236 THE MOVEMENT OF WATER THROUGH THE PLANT 



fusion pressure deficit of water in the soil is less than that of the water in 

 the epidermal cells and root hairs in the absorbing zone of the root (Chap. 

 XVH). 



In most trees practically all upward movement of water occurs in the 



vessels or tracheids of a few of the outer- 

 most annual rings of the sapwood and 

 in some species, especially those with 

 ring-porous wood (Huber, 1935) is al- 

 most entirely confined to the outermost 

 ring. As is well known, in some 

 species of trees (beech, sycamore, etc.) 

 the heartwood of the trunk may dis- 

 appear completely by decay. The fact 

 that such hollow trees continue to live 

 and thrive is conclusive evidence that 

 the heartwood plaj's no essential role 

 in the upward movement of water in such 

 species. 



The water-conductive system of plants 

 should be regarded as a unit system; 

 the individual threads or columns of 

 water not operating separately but each 

 as a part of a more or less complicated 

 meshwork. Fig. 63 illustrates the con- 

 tinuity of the water columns through a 

 woody tissue, in which part of the 

 elements have become blocked with air. 

 The air-plugged tracheids or vessel sec- 

 tions become merely "islands" in the mesh- 

 work of intact water columns, and the 

 capacity of the tissues as a whole to con- 

 duct water, although somewhat impaired 

 by the presence of air in some of the con- 

 ducting elements, is by no means destroyed. 

 Magnitude of the Cohesive Force of Water. — If water is to move 

 through the xylem ducts as postulated by this theory, its cohesive force must 

 be of sufficient magnitude to resist the stresses which are imposed upon it. 

 The maximum height to which water ever moves in plants does not exceed 

 400 feet. This height is equivalent to about 12 atmos. which represents the 

 maximum stress which the water columns could develop as a result of their 



Fig. 63. Continuity of water 



columns through the wood of a 



conifer. Redrawn from Dixon 

 (1914). 



