286 DEVELOPMENT OF SIMPLE LEAVES. [BOOK i. 



the base of the leaf. The lobes by degrees disappear and the 

 leaf then has only seven slight marginal undulations. At 

 the same time the point where the petiole and blade origin- 

 ally touched, is carried from the periphery to the centre, 

 in consequence of the more considerable development of the 

 lower half of the lamina. 



The edges of a young leaf (even if it becomes perfectly 

 entire), appear to be always more or less undulated or crenated 

 in consequence of the cells being disposed in series, and being 

 also spherical. When the leaf begins to fold itself, there is 

 no trace to be found of teeth or crenels, which afterwards 

 appear on its edges. But when the edges touch, we find pro- 

 jections of variable size on them, which are nothing but 

 nascent teeth or crenels. 



When a simple leaf is lobed or cut deeply at its margin, 

 the incisions are, as I have already stated, manifest from the 

 very beginning. There are, however, some exceptions to this 

 rule; the small lobes of unequally lobed leaves, of Acer 

 campestre for example, do not appear until after the forma- 

 tion of the larger lobes. 



In Liriodendron tulipifera, that part which in the nascent 

 leaf corresponds with the blade, resembles a subulated petiole 

 bent upon the axis. At the time when the stipules almost 

 completely envelope the axis, we find on the inferior surface 

 of the lamina two very delicate lamella, narrowed at the 

 base and continuous with the stipules : these are the two 

 halves of the leafy lamina ; they touch one another above, 

 and form a double straight line which has not as yet any 

 traces of the lobes found there at a later period. At a more 

 advanced stage when the petiole begins to be distinguished 

 from the blade, we find that the edge of the two lamellae is 

 slightly undulated. Later, lastly, the two halves are evi- 

 dently lobed and applied to each other by their internal 

 surface. The subulate body then occupies the place of the 

 medial nervure, and its size has manifestly decreased in pro- 

 portion. 



The young leaf of Acer campestre has at first five crenels, 

 and then five lobes. These lobes are cylindrical, dilated at 

 their base ; below their summit, which is obtuse, we find 



