46 THE RUBBER TREE BOOK 



perforated by a large number of minute holes or pores, which 

 permit free access of air into the inside of the leaves. A thin 

 film of water is always present on the surface of the cells inside 

 the leaves, and this dissolves the carbonic acid gas, which is then 

 absorbed into the cells and acted upon. 



The cells of the leaves, especially the upper cells, contain 

 small green bodies which are termed chloroplasts. These 

 chloroplasts are able to use the energy of the sunlight to supply 

 the chemical energy necessary to bring into combination the 

 elements that go to make starches and sugars. Though the 

 mineral salts brought up from the roots do not enter into the 

 composition of starches and sugars, certain of them must be 

 present to play a part in their manufacture. When darkness 

 falls the leaves stop working, just like a factory in which the 

 engines have been stopped, and, therefore, operations must 

 cease. It is only during daylight, and especially when the sun 

 is shining, that the manufacture of the simpler plant-foods from 

 carbonic acid gas can proceed. When there is an absence of 

 light, such as is caused by close-planting, leaves which are always 

 in shade cannot perform their work ; they die off and fall away. 

 The tree is thus something like a steamship which, while it has 

 a dozen furnaces and boilers, is reduced to working with two or 

 three, and can only proceed at quarter-speed. 



The chloroplasts are, as yet, but little understood, and the 

 marvels they perform are still something of a mystery. They 

 seem to bear some analogy to the bacteria which work so 

 actively in the soil to supply the roots with nitrogen in the 

 form of salts in solution, and also in some degree in the form of 

 ammonia. For the growth and nourishment of the tree there 

 are thus active processes, little comprehended, yet working in 

 combination. 



One more fact should be noted. It is a common error to 

 imagine that plants do not breathe in the way that animals 

 absorb or inhale oxygen and give out or exhale carbonic acid 

 gas. This process of " breathing " should not, however, be 

 confused with the exchange of gases in carbon assimilation, in 

 which it is carbonic acid gas that is absorbed and oxygen that 

 is given out. During the day the latter process swamps the 

 former, and any oxygen required in the leaf is obtained from 

 that set free in the leaf, while the carbonic acid gas produced is 



