68 STUDY OP COMMON PLANTS. 



therefore, of the general facts of leaf structure and physi- 

 ology is essential to an understanding of some of the most 

 fundamental facts of biological science. 



Beginning with form and position, we have seen that, 

 as a rule, leaves are so constructed and placed as to secure 

 Form and ^^® exposure of a large surface to the air and 

 position. light. The blade of the leaf is raised on a 

 petiole whenever this is necessary to more readily accom- 

 plish the end to be attained. Furthermore, the position of 

 leaves on the stem is such as to aid in securing the great- 

 est exposure. If we inspect a large tree in full foliage, 

 such as a maple or basswood, it will be seen that the leaves 

 are placed so as to result in a minimum of interference 

 with each other. It will also be noticed, as Sir John 

 Lubbock points out, that there is a manifest correlation 

 between the form of the leaves and their arrangement on 

 the branch, so that in many cases it would be a decided 

 disadvantage to replace the leaves of one species by those 

 of another unless the leaf arrangement were changed. 

 Further, an examination of buds that have not yet opened 

 shows that the leaf arrangement is such as to economize 

 space. These two principles, compact disposition in the 

 bud, and a position on the stem that will secure full expos- 

 ure of leaf surface, are the determining factors in the 

 arrangement of leaves.^ 



An examination of the anatomical structure of an ordi- 



1 Incidentally it results that the leaf arrangement of many plants is so 

 definitely fixed that it may he expressed by a mathematical formula. 

 Phyllotaxis, however, as usually presented, is a curious rather than a 

 fruitful study. ' ' We must now acknowledge that there is no general 

 law which can he formulated for the arrangement of the organs on a 

 parent axis ; that, on the contrary, according to circumstances in each 

 case, special causes determine whether the relations of position turn out 

 to be this or that." — Sachs, Physiology of Plants, pp. 500, 501. 



