296 THE INTERNAL WATER RELATIONS OF PLANTS 



critical sense of the word. The fundamental concept referred to under this 

 term is essentially that of the migration of water within the plant from regions 

 of its greater to regions of its lesser diffusion pressure. Whenever absorption 

 of water from the soil has ceased, or is occurring at a relatively slow rate as 

 compared with the rate of loss by transpiration, the water in the plant behaves 

 essentially as if it is an isolated hydrostatic system within which water will 

 be continuously moving from regions of its greater to regions of its lesser 

 diffusion pressure. The water in the "competing" organs or tissues is in 

 intercommunication through the continuous water conductive system. Within 

 any plant, therefore, in which the volume of water present has been reduced 

 sufficiently water will migrate most rapidly towards those organs or tissues in 

 which the greatest diffusion pressure deficits have developed. 



The actual rate of movement will also be influenced by the resistance of 

 the tissues through which the water must pass. Given a certain diffusion 

 pressure deficit in the cells of a certain organ, water will move more rapidly 

 toward that organ if the resistance offered by the conductive tissues leading 

 to that organ is relatively small than if it is relatively large. The resistance 

 factor affects only the rate and not the direction of the movement of water. 



Generally speaking, when a severe internal water deficit develops within 

 a plant, tissues in which the cells possess relatively high osmotic pressures will 

 gain water from tissues in which relatively low osmotic pressures prevail in 

 the cells, since under such conditions the diffusion pressure deficit of a tissue 

 usually approximates its osmotic pressure. Young leaves usually have higher 

 osmotic pressures than older leaves on the same plant, although there are ap- 

 parently some exceptions to this statement. On most stem axes, therefore, 

 the closer the point of attachment of the leaf to the tip, the greater its omotic 

 pressure in relation to the other leaves on the same axis. The stem tips 

 usually possess relatively high osmotic pressures, equalling or slightly exceeding 

 the values for young leaves on the same plant. 



The osmotic pressures of green fruits seem to be invariably less than that 

 of leaves on the same plant (Chandler, 1914). This relation is sometimes 

 although not frequently reversed as the fruit ripens. The osmotic pressures 

 of root cells are in general low as compared with the other tissues of the plant 

 (Hannig, 1912). Some exceptions to this principle have been discovered, 

 however, notably in the garden and sugar beet, in both of which species the 

 osmotic pressure of the "roots" ^ is appreciably in excess of that of the leaves. 



The above discussion indicates that under conditions of stress in the 

 internal hydrostatic system the younger leaves and stem tips frequently gain 



1 The "fleshy roots" of beets and a number of other species actually consist 

 principally of enlarged hypocotyls. 



