GLIMPSES INTO PLANT STRUCTURE 55 



of a leaf stalk there are many plants which pro- 

 duce only a single leaf and the leaf stalk in turn 

 bears the same relation to the mid-rib of the leaf. 

 So, in Fig. 37, I show the internal structure of 



The structure of the mid-rib, or central vein of a 

 rhododendron leaf 



the mid-rib of the leaf of a rhododendron. There 

 are some leaves, moreover, which apparently have 

 no mid-rib, or are all mid-rib. It would be diffi- 

 cult, for instance, to say how much of the needle 

 of a pine-tree was " blade " and how much " mid- 

 rib " and "nerves " when viewed externally, but 

 the microscope reveals, when the leaf is seen in 

 section, that the vascular strands of the mid-rib 



