76 



A TEXTBOOK OF BOTANY [Cn. Ill, 9 



their chlorenchyma, and the changes are chiefly in form. 

 Thus our native Pitcher Plant, or Sarracenia (Fig. 48), seems 

 to represent a leaf in which the margin has grown up around 

 a central-standing petiole, forming as it were first a saucer, 

 then a cup, and finally a pitcher. In the Nepenthes, most 



elaborate of Pitcher Plants (Fig. 49), 

 there occurs a partial division of 

 labor between the pitcher and foli- 

 age functions, for a very perfect 

 blade exists in addition to the 

 pitcher. Doubt still exists as to 

 the precise morphology of the parts 

 in this remarkable leaf, though it 

 seems most probable that the pitcher 

 represents a blade transformed as in 

 Sarracenia, with the lid a special 

 outgrowth and the seeming blade an 

 expansion of the elongated petiole, 

 which often serves also as a tendril. 

 But we must guard against push- 

 ing such homologies too far, be- 

 cause leaves and other parts, while 

 strongly influenced in development 

 by the characteristics of the part 

 from which they have evolved, are 

 by no means limited to the charac- 

 teristics thereof, but often break loose, as it were, and develop 

 new features upon their own account. In another well-known 

 insect-trapping leaf, that of the Venus Fly-trap (Fig. 50), the 

 morphology is obvious, the petiole becoming expanded much 

 like the blade. 



Another function performed by leaves is that of support 

 to climbing plants, in which case they form TENDRILS, which 

 are characteristic organs of most vines. Tendrils are very 

 slender almost thread-like structures, fitted to twine around 

 supports, to which they thus attach their plants. In the 



FIG. 49. Nepenthes, an 

 East Indian Pitcher Plant ; 

 X . The slender stalk be- 

 tween blade and pitcher 

 often serves as, a tendril. 



(From Le Maout and 

 Decaisne, Traite General de 

 Botanique.) 



