VII] EPIDERMIS, VASCULAR SYSTEM, ETC. 81 



and may then be readily washed away from the sup- 

 porting network. But in practice it is usually found 

 that the removal of this rotted pulp is for a long time 

 obstructed by a thin transparent skin continuous over the 

 whole of both surfaces of the leaf, and which contains 

 within it, as in a flattened bag, both pulp and network. 

 Sooner or later, however, this skin also rots and with the 

 pulp can easily be removed from the more resistant net- 

 work by gentle brushing with a camel's hair pencil under 

 a stream of water. 



The enveloping skin of the leaf is the Epidermis : the 

 network of pipes and fibrous strands is the fibro-vascular 

 system or venation, and will here be called the Vascular 

 system : the pulp between, which in the fresh living leaf 

 forms the green tissue of the lamina, is termed the 

 Mesophyll. 



It is not necessary here to carry far either of the two 

 following amplifications of our subject, because my purpose 

 is neither to describe in detail all the structural variations 

 to be met with in the leaves of different plants, nor to 

 enter upon discussions of theoretical morphology : the 

 first amplification must here be satisfied by the statement 

 that in many leaves the vascular system does not form 

 copious networks of the kind here referred to, while in 

 others the quantity of fibrous supporting tissue may be 

 much greater or less than that met with in the common 

 typical leaf such as that of the Beech, Oak, Lime, Maple, 

 Poplar, Elder, Lilac, Gooseberry, &c. here concerned. 



The second amplification must be satisfied by the 

 statement that no useful purpose would be served here by 

 trying to classify the tissues or structures I have called 

 Epidermis, Vascular system, and Mesophyll into different 

 groups, such as would have to be done were we concerned 

 with recondite questions of histology and morphology. 



w. ii. 6 



