CHAPTER VIII. 



THE VASCULAR SYSTEM AND VENATION. 



Terminals end blindly in the mesophyll Exchanges of water and 

 food- substances "Circulation of sap" Afferent and efferent 

 pipes Source of carbon. 



ON examining a very thin leaf, such as that of a Bilberry, 

 Lilac or Honeysuckle, or Salix herbacea, under a good 

 lens or a low power of the microscope, especially if the 

 leaf be first heated in water and all the entangled air 

 driven out, it will be seen that the ultimate branches of 

 the vascular network i.e. the smallest veins or terminals 

 end blindly in the mesophyll, 'tailing off as they do so into 

 two or three, or even a single pipe or vessel or similar 

 structure. The blind end abuts closely on to one or more 

 delicate bags, or cells, and so comes - at its end and sides 

 into direct contact with the mesophyll, which is composed 

 of innumerable thousands of such bladder-like bags or 

 vesicles (cells). 



It is at these blind ends, which are to be found in 

 hundreds and thousands all over the leaf-area, that the 

 exchanges of fluids between the mesophyll and the vas- 

 cular system take place. Whenever the vascular pipes 

 are full of water and by water we are always to under- 

 stand water with traces of minerals dissolved in it, like 

 ordinary drinking water while the cells on which they 

 abut are for any reason short of water, then such water 



62 



