[Chap. X 



HEREDITARY DIFFERENCES IN LEAVES 



85 



time consider how heritable variations occur, but Hmit our attention to 

 a few types of them in leaves. 



Size and form. The smallest flowering plant in North America is a 

 little globular duckweed ( Wolffia ) about 2 millimeters in diameter, with- 

 out distinct root, stem, or leaf (Fig. 11). The opposite extreme is repre- 

 sented by palms with leaves 20 feet long. The native fern from which 

 the Boston fern was derived grows in the Everglades region of southern 

 Florida and has been found with leaves over 20 feet in length. Pines have 

 needle leaves which in some species may be more than a foot long; 

 structurally they are quite different from the leaves thus far described 

 ( Fig. 324 ) . Iris is readily recognized by its sword-like vertical blades and 

 the tightly folded sheath enclosing the stem and younger leaf bases. 

 The cactus (Opuntia) has small temporary succulent leaves (Fig. 43). 

 Victoria regia of the Amazon River, the largest of the water lilies, may 

 have floating blades 5 feet in diameter with petioles 2 inches thick and 



Fig. 43. The cactus {Opuntia) in which the younger stem segments and fruits bear 

 small, awl-shaped, succulent leaves. Photo by G. S. Crowl. 



