62 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 8 



small size of the leaves of Mosses, despite their occurrence in 

 protected places, seems structurally determined by the very 

 imperfect water-conducting system of those plants. The com- 

 pounding, with the consequent small leaflets, of our under- 

 growth Ferns seems probably an 

 hereditary survival from tree-like 

 ancestors. And other minor factors 

 enter into these problems. 



In shapes, leaves are equally 

 diverse, seeming -to defy classifica- 

 tion. Yet comparative study re- 

 duces them to modifications and 

 combinations of three primary forms, 

 which are the orbicular, linear, and 

 ovate. 



Orbicular leaves are well typified 

 by the Garden Nasturtium (Fig. 33), 

 with its nearly circular blade and 

 central-standing vertical petiole from 

 which the veins radiate to the mar- 

 gin, giving off a network of veinlets. 

 In this leaf the blade is unbroken, 

 but in most others a gap or slit runs 

 from margin to petiole, as illustrated 

 by the Pelargonium (" Geranium"), 

 the difference apparently represent- 

 ing a different mode of evolution 

 from ancestral forms which had mar- 

 ginal petioles. Structurally the orbic- 

 ular form serves best the leaf function, since it combines 

 the most green surface with the least lateral spread, and pro- 

 vides the shortest paths of conduction for water and food 

 through the blade. Orbicular leaves are found oftenest upon 

 low-growing or flat-growing plants, where each blade has 

 room for exposure to light unshaded by its neighbors, as in 

 "stemless" herbs, in creeping vines like Ground Ivy, and in 



Fig. 32. — Bidens Beckii, 

 which grows partly im- 

 mersed in water and bears 

 simple leaves above, and 

 compound leaves below the 

 surface. (After Goebel, 

 Biologische Schilderungen.) 



