234 Prof. J. B. Farmer. On the Quantitative Differences in the 



region the lateral leaf-bearing shoots are chiefly produced. A relatively 

 larger proportion of the wood at the base is devoted to mechanical con- 

 struction, and less to conductive purposes. It may easily happen, and, 

 indeed, commonly does, that the absolute volume of transmitted water is 

 greatest at the base, owing to its greater cross-sectional area. Hence, in 

 endeavouring to get an idea of the specific conductivity of the wood as 

 a whole, and for comparative purposes, it is best to test consecutive lengths of 

 such branches, and then to average the lot for each shoot. The averages thus 

 obtained coincide with one another more closely than was anticipated, and 

 the resultant figures agree nearly with those which best express the character 

 for a given species. In the earlier stages of the work, before this had been 

 recognised, the figures, though obviously grouping themselves more or less 

 round a mean, appeared to be broken up into puzzling sub-groups. This was 

 largely due to the chance positions in a branch from which the specimens to 

 be tested had been selected. This plasticity of the wood is a matter of 

 great importance, which the forester cannot too clearly recognise, for, by 

 understanding the limits of variability and the conditions that affect them, 

 he is obviously in a better position to produce the largest yield of the most 

 desired kind of timber. It becomes plain that, while the deciduous trees are 

 far more plastic in respect of the quantitative distribution of the tissues 

 composing their wood than are the evergreens, they also differ a good deal 

 amongst each other in respect of this plasticity. 



After examining the stems of Birch, Hazel, Ash, and Sycamore in suffi- 

 ciently large numbers (from 100 to 300 for a single species), and plotting the 

 results as curves, it became evident that several independent factors were 

 concerned in producing the irregularities observed. The water conductivity 

 was found, generally speaking, to be lower in young plants and stool-shoots, 

 and to be often very much higher, and also far more variable {i.e. the wood 

 is much more plastic), in rapidly-growing older trees. In the shoots formed 

 on adult trees, however, the value again sank, and a closer approximation to 

 a particular mean was obtained, in other words, individual variation became 

 more restricted. The leading shoots, in spite of their wider average diameter, 

 were commonly of lower specific conductivity than the laterals. This is very 

 marked by the case when, as in the Ash, the leader commonly becomes more 

 or less abortive, its function being assumed by one or more of the lateral 

 branches below it. But, in any event, the laterals usually produce the bulk 

 of the leaves, either directly or by further branching, and it is these leafy 

 branches which always display the highest specific conductivity, at any rate, 

 in young trees. 



The methods employed in dealing with the experimental data acquired 



