THE RESPIRATION 'OF PLANTS 85 



The foodstuffs that are stored instead of being 

 directly consumed, undergo a transformation which is 

 the opposite of that to which the enzymes give rise. 

 The sugar is converted again into starch, a process 

 carried out by certain plastids much like chloroplasts, 

 but without any green pigment. These leucoplasts, as 

 they are called, are present in the cells in which the 

 storage takes place. The amides are built up again into 

 proteins and deposited in the cells. In seeds they appear 

 as peculiar grains of protein matter, which have long been 

 known under the name of aleurone grains. They are formed 

 by the protoplasm of the cell and not by any plastid. 



The transport of these streams of food material is 

 effected chiefly by those soft parts of the vascular 

 strands of which we have spoken as bast or phloem. 



CHAPTER X 



THE RESPIRATION OF PLANTS 



IT is a matter of common experience with us all that a 

 certain process called breathing must take place. We 

 know that we are continually taking air into our bodies 

 and passing it out again. What we are not perhaps 

 aware of is that the air we give out differs in two im- 

 portant particulars from that which we take in it has 

 gained some carbon dioxide and it has lost some oxygen. 

 What is true of ourselves is true also of plants. 

 During the whole time of their lives they are absorbing 

 oxygen and giving out carbon dioxide, two processes 

 which constitute the beginning and the end of another 

 very complex internal one which is known as respiration. 

 So long as active life lasts this interchange of gases can 

 be detected by appropriate methods, though it is 

 observed with difficulty during the daytime on account 

 of the absorption of carbon dioxide and liberation of 



