88 A TEXT-BOOK OF BOTANY 



which it is stored. Accordingly it must be changed into a 

 soluble form; and this work is commonly done by a sub- 

 stance called an enzyme, which is produced by the living 

 substance (protoplasm) of the cell. There are numerous 

 enzymes, which act upon different substances; but the one 

 most frequently found in seeds is that called diastase, which 

 has the power of converting starch into one of the soluble 

 sugars. This process of converting insoluble food into a 

 soluble form is digestion, and in ordinary seeds the starch is 

 digested and becomes sugar. All of this work preparatory 

 to growth accounts for the activity noted in the two preced- 

 ing sections. The food being in the form of a soluble sugar 

 can leave the storage cells and pass to the regions where 

 growth occurs. 



48. Assimilation. In a germinating seed the soluble 

 sugar produced by digestion passes in solution from cell to 

 cell, according to the laws of osmosis, until it reaches cells 

 where growth is taking place; that is, where the protoplasm 

 is forming new cells by dividing those already formed, and 

 enlarging the new ones until each one is as large as the 

 cell of which it was a division. This cell division and cell 

 growth are going on very actively in the hypocotyl and 

 plumule of the germinating seed; and when the sugar in 

 solution reaches the active cells, it is used in building up the 

 active protoplasm, which is being broken down by its 

 activity. This transformation of food into protoplasm, 

 by numerous intermediate steps, is assimilation. 



49. Proteids. Thus far we have considered only carbo- 

 hydrate foods, but in building up protoplasm the carbo- 

 hydrates are first used in the manufacture of proteids. 

 Just how proteids are formed is very uncertain, but they 

 are more complex than carbohydrates; and in addition to 

 the carbon, hydrogen, and oxygen of the carbohydrates, 

 proteids contain other elements, notable among which are 

 nitrogen, sulphur, and phosphorus, and these enter the 



