106 SHADE-TREES IN TOWNS AND CITIES 



minerals taken up by the roots, and helps to convey these 

 nutrient substances of the soil into the tree body. A large 

 quantity of the water taken up by the roots passes through 

 the tree merely as a medium for the transport of nourish- 

 ment, and is again discharged through the leaves by evap- 

 oration. This evaporation of water through the leaves is 

 called transpiration. 



The watery fluid absorbed by the roots is carried by the 

 transpiration current to the leaves. These in turn take up 

 the carbonic acid from the air, and under the action of sun- 

 light the carbonic acid is decomposed, the carbon combined 

 with the minerals from the soil into food materials used in 

 building up the tree. This process is called assimilation. 

 The leaves, therefore, perform a very vital function in the 

 life of the tree, and it is evident that defoliation by insects 

 or other causes will seriously affect its growth. 



Besides, trees, like animals, in order to live must have 

 air to breathe, and in this process of respiration they 

 take up oxygen and give off carbonic acid. Respiration 

 and assimilation are two distinct vital processes, carried on 

 independently by trees and other plants. The process of 

 assimilation is carried on only in the light, carbonic acid is 

 decomposed, and oxygen given off. The process of respira- 

 tion is carried on both by day and by night, oxygen is taken 

 up, and carbonic acid given off. Furthermore, not only the 

 leaves but the twigs, the branches, the trunk, and the' roots 

 have breathing pores, and require air for the maintenance 

 of life. 



It will, therefore, be seen how important it is to keep the 

 soil in a state of culture, and to see that the supply of air is 

 not cut off from the roots by pavements, by filling in around 

 the base, or by flooding of the roots. 



