25 



Whilst agreeing in a measure with the opinions of these authorities, yet, 

 to us it appears that the physiological significance of this leaf 'decurrence, is a 

 provision of nature to ensure protection against excessive transpiration by the 

 stomata, and so prevent a loss of water through the activity of these organs. 

 This furnishes an- 

 other illustration of 

 adaptation to physi- 

 cal conditions or en- 

 vironment. It is 

 only on some coast 

 varieties that the 

 decuiTent channel is 

 absent. 



By such an ar- 

 rangement, a too 

 energetic transpira- 

 tion can be obviated 

 during times of 

 drought, when the 

 soil has scarcely 

 sufficient moisture 

 for the tree's re- 

 quirements, for the 

 leaves, by taking a 

 decurrent form, 

 place the stomata 

 surfaces on a fixed 

 under side, and at 

 the same time are 

 further protected, if 

 necessary, by the 

 edges of the con- 

 crescence acting as a 

 door to the channels 

 thus formed, a move- 

 ment which would 

 also, most probably, 

 be brought into use during wet weather. 



Figure 2.- 



-.\ more advanced plant than Figure I, s!i 

 period of retention of primordial leaves 

 axis. Nat. size. 



owing a longer 

 on the central 



The absence of palisade parenchyma and the finer structure of the type 

 material, of the ventral surface, are aiding factors in this instance of leaf movement. 

 As we see that nature responds to climatic adversities in xerophilous plants by 



