84 BOTANY 



more is made than is needed for immediate use. The 

 surplus is immediately deposited in the chloroplasts in 

 the form of grains of starch. Starch is another carbo- 

 hydrate which very readily becomes transformed into 

 sugar by the action of dilute mineral acids or of a 

 peculiar body known as diastase, which is a digestive 

 ferment or enzyme. Probably an excess of protein is 

 also made while the conditions are favourable. 



The cells in which these foodstuffs are made have to 

 manufacture sufficient food not for themselves only, but 

 for the whole plant, many of whose cells, as we have 

 seen, are set apart for other purposes altogether. It is 

 necessary accordingly for the food to be made in large 

 quantities and for the surplus to be transported from 

 the cells in which it is made to the other cells of the 

 organism. Part of it is devoted to the nutrition and 

 growth of the body of the plant, and a large amount is 

 stored in various places to nourish the reproductive organs. 



There is consequently a continuous movement of the 

 manufactured food substances about the plant. The 

 sugar temporarily deposited as starch grains in the 

 chloroplasts is taken away as soon as darkness falls. 

 The ferment or enzyme diastase, which is present in the 

 cells, converts the starch into another form of sugar, 

 which diffuses outwards. A stream of such sugar is thus 

 leaving the leaf during the night and passing to other 

 cells of the plant, either to be consumed in the growth 

 processes or to be stored up till wanted. Probably this 

 stream of sugar is passing also during the day, but while 

 the manufacture is going on it is not easy to detect it. 



When proteins leave the cell they are first converted 

 by a similar ferment into the amides we have spoken of 

 and pass through the plant's tissues as such. 



Sugar and either proteins or amides are taken up by the 

 living protoplasm and incorporated into its substance. 

 This is the true assimilation, or nutrition of the plant. 



