184 PROCESS OF NUTRITION. CHAP. III. 



copia 9 furnished with its leaves and seed-vessels, 

 was placed in a decoction of Madder-root, so as that 

 the lower portion of the stem and two of the in- 

 ferior leaves were immersed in it. After remaining 

 so for several days the colour of the decoction was 

 distinctly discerned passing along the midrib of each 

 leaf. On the upper side of the leaf many of the 

 ramifications, going from the midrib towards the 

 circumference, were observed to be tinged vith red ; 

 Channel but on the under side there was observed a system 

 f branching vessels, originated in the extremities 

 ^ tne * ea ^ anc * carrvm g not a re d but a P a ^ 



and leaf- fluid, which after uniting in two sets, one on each 



cording to side the midrib, descended along with it into the 



leaf-stalk. These were the vessels returning the 



elaborated sap.* The vessels observable on the up- 



per surface Darwin calls arteries, and those on the 



under surface he calls veins ; the propriety of which 



appellations is questionable, though the discovery of 



the different sets of vessels conducting the sap and 



proper juiceis important; because it points out the 



intention of the peculiar structure of the leaf as dis- 



coverable in the skeleton, which has been already 



described as consisting of two, or, as in the case of 



the Orange-leaf, of three layers of net-work. 



According To this may be added the more recent discoveries 



Knight. f Mr. Knight who in his experiments, instituted 



with a view to ascertain the course of the sap, de- 



tected in the leaf-stalk, not only the vessels which 



* Pbytologia, sect. iv. 



