THE WATER-RELATION 105 



of unnecessary loss of water by transpiration during a period when 

 there is little need for gaseous exchange with the atmosphere. It 

 was formerly believed that the stomata exercised a close control 

 over the transpiration from the leaf during the whole day and adjusted 

 it in accordance with the available supply of water. It was supposed 

 that if transpiration tended to exceed supply, the turgor of all the 

 leaf cells including the guard-cells would tend to fall, leading to 

 reduced stomatal opening and to a check to transpiration. Experi- 

 ment has indicated, however, that frequently a very considerable 

 reduction in stomatal aperture must occur before transpiration is 

 itself reduced, the reason being that when the stomata are wide 

 open their diffusive capacity may not be fully utilised. Transpiration 

 at such times is under the control of other factors, such as atmospheric 

 conditions or the water supply at the seat of the process (see below). 

 When, however, transpiration is so excessive as to produce visible 

 loss of leaf turgor, as shown by flagging or wilting, the stomata 

 usually tend to close completely and leaf moisture is thus conserved. 

 Independently of any stomatal regulation, transpiration does tend 

 to keep step with the supply of water to some extent : for if the latter 

 becomes inadequate the cell-walls in the leaf become somewhat 

 drier, and evaporation is correspondingly reduced. 



Ascent of Water through the Plant. 



Since in our climate leaves usually present a turgid appearance, 

 except perhaps on a hot summer's day, it is obvious that the water 

 lost by transpiration from them is continuously replaced. The 

 replacement is effected by an upward transport of water absorbed 

 by the root from the soil. This current of water, flowing through 

 the plant from the root hairs to the leaves, is known as the Trans- 

 piration Stream. Experiment proves that the stream of water ascends 

 the plant through the vessels and tracheides of the xylem, a tissue 

 which forms a continuous system through root, stem and leaf ; more- 

 over, its elaborate system of pitting gives the impression of its being 

 adapted for aiding the conduction of liquid. If plants or cut shoots 

 are stood in dye solutions until signs of the dye are seen in the upper 

 parts of the shoots, sections then taken from the stem will show that 

 the xylem tissues alone are stained. The water of the transpiration 

 stream thus indicated contains small quantities of mineral salts, 

 and of organic substances, and the contents of the xylem elements 

 are for this reason sometimes referred to as sap. 



