264 BAOTEEIOLOGICAL AND ENZYME CHEMISTRY 



are acquainted with, has its origin in pre-existent life, but the 

 distinction between the changes taking place in the seed, and 

 those which occur in the leaf, is that the former are not 

 directly dependent upon sunhght, unless indirectly, it may 

 be, for warmth. 



The chemical changes taking place have been fully illus- 

 trated in the study of the barley grain, to which Chapters V 

 and VI have been devoted. The seeds of all plants contain 

 in the endosperm a store of reserve material, which has been 

 elaborated by the growing plant. The embryo, as we have 

 seen, has the power of secreting various enzymes, viz., cytase, 

 which breaks down the cell walls of the endosperm, and 

 amylase, which converts the starch into sugar. There are 

 also present proteolytic enzymes, which break down the stored 

 albumin of the seeds, and, in the case of fat-containing seeds, 

 such, e.g., as those of the castor oil plant, lypolytic enzymes 

 are present, which break down the oils or fats. All these 

 changes, it may be seen, are essentially concerned with the 

 breaking down of material already elaborated, i.e., they 

 are what is known as cataboUc ; unless fresh nutriment is 

 suppHed, on the one hand, and fresh energy on the other 

 hand, growth will cease. Nutriment is supphed to the 

 plant, as abeady stated, by the roots, and by the leaves ; 

 energy is supphed by the leaves only, and it is in the leaf cell 

 that we have to look, to find what we may describe as the 

 power house of the plant. 



The chemistry of the leaf cell is one of the most fascinating 

 problems which has occupied the attention of chemists, but 

 in spite of numerous researches by highly quahfied workers, 

 it is still very imperfectly understood. 



The simple beginnings and ends of the process have been 

 known for a long time ; they may be studied without difiiculty, 

 and indeed form one of the subjects of most ' nature study ' 

 classes. The following experiments are easily carried out. 



A portion of American pond weed, Elodea canadensis, is 



