2o6 SECTIONAL ADDRESSES 



The first movement of water into the bud seems to be brought about 

 by osmotic systems consisting of differentiating tracheae developing in 

 the bud itself and then extending dovs^n wards from the bud over the surface 

 of the old wood. Then these new tracheal systems, as they lose water 

 to the expanded foliage faster than it can be supplied, are able gradually 

 to draw upon the water in the old wood through its displacement by 

 water vapour, thus mitigating the tensions developing in the water columns 

 in tracheae which remain full of water. Up till the end of the growing 

 season, the water content of the wood shows that many tracheal elements 

 in the sap wood remain full of water, and during the autumn and winter 

 the tensions in these columns, as well probably as in the Hving parenchy- 

 matous elements interspersed throughout the wood, may play a part in 

 filling the remaining tracheal elements once more with water instead of 

 water vapour. 



The problem is a difficult one — a tracheal system would obviously fill 

 again with water to any height to which water could be driven by the 

 pressures available in the root supply system, but at heights beyond this 

 it is not at present clear how the tracheal elements are once more 

 refilled. 



The present discussion of the problem, however, should serve to 

 emphasise a consideration that is too little regarded at the present day. 

 Water movement through the tree is associated with the grovith of the 

 tree ; the mechanism of movement is inseparable from the processes of 

 gro'Q^th and differentiation, and the movement is not equivalent to the 

 passive flow of a liquid along a pipe driven either by a pressure below or 

 a tension developing above. 



Within the tracheal sheet laid down in the current season, which alone 

 has direct continuity with the leaves, is an inner core of sap wood which 

 acts as a reservoir of water of which the contents fluctuate daily and with 

 the seasons, under the influence of a supply and demand determined by 

 the activities of the growing tissues of root and shoot. 



The Movement of Organic Solutes. — Finally we would suggest that the 

 movement of solutes throughout the tree similarly cannot be adequately 

 interpreted unless the grovi^th processes of the tree are borne in mind. 

 The movement of inorganic solutes will not be considered ; the available 

 data are too few, but one consideration is emphasised in relation to the 

 organic substances which are manufactured by the leaves during their 

 season of activity. Undoubtedly these substances are mainly transferred 

 downwards from the leafy shoot to branches, trunk and roots, where they 

 are stored. Both the path of transfer and the mechanism of movement 

 are controversial subjects which cannot be fully examined here. There 

 is very general agreement that the phloem plays a role in this movement, 

 and much discussion centres around the problem as to how so much 

 material can move through a tissue containing elements of such peculiar 

 and characteristic structure as the sieve tubes. The data supplied by 

 Ramann and Bauer show that the gain in dry weight of stem and root 

 system takes place relatively late in the growing season. The point it is 

 desired to emphasise is that this gain in dry weight appears to synchronise 



