238 VEGETABLE ORGANOGRAPHY. 



leaf. In cylindrical leaves, as those of Mesembryanthe- 

 muin calamiforme, neither the edges nor the surfaces can 

 be distinguished. Lastly, there are some leaves so folded 

 upon themselves, that all their external part is formed 

 by the lower surface ; such are those of the Iris, which 

 are called Ensiform ; they seem flat like ordinary leaves, 

 but compressed laterally, instead of being depressed; 

 they are in reality folded one half upon the other ; their 

 lower angle is the middle nerve, the upper one is the 

 junction of the two edges. 



The edges of leaves present some peculiarities worthy 

 of notice ; they are frequently indurated : when they 

 bear hairs, these are generally stiifer than ordinarily, and 

 have received the name of Cilias ; these ciliae indicate the 

 existence of a marginal nerve. Sometimes, especially in 

 leaves with ramifying nerves, the ramifications are more 

 or less directed towards the edges, and their extremity 

 frequently presents a point or small tubercle, from which, 

 in favourable cases, may arise either roots, or even young 

 individuals, as in Bryopliyllum Calycinum. It is also 

 from points analogous as to their position, that the mar- 

 ginal spines are sometimes prolonged. But these facts 

 can only become clear after we have examined the 

 disposition of the n'irves. 



Section II. 



Of the Distinction between the Petiole and the Limb of 



Leaves. 



The Petiole, or, as it is commonly called, the stalk of 

 the leaf, is not the prolongation of the limb, as the popular 

 terra would seem to indicate, but, on the contrary, the 

 base or support of the limb, the as yet unexpanded 



