194 AERATING SYSTEM [CH. xiv 



evidence for the view that the aerenchyma in this plant is not 

 respiratory in function, Schrenk, who described it, points out 

 that in old stems the surface of this tissue is covered by a layer 

 which is air-tight and suberised, and that a similar layer is also 

 sometimes found separating it from the interior of the stem. 

 He accounts for its occurrence in regions where it cannot serve 

 for flotation, by supposing that the meristem spreads there 

 automatically from the floating parts. 



To the present writer, however, the question whether the 

 secondary air-containing tissues of water plants serve mainly for 

 aeration or for flotation, seems to be a matter of minor import- 

 ance. It appears to her that the evidence as a whole points 

 rather to a fundamentally different interpretation namely, that 

 the formation of the secondary air-tissues is directly induced by 

 environmental conditions, and that their serving any purpose 

 is to be regarded as quite fortuitous. In the case of the 

 -primary lacunar system, the position is somewhat different, and 

 it seems difficult to escape the conclusion that we have here an 

 example of the inheritance of acquired characters. There is 

 some experimental evidence tending to show that this system 

 was initiated as a direct response to the aquatic milieu; its 

 elaboration may either be attributed to natural selection or to 

 the inherited effects of use. There is no doubt that the habit 

 of developing an elaborate aerating system has now become in 

 many cases an inherited character, for though it can be modified 

 and reduced by terrestrial conditions, it cannot be altogether 

 eliminated. 



