142 TIMBERS AND THBIE USES 



A word or two concerning plant physiology- 

 may lead to a better understanding of these 

 harmful parasites. In the case of ordinary 

 green plants, the hosts of the parasites with 

 which we are concerned, all food substances, 

 with the exception of carbon, are absorbed 

 through the roots in the form of watery solu- 

 tions. Carbon is taken up by the leaves in the 

 form of the gaseous compound carbon dioxide. 

 Without proceeding further it is clear that the 

 roots and leaves are most important plant 

 organs. Actual root absorption takes place 

 through the root hairs, very fine threadlike roots 

 with thin walls which occur just behind the 

 growing points of the root branches. From the 

 root hairs the water with its dissolved food 

 passes to the roots proper and enters the water- 

 conducting system which we have described else- 

 where. From the finest roots, through the stem, 

 to the extreme leaves this liquid food passes. 

 In the leaves starch and sugar is built up from 

 this liquid food, and from the carbon of the 

 carbon dioxide taken up from the air, at the 

 same time, a great deal of the water which has 

 served its purpose as a vehicle for the transport 

 of the food passes off into the air. A fair-sized 

 tree wiU give off more than 150 lbs. of water a 

 day. The . food manufactured in the leaves 

 passes by other channels to the growing points 



