♦ 



Water- Conductivity of the Wood in Trees and Shrubs. 245 



with the view of finding out what influence manuring and pruning might 

 exert on the character of the wood. The results as a whole are still some- 

 what obscure, but it is clear that the effect of manuring is' greatly to increase 

 the amount of water-conducting tissue. This may account for the exuberant 

 vegetative development and loss of fruitfulness which often occurs as the 

 result of too liberally manuring fruit trees (plums, apples, pears). 



The observations on the results of root-pruning were not very conclusive, 

 though they all pointed in the direction of a lowering of the specific con- 

 ductivity as a result. The actual amount of wood produced on two adjacent 

 rows of trees respectively root-pruned (in 1916-17) and not so treated, was 

 very striking. There was a large falling off in the absolute conductivity of 

 the total shoots for the current year, and their feeble development was 

 clearly a consequence of the pruning. But the quality of the wood in the 

 pruned trees, as measured by its specific conductivity, was only slightly 

 less efficient as a water-conductor than that of the unpruned set of pears 

 and apples which were used as controls in the experiments. 



During the course of the investigation it was observed that a seasonal 

 difference in the water-contents of the wood is clearly apparent in most 

 deciduous trees. For whereas twigs and branches cut off during the summer 

 always float in water, those cut during the autumn and winter as regularly 

 sink at once. This means that the air which is present in many of the water- 

 conducting elements of the wood is got rid of soon after the faU of the leaf. 

 In the summer, while transpiration is active and the water in the tree is 

 under low pressure, bubbles of air are formed. As was shown by Dixon, 

 these may be re-absorbed into solution under pressure. This is doubtless 

 accomplished by means of root-pressure in early autumn while the soil is still 

 warm enough to enable the roots to pump up water into the trunk and 

 branches of the tree at a time when no corresponding amount is being lost 

 through the leaves. In this way the lacunse (localised in individual tracheids 

 and vessels) in the water column within the tree are again filled up with 

 water, and the wood is already preparing to meet the requirements of the 

 unfolding leaves in " the following spring. A few trees, however, possess 

 branches which always float, but they are those in which, as in Willows, the 

 pith and cortex contain considerable air spaces. Of course, it is not to be 

 expected that branches old enough to have heart wood will sink in this way, 

 inasmuch as the heart wood has ceased to conduct water and always contains 

 some air. But in every example I have been able to examine personally I 

 have found the specific gravity of lengths cut from trunks containing heart 

 wood to be very much greater in winter than when cut from trees felled in 

 summer. This difference is largely, but not perhaps entirely, due to relative 



