The Tre-es 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 can 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 light 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 sucn 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 



