot be 

 ignored. The matter can readily be settled by experiment, 

 which involves this problem: 



Do plants obtain the nitrogen of their proteids from the free 

 nitrogen of the air, or from the combinations in the soil? 



This may be determined, indirectly, by so growing two sets of 

 similar plants that both have full supply of atmospheric nitrogen, 

 while one is supplied with combined nitrogen through the roots and the 

 other lacks it ; then the comparative growth of the two sets must show 

 whether or not the soil nitrogen is needed. This can most readily 

 be effected through water-culture. "~ . 



EXPERIMENT. On each of the covers of two water-culture vessels 

 place ten Oats germinated by the method described later under Growth; 

 fill one vessel with a standard water-culture solution, and the other 

 with the same, excepting that the nitrogen salt is replaced by a calcium 

 salt. Give favorable conditions for water-culture growth, and com- 

 pare their relative progress; finally compare the relative dry weights. 



