2 48 TEXTBOOK OF BOTANY 



thus the special work of the leaf -cells. They also take an 

 important part in the building up of higher compounds, 

 including proteins and living matter, out of the carbohy- 

 drates and the food substances derived from the soil. But 

 this latter work is not peculiar to the cells of the leaf ; it is 

 probably going on more or less actively all the time, in all 

 the living cells of the plant. 



260. Storage of Food in Leaves. — In general, the busi- 

 ness of leaves is the manufacture of food ; surplus food which 

 is to be kept for a long time is usually stored in other organs 

 of the plant. Nevertheless, there are not a few plants 

 whose leaves are important storage organs. Leaves in 

 which any considerable amount of food is stored are likely 

 to be thicker than those which merely manufacture food and 

 pass it on to other organs. The very thick leaves of the 

 century plant, as we have seen, serve for the storage of water ; 

 they contain also other food substances, some of which are 

 dissolved in the water. The bulb scales of lilies and onions 

 are examples of underground leaves that are important 

 storage organs ; in the case of the onions, we profit by the 

 provision which the plant has made for its own future needs. 



261. Respiration. — This is another kind of work in 

 which leaves play a large part, although it goes on in all parts 

 of the plant where there are living cells. The importance 

 of leaves in this process results from the fact that they expose 

 a larger surface to the air and so can take in the oxygen that 

 is needed for respiration more rapidly than can the other 

 organs of the plant. The oxygen that is taken in by the 

 living cells of a plant, whether it comes directly from the 

 air as in the case of most seed plants, or from the water in 

 case the plant is submerged, must pass through the cell 

 walls in solution in water, just as do all other substances 

 soKd, liquid, or gaseous (excepting water itself), that enter or 

 pass out of cells. Not all the oxygen is necessarily used in 

 the cell that first takes it in ; it may be passed on to cells 



