The Trees of Texas 3 
function in nature depends upon sunlight for its realization, we 
might well expect to find provision in the arrangement of leaves 
to facilitate an optimum light exposure. 
This expectation is borne out by the facts, and can be easily 
verified by an examination of the leaves of a few of our native 
trees with this point in mind. One ean not fail to be impressed 
with the wonderful adjustment by which each leaf on the tree 
is so placed as to bring it into just the proper exposure to light. 
Obviously, one of the great difficulties to be overcome is the 
danger of shading of the lower leaves by those growing above 
them. The leaf is in its best hght relation, on most plants, when 
its blade is exposed at an angle of about ninety degrees, to the 
source of light and is free from the danger of being shaded. The 
exact arrangement of leaves on the stem is a constant character- 
istic for any plant. In some species the leaves are borne in pairs, 
one leaf attached on the shoot opposite the other. In this man- 
ner of attachment the pairs are arranged in such a way that 
when looking down on the stem from above there can be seen 
four distinct rows of leaves separated from each other by greater 
or less distance, depending upon whether the leaf blade is narrow 
or wide. In the great majority of trees the leaves are attached 
singly, and not opposite. This arrangement is called the alter- 
nate arrangement, and they are spaced in definite cyclic order 
upon the shoot in such a manner as to afford sufficient space be- 
tween the lower leaves and the ones growing above them. 
The blades of many leaves are variously cut, toothed, notched, 
lobed or compounded, and through these openings the light is 
enabled to filter down to the lower parts of the tree. It is not 
to be expected that every leaf on a tree can be so placed that it 
will be in its optimum light relationship at every minute in the 
day. This would only be possible if the leaf were able to swing 
about on its petiole and follow the daily march of the sun. When 
the leaf has reached maturity, its position is relatively fixed ana 
its position with reference to light has been determined by the 
direction from which it was illuminated during the greater part 
of the day as it developed. It is, therefore, in the shadow during 
a part of the time only. Some plants require much more light 
than others, and many find their best conditions for growth in the 
subdued light of our deep forests. The shade loving habit of 
