110 DEVELOPMENT OF COMPOUND 



of growth is at its maximum, and that they take a simpler 

 form, their fibrous portion or framework being less developed, 

 as the vegetative force diminishes in intensity, or the wave 

 of growth is gradually depressed along the axis of the shoot. 

 Hence, this variability in the form of the leaves on such 

 shoots and branches as are purely vegetative, arises from the 

 operation of the same common laws of accelerated and re- 

 tarded growth which pervades the branches. 



On some trees, such as the sassafras and paper mulberry, 

 with polymorphous foliage, if we collect and .compare the 

 different varieties of leaf we shall find that they invariably 

 take a higher form of organization in proportion to the 

 development of the fibrous portion of their lamina. The 

 truly simple leaf is in fact the one with an entire edge, and 

 all other varieties of leaf having irregularities of margin, such 

 as lobes, dentations, crenatures, serratures, must be regarded as 

 leaves in a certain stage of progress to a compound condition, 

 for the above marginal irregularities are in reality an effort 

 at new leaflet formation arrested in its first stages. 



We commence this demonstration with the case of lobed 

 leaves, which is the most obvious. 



The lobed variety of leaf is caused by two or more partially 

 formed leaflets, which are organically united with each other 

 and develop in the same plane. The upper part of the lamina 

 of these leaflets is the only portion of them which is fully 

 formed, and it is this which gives to such leaves their lobed 

 appearance. 



For let the polymorphous foliage of the common blackberry 

 (Rubus villosus), see Plate III., be carefully studied, and a com- 

 parison instituted between the different varieties of its leaves. 

 Every intermediate grade of development may be traced 

 between lobes and leaflets, the various vegetative stages in 

 the passage of the former into the latter having been left 

 visible in the leaves of this plant. We have monographed 

 a few of the more prominent forms to show their connection, 

 and give the reader an idea of our method of observing 

 Nature. But all the intermediate varieties can be collected, 

 and thus the truth of the above proposition may be verified. 



