298 ANATOMY OF LEAVES. 



be readily demonstrated by dissection, with the aid of 

 the microscope. 



Whether the communication of the cavities of these 

 united vessels be direct, as in the vessels of animals, 

 so as to allow the fluids they convey to flow in an un- 

 interrupted stream from the one to the other, is not 

 easy to determine. It is, however, evident that in 

 the leaves of dicotyledons, as in those of monocotyle- 

 dons, all the vascular ramifications of the foliar ex- 

 pansion are not prolongations of the vessels forming 

 the petiolar fasciculi ; but that many of them are dis- 

 tinct vessels anastomosing with others, although in a 

 different manner from this kind of union as it occurs 

 in vessels in animal bodies. It is probable that the 

 inosculation which occurs in the proper or returning 

 vessels, more nearly resembles that which we find in 

 the vessels of animals ; for, as the proper vessels are 

 simple membranous tubes, any communication be- 

 tween them must be by direct openings, such as are 

 found to exist in the vessels of Marchantia. 



2d. The thick and fleshy leaves of dicotyledonous 

 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 generally thicker and more succulent 

 than the petiolated. If we take the genus Mesem- 

 bryanthemum, as afibrcling specimens illustrative of the 

 character of these sessile leaves, we find that the ves- 

 sels pass from the stem into the leaf in one or more 

 fasciculi, according to the figures of the leaves. Thus 

 in the Hatchet-leaved Mesembryanthemum (M. dola- 

 briforme), the leaves of which are connate, the sap- 

 vessels enter the leaf in one bundle, which extends in 

 the direction of its axis, the whole length of the leaf. 



