CHAPTEE VII. 



The leap with the entire edge is alone to be ee- 



GAEDED AS A SIMPLE LEAF — THE LEAP TAKES A HIGHEB 

 POEM OP OEGANIZATION, AND BECOMES COMPOUND IN 

 PEOPOETION TO THE DEVELOPMENT OP THE PIBEOUS POR- 

 TION OP ITS LAMINA — ALL THE lEREGULAEITIES OP ITS 

 MAEGIN, SUCH AS LOBES, TEETH, CEENATUEES, SEEBA- 

 TUEES, RESULT FEOM AN EFPORT AT NEW LEAFLET- 

 FORMATION AERESTED IS ITS FIRST STAGES. 



The commonest leaf-form in tlie vegetable world, wliich 

 must therefore be regarded as tbe fundamental type or normal 

 form, consists of two parts — a stalk or support, called tbe 

 petiole, and an expanded part called the lamina or blade. To 

 such organs the term leaf was formerly restricted. But it is 

 now admitted by all physiologLsts, that this term is equally 

 applicable to all the organs of the periphery of the plant 

 . which develop laterally during the growth of .the axis, and 

 which are distinguished from the ordinary stem leaves by the 

 use of the terms bud-scales, stipules, bracts, sepals, petals, 

 stamens, and pistils. All these different varieties of leaf-form 

 pass by intermediate gradations into one another, so that this 

 classification is wholly arbitrary, and the terms are merely 

 used as convenient references to those varieties which are the 

 most decidedly marked. Now, what is the evidence on which 

 this doctrine rests? Is it not transition forms? Does not 

 the whole science of Vegetable Morphology rest on this 

 foundation? The following facts presented in this chapter 

 are only the fruit of a careful application of precisely the 

 same principles of research. Transition forms show ties of 

 relationship subsisting between the most dissimilar organs 

 and often lead to conclusions at which Naturalists themselves 



(107) 



