21 



In gene'('al densit" of arrangemeiit is dependent on size and 

 shape of leaves. The larger th-? leaves the fewer numljer, the 

 smaller the leaves the greater number. 



Kerner savs numher and size of leaves are related to phyliotaxy, 

 hut Dr. Cowles considers this not absolutely true. That the 

 arrangement of leaves on the tree is dependent upon widely different 

 causes. The mint faraily. 



In Maple this difficulty may be i_:otten over "by the length of 

 petiole and its twisting. The question of number of rows is 

 comparatively unimportant with regard to that of light. \fiien 

 plants have many vertical ^j^^/j^/ rows of large leaves, .the petiole 

 may get shorter, the leaves smaller toward the top, and wide space 

 between leaves. 



In most plants there is a tendency toward the "mosaic arrange- 

 ment". This reaches its highest development when shape of leaves 

 are modified to fit into each other. Hackberrjr, begonia, etc. 

 Another case of mosaic arrangement is fitting angular leaves into 

 each other, as Ivy. Still another fitt/ing smaller into larger leaves, 



Reading in Atkinson, Chapters X.and XI., pages 13-58. 

 Field lesson, July 16, 1900. 

 Petioles . As a rule monocotyl/s and gymnosperms have no petioles. 

 Dicotyls and ferns have. Exceptions goldenrods and asters. General 

 shape of the petiole flattened, or grooved ) rounded only in a few, 

 such as the cucumber and squash. Petioles are more apt to be 

 colored than leaf blades. In cottonwood the petiole is flattened 

 vertically giving; the opportun|ty for free movement. 



