ss 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, 1s 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 
