480 CONSERVATIVE ORGANS. [liECT. IX. 



We find that it consists of two parts ; the one, a. 

 (see the figure) thin and expanded, and which in 

 common language is named the leaf (folium); the' 

 other b. long, equally thick as broad, and stalk- . 

 like, which is denominated the footstalk or petiole 

 (petiolus). The footstalk and the expansion, 

 however, constitute but one organ or proper leaf, 

 the footstalk being merely a prolongation of the 

 mid-rib c. c. which in this leaf divides the expansion 

 into two equal portions. This is further proved by 

 the fact, that the expansion cannot i>e separated 

 from the footstalk without tearing or cutting ; and 

 that in autumn, when a leaf withers, both parts 

 fall together, the whole leaf separating, on the 

 slightest touch, at the point where the foot- 

 stalk is attached to the branch*. But many of 

 the leaves before us have no prolongation of the 

 mid-rib, thence we conclude that the petiole is not 

 universal. Continuing our examination, we observe 

 that the two surfaces of this leaf are not alike; 

 that one is of a deep green colour and smooth ; 

 the other is a pale green and marked by a num- 

 ber of elevated ridges (costulce), branching off 

 from the mid-rib : the deeper green and smoother 

 surface is always turned upwards or towards 

 the light, and is named the upper disk (pa- 



* The error of Linnaeus's definition of the footstalk is very 

 apparent: " Petiolus, trunci species, adnectens folium, nee 

 " fructificationem.'' PhlL Bol. SI. F. 



