304 ANATOMY OP LEAVES. 



cular and cellular systems of leaves, the affinity which 

 exists between the stem and the leaf is very obvious. In 

 the stems of monocotyledons, the vessels run nearly in 

 straight lines in distinct fasciculi, imbedded in a cel- 

 lular pulp ; and a similar vascular arrangement pre- 

 sents itself in the leaves of this tribe of plants. In di- 

 cotyledons, on the other hand, the vascular fasciculi of 

 the stem are not distinct, but form a reticular tissue 

 which covers the whole circle of the stem ; and, in 

 like manner, in the leaves, the vessels ramify in ev- 

 ery direction, forming a most complicated and beauti- 

 ful net-work, the interstices of which are filled with 

 the cellular pulp. The leaf, therefore, may be re- 

 garded, in some respects, as a mere expansion of 

 the stem ; and, consequently, in aphyllous plants, we 

 perceive that the stem is adapted to perform all the 

 functions of the leaf. The internal structure of the 

 floral leaves or bractea, and of those more temporary 

 foliar appendages, which are termed sfyndce, is near- 

 ly the same as that of the real leaf; even the scales 

 that envelope buds (sometimes indeed true stipules), 

 and described as deriving their origin from the corti- 

 cal part only of the stem, and consisting chiefly of cel- 

 lular matter, have in every respect the same structure 

 as leaves, as far, at least, as relates to their vascular 

 and cellular systems- 



THE CUTICULAR SYSTEM OF LEAVES. 



Every leaf is covered with a real skin or epider- 

 mis, which not only guards the vascular and the cel- 

 lular matter from external injury ; but is the medium 

 by which it performs the important functions of ab- 

 sorption and exhalation. In the majority of leaves, 

 the epidermis can be separated from the parts it 

 covers ; and appears to be a compound organ, or to 



