On the quantitative differences in the water-conductivity of the wood in trees and shrubs. Part I. -The evergreens

The present communication aims at presenting, in a necessarily curtailed and somewhat preliminary form, the results of an enquiry into the comparative 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 corre­sponding 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 been 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 tissues in the wood which materially determine the nature and commercial value of timber.