4 Bulletin of the University of Texas 



the ferns, and of many herbs and shrubs, gives us the charac- 

 teristic ground plants or undergrowth of the woodlands. Trees 

 growing in open situations, where they are equally illuminated 

 from all sides, are usually lower, more bushy topped and sym- 

 metrical than members of the same species which grow crowded 

 together in forests. This is to be accounted for by upward 

 growth of the stem, under the stimulus of light from above only. 

 It usually happens, also, that the branches of trees growing close 

 together die away from the older portions, resulting in tall naked 

 stems with a crown consisting of a few branches at the top. 



The second important function of leaves is transpiration, or 

 the evaporation of water. The life of the tree depends upon 

 the ability of the leaf to perform this function properly. The 

 amount of water thrown off depends upon the structure of the 

 leaf itself, and upon the physical environment in which the tree 

 grows. The total amount of water transpired by plants is sur- 

 prisingly great. It has been estimated that an oak tree with 

 seven hundred thousand leaves would give off 244,695 pounds of 

 water from June to October. A birch tree with 200,000 leaves 

 transpired 700 to 900 gallons per day on hot summer days. An 

 acre of such trees would, therefore, transpire 3,168,000 pounds 

 of water in a single season. It is estimated that 200 to 500 pounds 

 of water is transpired in the manufacture of one pound of dry 

 substance. This accounts for the cooling and refreshing effect 

 which vegetation exerts on the atmosphere. It is a well known 

 fact that plants die if they are unable for any reason to take 

 in a supply of water from the roots, sufficient TO meet the 

 enormous and continual drain from the leaves. This accounts 

 for the disastrous effects which hot, dry winds exert on many 

 species of plants, and raises the question whether such quantities 

 of water >as are ordinarily thrown off are an advantage or a dis- 

 advantage to the tree. The opinion is now widely held by bot- 

 anists, that much greater quantities of water are thrown off by 

 plants than is necessary for the carrying on of their vital activi- 

 ties, and that such great water loss is due to the fact that plants 

 have not fully solved the problem of conserving water. In order 

 for the leaf to carry on the process of photosynthesis, it must of 

 necessity be provided with some means which will permit the 

 entrance of air containing the necessary carbon dioxide, and the 



