FROM SIMPLE LEAVES. 109 



species of Solidago, or Golden-rod, and the passage of lobes 

 into crenatures in the Swamp mallow (Hibiscus moscheutos). 



The stem leaves, however, not only vary in form in pro- 

 portion as we pass from the vegetative to the reproductive 

 region, but they vary in the midst of the vegetative region 

 itself. The polymorphous condition of the foliage of the 

 paper mulberry (Brousonettia papyri/era), and the common 

 Sassafras (Sassafras officinalis), is well known. On these trees 

 scarcely any two leaves can be collected which have precisely 

 the same figure. The variability of the foliage in this case 

 cannot arise, as in the former, from the gradual decrease in 

 the vegetative tendencies of the leaves in consequence of the 

 tendency to the exercise of the reproductive function there 

 must be some other cause. 



In Chapter VI. we have shown that the growth of trees 

 may be compared, not to the steady and continuous flow of 

 a stream, but to the rising and falling of a wave ; and that the 

 tree, whilst growing, is in a state of continual oscillation, as it 

 were, between a condition of progress and one of stand-still. 

 Now, the difference of growth amongst the shoots, not only 

 during the same year, but for a succession of years, may be 

 traced to this accelerated and retarded wave of growth, or the 

 ever- varying condition of the vegetative force throughout the 

 season; and the fluctuations of the same wave leave their 

 traces in the foliage of the shoots. In herbaceous annuals 

 and perennials this wave obtains its maximum elevation 

 where the leaves are most developed, and is depressed to a 

 minimum in the flower. The same wave is visible in the 

 leaves of . ligneous perennials or trees, which are generally 

 much smaller toward the bottom of each new shoot, and 

 larger toward its centre and summit. 



Now, when polymorphous leaves appear on such of the 

 shoots and branches of a tree as are purely vegetative, this 

 wave of growth must be taken into consideration in com- 

 paring them with each other. It will be found that the leaves 

 with the greatest number of lobes and other marginal irregu- 

 larities are invariably developed at those points of the shoot 

 where the vegetative force is the greatest, or where the wave 



