FOLIAGE LEAVES : THE LIGHT-RELATION. 



15 



relation is to be observed, therefoi-c;, bc^twct-^n the length 

 of leaves and their distance apart in the same vertical I'ow. 

 The same kind of relation can be observed in reference 

 to the breadth of leaves, for if leaves are not only short but 

 narrow they can stand very close together. It is thus seen 

 that the length and breadth of leaves, the number of ver- 

 tical rows on the stem, and the distance between the leaves 



Fig. 8. A di-agon-trec, showing narrow leaves extending in all directions, and numer- 

 ous vertical rows. 



of any row, all liave to do with the light-relation and are 

 answers to the problem of shading. 



19. Elongation of the lower petioles. — There is still 

 another common arrangement by which an etreotive light- 

 relation is secured by leaves which are In-oad and placed 

 close together on the stem. In such a case the stalks 

 [petioles) of the lower leaves become longer than those 

 above and thus thrust their blades lieyond the shadow (see 

 Fig. 9). It may be noticed that it is very common to 



