PRESIDENTIAL ADDKESS SECTION C. 63 



Farmer" has emphasised the importance of tlie varying re- 

 si-stance offered by wood to the passage of the transpiration 

 current. As a result of numerous measurements he has made 

 the important generahsation that the wood of most xerophilous 

 plants offers a relatively high resistance to the flow of water. 

 The significance of this fact can hardly be sought on adaptational 

 lines. In the light of it, however, it is easier to underetand why 

 the leaves of different plants growing in the same habitat should 

 be so unequally protected against excessive transpiration. More- 

 over, any structural feature which reduces the resistance to flow 

 of water to the mesophyll of the leaf will diminish the danger 

 from rapid water loss. Yapp's observations can be interpreted 

 accordingly. Those parts of a broad leaf which are farthest from 

 the msain channels of supply have the greatest resistance to 

 overcome. 



Now in a dissected leaf, especially one with slender cylindri- 

 cal segments, all the cells of the mesophyll are close to principal 

 veins. The same applies also to small leaves generally, and 

 especially to the extreme pinoid and ericoid types. 



Cupressoid types represent a retention of the mesophyll still 

 nearer to the main channels of supply in the stem. In this case 

 the foliage as a whole almost certainly exposes a relatively reduced 

 leaf surface; but to what extent this is true for microphyllous 

 plants in general has yet to be detennined. 



With these suggestions I conclude my address. It has I fear 

 been in pai-ts abstruse, an parts rather speculative; but I hope 

 that, following the example of our microphyllous leaves in relation 

 to tlieir water svipply, I have not ventured to expand too far from 

 established facts. 



Proc. Roy. Soc. B. 90, 1918. p. 218 and p. 232. 



