258 ANATOMY OF LEAVES. [BOOK i. 



the predisposition to assume particular appearances or func- 

 tions is absolutely fixed, and will not change in the ordinary 

 course of nature. This is a fact of high interest for those 

 who are occupied with researches into the causes of what is 

 called vegetable metamorphosis. 



The parenchym is, if casually examined, or even if viewed 

 in slices of too great thickness, apparently composed of heaps 

 of small green bladders, arranged with little order or regu- 

 larity ; but if very thin slices are taken and viewed with a 

 high magnifying power, it will be seen that nothing can be 

 more perfect than the plan upon which the whole structure 

 is contrived, and that, instead of disorder, the most wise order 

 pervades the whole. Upon this subject I extract the words 

 of Adolphe Brongniart : 



" There exists beneath the upper cuticle two or three layers 

 of oblong blunt vesicles, placed perpendicular to the surface 

 of the leaf, and generally much less in diameter than the 

 bladders of the cuticle ; so that they are easily seen through 

 it. These vesicles, which appear specially destined to give 

 solidity to the parenchym of the leaf, have no other intervals 

 than the little spaces that result from the contact of this sort 

 of cylinder : nevertheless, in plants that have stomates on the 

 upper surface of their leaves, as is the case in most herba- 

 ceous plants, and in such as float on the surface of water, 

 there exist here and there among the vesicles some large 

 spaces, through which the stomates communicate with the 

 interior of the leaf. This parenchym is entirely Different 

 from what is found beneath the cuticle of the lower side. 

 There, instead of consisting of regular cylindrical vesicles, it 

 is composed of irregular ones, often having two or three 

 branches, which unite with the limbs of the vesicles next 

 them, and so form a reticulated parenchyma; the spaces 

 between whose vesicles are much larger than the vesicles 

 themselves. It is this reticulated tissue, with large spaces 

 in it (to which the name of cavernous or spongy parenchyma 

 might not improperly be applied), that, in most cases, oc- 

 cupies at least half the thickness of the leaves between the 

 veins. The arrangement of the vesicles is very obvious if the 

 lower cuticle of certain leaves be lifted up with the layer of 



