116 AERATION AND AIR-CONTENT. 



(1) a transpiration optimum; (2) physiological dryness in wet, cold 

 soil; (3) poor soil-aeration; (4) water-retention in peat soil; (5) 

 chemical substances; (6) free humous acids and other dissolved sub- 

 stances that chemically affect the roots and are regarded as the chief 

 cause of physiological dryness. These are thought to depress the 

 root's acitivity and consequent absorption. Warming, however, 

 also recognizes the presence of hydrophytes in moors, which are not 

 in harmony with the supposed dryness of the habitat, such as Rubus 

 chamcemorus, Caltha palustris, and Viola palustris. 



Stenstrom (1895 : 117) discussed in an exhaustive fashion the re- 

 lation of species to different climates and habitats, and reached the 

 conclusion that Kihlmann's explanation of bog xerophytes was not 

 true. He cited Burgerstein's results with humus extract to support 

 the view that such substances explain the xerophily of bog-plants. 

 His explanation of the latter was based chiefly upon the transpira- 

 tion relation and the fixity of inherited structures. His views upon 

 transpiration seem to be unsound, and he himself admitted the 

 paradox involved in them (p. 184), stating further that the effects 

 supposed to be due to transpiration might well be caused by poor 

 aeration. As to the origin of unplastic or stable plants, such as 

 Ledum palustre and Pirola rotundifolia, he pointed out that they were 

 probably of great age phylogenetically, and that the evergreen char- 

 acter had persisted under widely different conditions, since the period 

 of tropical climates in high altitudes. 



Schimper (1898, 1903) broadened the concept of non-available 

 water, and emphasized the distinction between physical drought, 

 in which the soil itself is dry, and physiological drought, where the 

 soil is wet but much of the water not available to the roots. He 

 mentioned soils rich in humous acids and those with temperatures 

 at or near freezing as examples of physiological dryness, which led 

 to xerophytic vegetation in such habitats. He also stated that many 

 plants that thrive in meadow-moors were completely absent from 

 high moors, apparently kept away by the great amounts of humous 

 salts in solution. The presence of xerophytes in swamps and bogs 

 was explained by the occurrence of humous acids, which hindered 

 the absorption of water by the roots and rendered the soil dry to 

 plants, and hence well-suited to xerophytes. Nothing is said as to 

 why humous acids have this effect, but it is possible that the assump- 

 tion was based upon a mistaken statement in Pfeffer's Physiology 

 (1897, 1:231; 1900, 1:249), to the effect that "transpiration is 

 decreased by the addition of small quantities of tartaric, oxalic, nitric, 

 or carbonic acid to the soil, whereas it is increased by alkalies, such 

 as potash, soda, or ammonia, as Sachs has shown and Burgerstein 

 has since established more in detail." 



