6 Bulletin of the University of Texas 



regions are characterized by small leaves, which do not permit 

 excessive losses of water, while trees growing in warm ; moist 

 localities bear numerous large leaves. Some trees bear leaves 

 which are reduced to mere scales, iand in some plants, as the cactus, 

 -the leaves have entirely disappeared, or have been reduced to 

 small outgrowths known as spines. This is to be explained on the 

 basis of water loss. Such plants grow natively in arid places, 

 where they are fitted by nature to live, and where plants provided 

 with structures which would not insure them against excessive 

 transpiration, could not survive. The distribution of trees is de- 

 termined largely by their adaptation to water, and this is in 

 turn determined by the root and leaf characteristics. 



The third function of the leaf is respiration or breathing. 

 Respiration is a very important vital process which takes place 

 in all living parts of the tree, and is not confined to the leaver 

 alone. Unlike photosynthesis, it does not depend upon light. The 

 gas consumed in respiration is oxygen, and the gas set free is 

 carbon dioxide. It will be seen, therefore, that the plant breathes- 

 in exactly the same way as animals, consuming oxygen and liber- 

 ating carbon dioxide. Respiration is the process in which energy r 

 locked up in foods, is released. It is closely akin to combustion r 

 but the two processes are not identical. The radiant energy from 

 Uie sun enables the green leaf, in the process of photosynthesis, 

 to decompose the simple inorganic compounds of carbon dioxide- 

 and water, and to recombine them into foods which represent 

 a certain amount of stored or potential energy. Respiration 

 breaks down the products of these foods and sets the energy free. 

 Life involves always and continually these transformations of 

 energy. The sun is the source of all life, for it is the source of 

 energy. Photosynthesis utilizes carbon dioxide, and releases 

 oxygen, thus keeping the air pure for breathing purposes. Res- 

 piration uses oxygen, liberates carbon dioxide, and sets energy 

 free, and thus makes possible the vital phenomena requiring en- 

 ergy. 



The roots of trees constitute the organs of anchorage and ab- 

 sorption. They anchor the tree firmly in the soil and absorb the 

 water and mineral solutions from the soil. The roots are of two* 

 kinds, surface and tap, depending on their shape and depth of 

 penetration. It may seem surprising that the roots are able to- 



