218 



On the Quantitative Differences in the Water- Conductivity of the 

 Wood in Trees and Shrubs. Part I. — The Evergreens. 



By J. Bretland Farmer, F.E.S., Professor of Botany in the Imperial College 



of Science and Technology. 



(Eeceived January 23, 1918.) 



The present communication aims at presenting, in a necessarily curtailed 

 and somewhat preliminary form, the results of an enquiry into the com- 

 parative efficiency of the wood as a water-conducting tissue in about sixty 

 species of plants. These consisted for the most part of trees and shrubs, but 

 a few herbaceous forms were studied as well. The investigation was under- 

 taken in the first instance in order to find out whether the efficiency 

 (regarded from the standpoint of its water-conductivity) of the wood could 

 be usefully expressed for a given species in a quantitative form, and if so, 

 what kind of deviation from the average or mean was to be expected, and to 

 what extent separate species might differ among themselves in this respect ; 

 secondly, whether the mean conductivity can be correlated with any obvious 

 character such as deciduous or evergreen habit ; thirdly, to ascertain if 

 possible whether definite changes of external conditions can evoke corre- 

 sponding change in the water-conductivity. 



It is a remarkable circumstance that, although the absorption of water by 

 the roots and its elimination during transpiration from the leaves and other 

 green surfaces are processes which have served as the starting-points of a 

 vast number of investigations, the behaviour of the wood as the intervening 

 conducting channel has beeii almost entirely neglected. It is true that the 

 path actually followed by the water has long been known, and that attempts 

 to discover an adequate physical explanation of the ascent had been 

 repeatedly made for many years before the fine researches of Dixon and Joly 

 showed clearly where the solution of this problem was to be sought. But 

 the limitation which the structure of the wood may impose on the volume of 

 water transmitted appears to have attracted no attention at all. And yet the 

 life and habits of a plant are so closely linked up with the problem of water 

 supply that every factor that can influence the complex adjustment of supply 

 of and demand for water deserves serious consideration. It cannot be devoid 

 of interest for those concerned in arboriculture and forestry, for not only 

 may it determine whether a particular species can flourish under given 

 natural conditions, but it also concerns the inter-relations between those 



