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6 Bulletin of the Unwersity 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, and 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 hving parts of the tree, and is not confined to the leaves 
alone. Unlhke 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 earbon dioxide. Respiration is the process in which energy, 
Jocked up in foods, is released. It is closely akin to combustion, 
but the two processes are not identical. The radiant energy from 
the 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 
