STUDIES OF SEED-PLANTS 189 



requires (1) great expanse of surface, which is secured by the 

 broad blades ; and (2) arrangement so that they will shade 

 each other as little as possible. This latter point deserves 

 some special, study. 



182. Arrangement of Leaves. One of the most im- 

 pressive points about leaves is their position on the branches 

 of plants. It would be a careless observer indeed who does 

 not notice that the leaves are commonly arranged in rows, 

 but perhaps only the student of botany observes that there 

 may be two, three, four, five and even more rows. This has 

 been already indicated in the study of twigs, on which the 

 buds mark the positions of future leaves ( 156). The 

 student should look at plants having different numbers of 

 rows, viewing the main stem from above, and note how the 

 number of rows- affects the shading of leaves by each other. 



The length of the internodes of stems affects light exposure 

 by separating the leaves. If the leaves are close together, 

 it is evident that they will tend to shade each other. 



The size of leaves and the number of leaves have a definite 

 relation to the arrangement in many plants. Some plants 

 have a few large leaves, while others have many small leaves. 

 If possible, contrast such trees as catalpa and willow, or 

 hickory and birch, looking at small trees of these species from 

 above and at large trees from below. 



Varying length of branches of the stem and of petioles of most 

 leaves of trees aid in making the best adjustment. Numerous 

 plants illustrate this. The lower branches of most trees are 

 longest and so hold their leaves out of the shadow of the upper 

 branches ; and also some of the leaves have long petioles so 

 that their blades avoid shading by leaves above. 



Twisting of stems and petioles brings some leaves into the 

 best position. This is most strikingly illustrated by the 

 horizontal branches as contrasted with vertical branches on 

 the main stem of the same trees. 



Most leaves face the source of light, as may be seen in plants 



