128 



LEAVES AND THEIR WORK 



of water to the tops of very tall trees. Experiments show that evapora- 

 tion of water through the stomata exerts a lifting power upon the fluids 

 within the stem of the tree, thus aiding in the raising of water to the leaves 

 in the upper branches. 



Diagrams of a stoma : a, surface view of an opened stoma ; 6, same stoma closed 

 (after Hansen) ; o, diagram of a transverse section through a stoma — dotted 

 lines indicate the closed position of the guard cells, the heavy lines the open 

 condition. (After Schwendener.) 



Y. 



Respiration by Leaves. — All living things require oxygen. It 



is by nieans~of the oxidation of food materials within the plant's 

 body that the energy used in growth and movement is released. A 

 plant takes in oxygen largely through the stomata of the leaves, 

 to a less extent through the lenticels in the stem, and through 

 the roots. Thus rapidly growing tissues receive the oxygen neces- 

 sary for them to perform their work. The products of oxidation 

 in the form of carbon dioxide are also passed off through these 

 same organs. It can be shown by experiment that a plant uses up 

 oxygen in the darkness ; in the light the amount of oxygen given 

 off as a by-product in the process of starch-making is, of coin-se, 

 much greater than the amount used by the plant. 



Summary. — From the above paragraphs it is seen that a leaf 

 performs the following functions : (1) breathing, or the taking in 

 of oxygen and passing off of carbon dioxide; (2) starch-making, 

 with the incidental passing out of oxygen; (3) formation of proteids, 

 with their digestion and assimilation to form new tissues; and (4) the 

 transpiration of water. 



Economic Uses of Leaves. — The practical use of green plants to 

 man is very great. Plants give off oxygen in the sunlight and use 

 carbon dioxide, which is given off by animals in the breath. We 

 should remember, as taxpayers, that money invested in public parks 

 is money well invested, bringing as it does a source of oxygen supply 

 where it is most needed, in the congested parts of our great cities. 



