260 ANATOMY OF VEINS. [BOOK i. 



that their parenchyma is cavernous, and that the air cavities 

 are uniformly in communication with the stomates. 



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

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

 partitions covered by the ribs and principal veins, so that the 

 air cavities have not actually a free communication in every 

 direction through the parenchym; but are, to a certain 

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

 Mirbel has described in Marchantia, where the leafy expan- 

 sions 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 vessels, to which is added 

 cinenchymatous tissue. In submersed leaves spiral vessels 

 are often wanting, the veins consisting of nothing but woody 

 tissue. 



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 parenchym, when leaves become 

 succulent) or from the non-development of it when they 

 become membranous, or from the total suppression of it, and 

 even of the veins also in great part, as in those which are 

 called ramentaceous, such as the primordial leaves of the genus 

 Pinus. Occasionally, the veins only are formed, the paren- 

 chym being deficient, as in Ranunculus aquatilis, the very 

 curious Hydrogeton or Ouvirandra, and various species of 

 Podostemads. 



It has already been seen that a leaf may consist of two 

 distinct parts ; the petiole, or stalk, and the lamina, or blade : 

 both of these demand separate consideration. These are, 

 however, not necessarily present ; the petiole may exist with- 

 out the lamina, as in leafless Acacias, or the lamina without 

 the petiole, in all sessile leaves. 



The BLADE, lamina (or limbus, as it is called by some) is 

 subject to many diversities of figure and division ; most com- 

 monly it forms an approach to oval, being longer than broad. 



