p 



On the Growth of Leaves. 277 



The growth of leaves by the expansion of the cells is subject to 

 iyarious modifications. 



■ In the Algae the expansion does not usually begin until the 

 growth by development is complete. It then commences in the 

 uppermost cell and extends gradually to the base. In those 

 branched filaments where the cells break up {Poly sip honia, &c.) 

 the uppermost cell falls ofi" first, and the process extends down- 

 ward to the others in succession. There are some few exceptions to 

 this rule, where the expansion of the cells is simultaneous, or even 

 begins in certain other parts instead of the apex. Those Algse 

 however which consist of single branched cells must of course be 

 excepted from the rule, as the expansion is the extension of one 

 dividual cell, and that proceeds from below upward. 

 In the Mosses and Hepaticce the expansion commences, after 

 e completion of development, at the apex and extends gra- 

 ally downwards. In Characece the terminal cells expand first, 

 the Lycopodiacea the expansion also proceeds from the apex 

 the base. 



In the Phanerogamia as a general rule both in simple and 

 mpound leaves, the expansion commences at the summit, but 

 is rule is not without exception. In some leaves the expansion 

 tolerably simultaneous, while in others, as in Utricularia, it 

 tends like the development from below upward. There does 

 t appear to be any rule for the expansion of the petiole of com- 

 ound leaves. 



Drawing the conclusion as to the origin of the leaf in the 

 igher classes from analogy, Nageli propounds the following 

 rmulse as the expression of his views : — 



1. The leaf originates as a simple cell. 



2. The growth by cell-formation occurs at the apex and on the 

 order, and proceeds, from the base, upwards and outwards. 



3. The growth by the expansion of the cells begins, on the con- 

 ary, at the apex and extends to the base. 



Now these formulae look remarkably definite and clear, and if 

 re could receive them, our knowledge of these structures would 



)e much simplified ; but unfortunately, although the laws of de- 



irelopment are simple, fundamentally, they are subject to innu- 

 lerable modifications in their application, and I cannot think 

 lat Nageli has taken all the conditions of leaves into consider- 



lltion, and I believe therefore that he has generalized much too 



"reely. 



In the first place it is a question whether the fronds of the 

 Llgse are always the analogues of leaves : if we have leaves in 



them, we also have leaves and stems, and probably often stems 



