WATOMY OF LEAVES. 291 



ciculi ; and between these are secondary fasciculi, 

 which are less elevated. To the unassisted eye they 

 all appear to go off from the midrib ; but viewed by 

 a magnifying lens, and with transmitted light, we per- 

 ceive that all of them do not proceed directly from 

 the fasciculi of the midrib, but that some of them are 

 branches of the others. At the margin they all inos- 

 culate, and form, as it were, one fasciculus, which, 

 extending from the base to the apex, is the real living 

 boundary of the leaf. 



Examining more closely, and placing a slice of the 

 petiole, cut transversely near the base of the expan- 

 sion, under the microscope with a glass of a moderate 

 power, we perceive that the vessels are arranged in 

 distinct fasciculi, which are nearly of the same size in 

 the centre of the section; alternately larger and smaller 

 near the circumference on the convex surface, or that 

 part of the petiole which is towards the under disk of 

 the leaf; and all small on the concave surface. The 

 costae are continuations of those on the concave sur- 

 face of the midrib, which are curved outwards in op- 

 posite pairs, at different distances between the basis 

 and the apex of the leaf; but the central fasciculi 

 pass on its apex. These vascular bundles are imbed- 

 ded in a cellular tissue ; besides which, the petiole 

 and midrib of this description of leaves contain pecu- 

 liar pneumatic or air cells closely resembling those 

 which constitute a great part of the substance of aqua- 

 tic plants. In a transverse section of a small part of 

 the expansion of the leaf we perceive that the vascu- 

 lar cords run nearly in the centre between the two 

 plates of cuticle, imbedded in an opaque green paren- 

 chyma ; and that, instead of the pneumatic apparatus 

 of the petiole and midrib, there is a transparent layer 

 of large cells immediately under the cuticle of the up- 

 per disk. These pneumatic cells, however, are not 



