586 CONSERVATIVE ORGANS. [LECT. X. 



they do not anastomose until, as I have already 

 stated, they approach the apex of the leaf. 



In this view of the arrangement and struc- 

 ture of the vascular system of the leaf, I have 

 not noticed those cutaneous vessels which Hed- 

 wig has described as lymphatics; but the exist- 

 ence of which has been denied by Mirbel and 

 others. If we admit their existence, these ves- 

 sels must undoubtedly be regarded as forming 

 a part of the vascular system of the leaf, and 

 ought now to be described; but as this involves 

 a knowledge of the entire cuticular system, the 

 consideration of this point must be deferred until 

 we examine that system. 



II. Cellular system of the leaf. On cutting a 

 thick, succulent leaf transversely, we immediately 

 perceive that it consists chiefly of a pulp ; which, 

 when placed under the microscope, or examined 

 by a good magnifying glass, is evidently com- 

 posed of cellular tissue. Extending our inquiries, 

 we find that this substance forms a large part of 

 the structure of leaves; filling up the meshes of the 

 network formed by the vessels in the thin and very 

 vascular leaves ; and, in all, occupying that space 

 which separates the two cuticular layers, which 

 constitute the upper and the under disks of the 

 leaf. 



The cellular substance of leaves differs very 



