LECT. X;] ANATOMY OF LEAVES. 583 



lar ramifications of the foliar expansion are not pro- 

 longations of the vessels forming the petiolar fas- 

 ciculi ; but that many of them are distinct vessels 

 anastomosing with others, although in a different 

 manner from this kind of union as it occurs in ves- 

 sels in animal bodies. It is probable that the in- 

 osculation which occurs in the proper or return- 

 ing vessels, more nearly resembles that which we 

 find in vessels of animals ; for, as the proper ves- 

 sels are simple membranous tubes, any commu- 

 nication between them must be by direct openings, 

 such as are found to exist in the vessels of Mar- 

 chantia (see page 550). 



2. The thick and fleshy leaves of dicotyle- 

 donous plants are seldom petiolated ; but when 

 they are so, the arrangement of the vascular 

 fasciculi, both in the petiole and in the expansion, 

 closely resembles that of the thin membranaceous 

 leaves. The sessile leaves of this division are ge- 

 nerally thicker and more succulent than the pe- 

 tiolated. If we take the genus Mesembryan- 

 themum, as affording specimens illustrative of the 

 character of these sessile leaves, we find that the 

 vessels pass from the stem into the leaf in one or 

 more fasciculi, according to the figures of the 

 leaves. Thus in the Hatchet-leaved Mesembryan- 

 themum (M. Dolabriforme) , the leaves of which 

 are connate, the sap- vessels enter the leaf in one 

 bundle, which extends in the direction of its 



p p 4 



