LECT. X.] ANATOMY OF LEAVES. 565 



in sandy, arid soils, chiefly derive their nou- 

 rishment : for, as very little moisture is taken up 

 by the roots, these plants are supported, almost 

 entirely, by cuticular absorption. The natural func- 

 tions of these tubular cells, also, being to absorb 

 and to carry fluids towards the centre of the leaf, 

 enable us to understand why the leaves of an 

 Aloe, when separated from the stem, are very 

 long in drying and losing their plumpness ; where- 

 as, if thrown into water, when they are very 

 much shrivelled, they almost immediately regain 

 their original size. The absorbing mouths by 

 which they are supplied will be demonstrated, 

 when we examine the cuticular system of leaves. 



In the leaves of those monocotyledonous plants 

 the costae of which, instead of being longitudinal, 

 run in transverse parallel lines, forming acute an- 

 gles with the midrib, we find that the arrangement 

 of the vascular framework resembles that of the 

 Grasses in some circumstances ; but differs from it 

 in other respects. Thus the costae are parallel to 

 one another, and communicate by small transverse 

 cords of vessels, so as to form meshes which are 

 rhomboidal or square according to the angles at 

 which these transverse cords are given off from the 

 costae, as in the Grasses. The petioles are., also, in 

 general sheathing, and many of them are furnished 

 with ligulse. But, in almost all of them, the peculiar 

 cartilaginous articulation, which divides the petiole 



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