CllAI'. I] 



WATER 



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

 well adapted for resisting- the danger of excessive loss of water, \\Jicther 

 t he danger arises fronj Jogliltla^biQrpiiQn or from excessive transpiration. 

 By a reducdon in the^'zeof the leaf and QLthe-JnterceUular spaces the 

 ti'anspiriiig surface becomes smaller for a given mass of plant-substance; 

 "t he c onductjoa_Qf water is facilitated by the increase _ in the vessels.;, 

 transpiration is reduced by^ a thici^er^cuticle, by the^ j)resence_ of ajr- 

 ContainThg felted or silky hairs, and_. b y the siijiking^of the stomata ; 

 waler-^cclls have a similar action, and besides this they rapidly fiF whenever 

 the water-supply is increased, and yield theiiL_contents_ to the assimilatin g 

 cells as the supply of water is reduced. 



A«U*«^^f^ 



'^^^'lu^ rfii 



Figs. 7 and 8. Xeiophilous structure. Alpine climate. Lcit hand : Myrica javanica. Part of 

 the transverse section of a leaf. Gedeh, Java, 3,900 m. Right hand : Fhotinia integrifolia. Trans- 

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

 Magnified 200. 



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

 butMncorrectly, 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, 



