SPECIAL FORMS OF LEAVES 



251 



59- Special Forms of Leaves. — It has already been 

 mentioned that the parts of flowers and, often, thorns and 

 tendrils are like leaves in 

 origin. From this point 

 of view, such organs may 

 be described as " special 

 forms of leaves." It 

 seems much better, how- 

 ever, to discuss the parts 

 of flowers in the chapter 

 devoted to flowers than 

 to discuss them under 

 the present heading. 



As to tendrils, it may 

 . be noted that the Vir- 

 ginia creeper is a familiar 

 plant whose tendrils arise 

 as leaves arise, while the 

 tendrils of sweet pea are 

 like leaflets in origin. 

 They show the same sen- 

 sitiveness to contact, and 

 the same general be- 

 havior as a consequence 

 of it, that has already 

 been noted in connection 

 with stem tendrils. In 

 the climbing nasturtium 

 the petiole often behaves like a tendril. 



As to thorns, the barberry is a commonly cultivated 

 shrub whose thorns are leaves as to origin. The thorns of 

 the locust trees are stipules as to origin. 



Fig. 88. — View in a swamp showing the ver- 

 tical arrangement of leaves, which gives the 

 maximum amount of illumination for the 

 vegetation as a whole. 



