414 The Physiology of Plants BOOK in 



The utilization of the products of digestion by the living 

 substance brings us back to the problem which met us at 

 the conclusion of the story of the original construction of 

 organic food material. How, and in what forms, are these 

 various substances incorporated into the living protoplasm ? 

 We have here the problem properly named assimilation, 

 which covers accordingly all questions of nutrition in the 

 strict sense. 



These problems were the subject of' close thought and 

 much speculation during the period we are discussing, but 

 the total outcome of actual knowledge was meagre in the 

 extreme. The study of its progress reminds us of the state 

 of ebb and flow of opinion before 1860 upon the questions 

 whose solution has been so greatly advanced since that 

 year. 



The range of thought and inquiry was mainly directed 

 to the relation of protein matter to living substance, and 

 naturally so, as evidently the latter demands supplies of 

 nitrogen-containing matter, of which protein most nearly 

 approaches to it in composition. 



Here we may first call attention to the writings of Pfliiger 

 in 1875. To him the relations between protein and proto- 

 plasm were so close that he regarded the latter as living 

 protein, holding that the difference between the two lies 

 in the power of auto-decomposition of the protoplasm, 

 ordinary protein being a fairly stable substance. He asso- 

 ciated this auto-decomposition with the power of fixation 

 of oxygen in some intramolecular combination which forms 

 part of the living molecule, pointing out how the addition 

 of oxygen increases the lability of the body with which 

 it combines. Pfliiger held that the fundamental difference 

 between living and dead protein lies in the way in which 

 its nitrogen is attached to the other elements in the 

 molecule, basing his opinion upon a comparison of the 

 decomposition-products of the two. Those which are yielded 



