UPON WBAT DOES A PLANT LIVE? 423 



and oxygen; and frdm this invisible source the 

 plant derives its carbon, which makes up nearly 

 half its entire weight, aside from the water. 



541. The materials taken in by the roots and 

 leaves cannot be used directly in the making of 

 plant tissue and in contributing 

 to growth. They must be 

 worked over, or formed into 

 organic compounds. This pro- 

 cess of elaboration takes place 

 in the green parts, chiefly in 

 the leaves, and in the presence 

 of sunlight. Plants destitute of 

 green, or chlorophyll, mostly use 

 food which is already organized : 

 they are parasites and sapro- 

 phytes. The withdrawal of sun- 

 light, as in the banking-up 

 of celery, destroys the chloro- 



Tmnk girdled by a wire. 



phyll. 



542. When the food has been elaborated, it can 

 be utilized, through further changes, for the build- 

 ing of tissue, and it is distributed throughout all 

 living parts of the plant, even to the roots whence 

 part of it came. The first process of changing in- 

 organic materials into organic materials — or pho- 

 tosynthesis, —r takes place only in light ; but the 

 transfer and subsequent use of the elaborated food 



