CHAP. II. LEAVES. 103 



are very small; but, nevertheless, here and tiiere, there are 

 often larger cavities, which either correspond directly with 

 the stomates, or are in communication with them. The same 

 thing happens in plants with floating leaves, where the stomates 

 placed on the upper surface correspond with the layer of the 

 cylindrical and parallel vesicles ; in such case there are, here 

 and there, between these vesicles, empty spaces which almost 

 always correspond to the points where the stomates exist, and 

 which permit the air to penetrate between the vesicles as far 

 as the middle of the parenchyma of the leaf" 



Thus much Adolphe Brongniart; who adds, that in sub- 

 mersed leaves there is no cuticle, but the whole consists of 

 solid parenchyma alone, in which there are no other cavities 

 than such as are necessary to float the leaves. The obser- 

 vations of Mohl and Meyen generally confirm this ; but, at 

 the same time, the latter mentions several cases in which the 

 texture of the leaf has been found to be nearly the same 

 throughout. 



Dutrochet states in addition [Ann. des Sc, xxv. 245.) that 

 the interior of a leaf is divided completely by a number of par- 

 titions, covered by the ribs and principal veins, so that the air 

 cavities have not actually a free communication in every 

 direction through the parenchyma ; but are, to a certain 

 extent, cut off" from each other. This is conformable to what 

 Mirbel has described in Marchantia, where the leafy expansions 

 are separated by partitions into chambers, between which, he 

 is of opinion, there is no other communication than what 

 results from the permeability of the tissue. 



The veins, being elongations of the medullary sheath, neces- 

 sarily consist of woody tissue and spiral vessels, to which are 

 sometimes added annular ducts. In submersed leaves spiral 

 vessels are often wanting, the veins consisting of nothing but 

 woody tissue. In these veins Schultz finds what he calls 

 vessels of the latex, or of the nutritive fluid ; concerning the 

 probable nature of which see p. 31. 



Such are the general anatomical characters of leaves ; but 

 it must be borne in mind, that, in different species, they 

 undergo a variety of remarkable modifications. These arise 

 either from the addition of parenchyma when leaves become 



H 4 



