+ Bulletin of the Unwerstty of Texas 
the ferns, and of many herbs and shrubs, gives us the charac- 
teristic ground plants or undergrowth of the woodlands. Trees 
erowing 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 
erowth of the stem, under the stimulus of hight 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 
