ANATOMY OF LEAVE?. 301 



the same causes, also, may be attributed, in a great 

 degree, the variety of figure which these cells exhibit ; 

 for, although they are in some instances globular, or 

 nearly so ; and in others triangular, or more or less 

 regularly hexagonal ; yet, it is probable, that the ma- 

 jority are originally spheroidal vesicles ; and that the 

 variations from this figure depend on the turgescence 

 of the vesicles, and the consequent compression which 

 must necessarily result from their contiguity. The 

 hexagonal figure being that which spheroidal vesicles, 

 mutually compressing one another, are naturally dis- 

 posed to assume, we find that a more or less regular 

 hexagon is the most common form of these cells ; and 

 this figure is generally more regular in the cells form- 

 ing the centre of the substance of the leaf, owing to 

 these being there more distended with fluid, than in 

 those towards either ol the cuticles. 



But that the diversity of figure in the cells of leaves 

 does not, altogether, depend on mechanical compres- 

 sion, is evident from the fact, that those towards the 

 upper disk of the leaf often differ in form from those 

 towards the under disk ; and yet in both these situ- 

 ations we may suppose the compression to be nearly 

 equal. This difference is probably necessary for the dis- 

 tinct functions of these two surfaces. When the tubular 

 cells of leaves are cut transversely, they appear to be 

 of an hexagonal figure, and not round, as might be 

 suspected from their longitudinal aspect. 



With regard to the individual structure of the cells 

 constituting the parenchyma of leaves, we find it is 

 the same as that of the cells in the other parts of the 

 plant. Each cell appears to be a distinct, transpar- 

 ent, membraneous vesicle, formed into the figure it 

 displays by the pressure of the contiguous celts, and 

 thence, the partition separating each cell, must be a 

 double membrane. This is more evident in the mi- 

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