CHAPTER VII. 



THE LEAF WITH THE ENTIRE EDGE IS ALONE TO BE RE- 

 GARDED AS A SIMPLE LEAF THE LEAF TAKES A HIGHER 

 FORM OF ORGANIZATION, AND BECOMES COMPOUND IN 

 PROPORTION TO THE DEVELOPMENT OF THE FIBROUS POR- 

 TION OF ITS LAMINA ALL THE IRREGULARITIES OF ITS 

 MARGIN, SUCH AS LOBES, TEETH, CRENATURES, SERRA- 

 TURES, RESULT FROM AN EFFORT AT NEW LEAFLET- 

 FORMATION ARRESTED IN ITS FIRST STAGES. 



THE commonest leaf-form in the vegetable world, which 

 must therefore be regarded as the fundamental type or normal 

 form, consists of two parts a stalk or support, called the 

 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 physiologists, 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) 



