ADJUSTMENT TO WATER 69 



turgidity. When the drouth is continued, the plant first wilts 

 and then dies. Plants often find themselves in conditions where 

 the drouth is sufficiently severe to render the temporary closing 

 of the stomata more or less ineffective, though not fatally so. 

 Such conditions result in modifications of various sorts, most of 

 which decrease water loss, though some are for the purpose of 

 increasing the water supply. 



86. Details of the adjustment. The water evaporated from 

 the cells bordering air spaces is supplied by osmosis from the 

 adjacent cells. The latter in turn draw upon cells nearer the 

 transporting bundles, until the demand for more water reaches 

 the bundles themselves. The vessels of the leaf meet the demand, 

 and the water given up is replaced in consequence of an upward 

 movement through the bundles of the stem and root. The deficit 

 thus caused is met by the movement of water into the root bundle, 

 and is consequently passed along until it reaches the root-hairs. 

 The last step in the process of replacement is taken by the latter, 

 which absorb the necessary amount of available water. As a matter 

 of fact, the root-hair is taking in water at the same time that it 

 is passing by diffusion into the adjacent cells. 



If the demand for water reaches the root-hair at a time when 

 it can not obtain an equivalent amount by absorption, it 3'ields 

 some water to the adjacent cell, but not as much as is needed. 

 This is due in part to differences of osmotic pressure in the two 

 cells, but in a large degree also, it would seem, to the strong attrac- 

 tion of protoplasm for water. Apparently as long as this attractive 

 power of the protoplasm of the root-hair is met by absorption, 

 water is permitted to pass readily into the adjacent cells. But 

 in case this affinity is only partially satisfied, the protoplasm of the 

 absorbing cell counteracts in a measure the pull exerted by the 

 adjacent cell, and insufficient water passes into the latter. As a 

 consequence, both cells become less turgid, and, if transpiration 

 continues without compensating absorption, finally lose their 

 turgidity entirely. A similar readjustment occurs throughout the 

 parenchyma of the root, the innermost cell drawing water from 

 the bundle. This loss is met by taking water from the leaf-cells 

 along the bundles, and the consequent loss of turgidity passes 

 throughout the leaf, finally reaching the epidermal cells. The 

 latter withdraw water from the guard-cells, which conse(iuenfly 

 close. 



