Chap. I] 



WATER 



ing of the palisade-cells, the above-mentioned modifications appear to be 

 well adapted for resisting the danger of excessive loss of water, whether 

 the danger arises from too little absorption or from excessive transpiration. 

 By a reduction in the size of the leaf and of the intercellular spaces the 

 transpiring surface becomes smaller for a given mass of plant-substance ; 

 the conduction of water is facilitated by the increase in the vessels ; 

 transpiration is reduced by a thicker cuticle, by the presence of air- 

 containing felted or silky hairs, and by the sinking of the stomata ; 

 water-cells have a similar action, and besides this they rapidly fill whenever 

 the water-supply is increased, and yield their contents to the assimilating 

 cells as the supply of water is reduced. 



V^> 



Figs. 7 and S. Xerophilous structure. Alpine cli»iate. Left hand : Myrica javanica. Part of 

 the transverse section of" a leaf. Gedeh, Java, 2,900 m. Right hand : Photinia integrifolia. Trans- 

 verse section (above) of lower, and (below) of upper surface of leaf: Ardjuno, Java, 3,300 m. 

 Magnified 200. 



The possession of protective means, such as those just mentioned, is usually, 

 but incorrectly, described as the result of strong transpiration. In reality they 

 accompany weak transpiration, as for instance on dry or saline soil, as well as 

 strong transpiration in dry air. On the other hand, plants on a damp soil transpire 

 energetically (Gain), and yet, as a rule, dispense with a xerophilous structure. It 

 is not the absolute strength of transpiration but its amount relatively to the water- 

 supply that leads to protective mechanisms. The causal-mechanical explanation 

 attempted by Kohl, according to which strong transpiration is the cause and modi- 

 fied structure the effect, as in a purely physical process, is refuted by such facts 

 as those just mentioned. With greater probability we might regard the varying 

 degree of concentration of the cell-sap as the first cause, that is to say the stimulus 

 acting on the protoplasm, for this is increased by an insufficient supply of water, 



