98 



LEAVES AND FOLIAGE 



142. Different forms of leaves from one 

 plant of ampelopsis. 



the leaves grow, it is not known that all forms and shapes 

 have been developed to adapt the plant to its environment. 



It is probable that the 

 toothing or lobing of the 

 leaf -margins is due to the 

 same causes which produce 

 compounding or branching 

 of leaves, but what these 

 causes are is not known. 

 It has been suggested that 

 leaves have become com- 

 pound in order to increase 

 their surface and thereby 

 to offer a greater exposure 

 to light in shady places, 

 but very many sun -loving 

 species have compound 

 leaves, and many shade-loving species have simple and 

 even small leaves. Again, it has been suggested that com- 

 pound leaves shade underlying leaves less than simple 

 leaves do. 



207. HOW TO TELL A LEAF.-It is often difficult to dis- 

 tinguish compound leaves from leafy branches and leaflets 

 from leaves. As a rule, leaves can be told by the follow- 

 ing tests: (1) Leaves are temporary structures, sooner 

 or later falling- (2) Usu- 

 ally buds are borne in 

 their axils. (3) Leaves are 

 usually borne at joints or 

 nodes. (4) They arise on 

 wood of the current-year's 

 growth. (5) They have a 

 more or less definite arrangement. When leaves fall, the 

 twig which bore them remains; when leaflets fall, the main 

 petiole which bore them falls also. 



143. Muskmelon seedlings, with the un- 

 like seed-leaves and true leaves. 





