200 SECTIONAL ADDRESSES 



an English spring this might mean the movement of water from the soil 

 through the activity of the root system, but in Rhodesia for instance, 

 where the spring foliage expands in early August over a dry soil in rainless 

 weather, the essential water movement must be from the branch to the 

 bud. Sachs long ago drew attention to a simple experiment which 

 suggests that such a movement of water will take place with rising tempera- 

 ture. In spring, when a woody branch is either dipped in hot water or 

 placed in a vacuum, the expansion of air in the tracheae will drive water 

 freely to the cut surfaces of the wood, especially of the youngest sap wood. 

 In the intact tree, therefore, a rise of temperature will drive water to the 

 ends of the tracheal system of the youngest sap wood which are to be 

 found, as a result of their method of differentiation, either at the leaf 

 scars or in the tracheal systems of the buds. At the leaf scars these ends 

 are blocked, so that all this water movement following upon rise of 

 temperature must find its goal in the buds which now resume their 

 growth. 



But, as Pringsheim has emphasised, when such a grovirth centre resumes 

 activity it is capable apparently of drawing water from any other portion 

 of the plant, and is the last to suffer from lack of water when supplies 

 are deficient. Thus even when the older leaves are wilted the growing 

 point may continue active and leaf primordia expand in size through the 

 Vacuolation and division of their cells. The mechanism drawing water 

 to such a growing point requires further elucidation, but a contributory 

 cause is ainjost certainly the osmotic system provided by the differentiating 

 vascular elements. In the bud in spring, between dormant cambial cell 

 and fully differentiated xylem element, are always left elements which are 

 not fully differentiated, whilst throughout the woody axis the dormant 

 cambium usually lies directly against fully differentiated and lignified 

 wood. As the water exiters the buds in spring the first visible change is 

 the swelling of these partly differentiated elements between cambium 

 and wood, and water is obviously attracted by the osmotic forces within 

 them. The shoot meristem as a whole now recommences growth, and 

 cambial activity spreads downwards to the base of the bud and into the 

 dormant cambium lying upon the surface of last year's wood. But as 

 the cambium thus awakens into life, this layer and the newly formed 

 tissues arising from it must be withdrawing water from the woody 

 tissues within. This will be true of both hardwood and softwood, 

 but it is particularly clear in the case of vessel differentiation in the 

 hardwood. ' 



In the leafy shoot of the hardwood are found differentiated, lignified 

 vessels, with cross walls all perforated and no protoplasmic contents, so 

 that if they contain sap under pressure it will flow out through the per- 

 meable lignified wall into the surrounding tissues. Traced downwards 

 this vessel system is in direct continuity, in the tissues growing upon the 

 surface of the old wood, with a vessel system which still contains proto- 

 plasm and in which water is accumulating under osmotic pressure sufficient 

 to force outwards the still plastic walls. This liquid, thus accumulating 

 under pressure, becomes continuous suddenly (with the collapse of the 

 cross walls and the coalescence of the liquid contents of the originally 



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