CONVERSION 121 



tion as to the method by which the reverse process is accom- 

 plished, viz., the formation of the insoluble substances from their 

 corresponding hydrolytes, and upon this matter the student 

 should inform himself. This will bring him to a consideration 

 of accumulation of reserve substances and the closely related 

 subject of secretion of substances of special function, to which 

 also he must here give attention. 



Closely correlated with this subject of conversion is that of 

 the function of the various minerals essential to the metabolism 

 of the higher plants, but the state of our knowledge of this sub- 

 ject is such that little can be said of it in this connection, and it 

 may best be treated later along with the phenomena of Absorp- 

 tion. 



Another related matter which should be considered con- 

 cerns any corresponding processes in the animal kingdom, and 

 especially the changes in the various classes of plant substances 

 when absorbed and digested by animals. 



The Idea and Terminology of Plant Food. As happens so often 

 in science, the advance in our knowledge of plant nutrition has introduced 

 difficulties with the older terminology. The term plant food was originally 

 applied to the minerals, water, and gases absorbed by the plant, but now we 

 know that the food of plants, using the word in the well-defined sense of 

 animal physiology, is not these substances, but the photosynthate and its 

 derivatives. Doubtless the old usage is too firmly fixed to be changeable, 

 but the real condition should upon all occasions be made plain. Some, 

 though an insufficient, distinction can be made by terming the former raw 

 food and the latter elaborated food. 



Literature of Conversion. Upon this subject the most impor- 

 tant work, in addition to those of Pfeffer and of Jost, is Czapek's 

 " Biochemie der Pflanzen'' (Jena, Gustav Fischer, 1905), an exhaustive 

 work upon every phase of this subject. There is an important article 

 upon the reserve substances of plants by Green in Science Progress, 

 2, 1894-95, 109, and 3, 68, 476. Upon starch in particular there is 

 the important work of Meyer, " Untersuchungen ttber die Starke- 

 korner " (Jena, Gustav Fischer, 1895), and in this country the subject 

 has been studied especially by Kraemer, Botanical Gazette, 34, 1902, 

 341, and 40, 1965, 305. Upon enzymes and their action, the most 

 important work is Green's " The Soluble Ferments and Fermenta- 

 tion" (Cambridge, 1901). 



