CHAP. XLVI XEROMORPHY. FORMATIONS 195 



kinds of soil there is some essential agreement, and that some of the 

 life-conditions under which marsh-plants exist compel them to deal 

 economically with water. Several different factors may be of moment 

 and co-operate : for instance 



Johow * and Kihlman 2 directed attention to the observation made 

 by Tschaplowitz 3 indicating the existence of a transpiration optimum, 

 and that therefore even marsh-plants may be forced to depress their 

 transpiration. 



Wet soil is cold, and therefore physiologically dry* Consequently, on 

 moors and swamps vegetation develops late, and flowering is late, 

 except in certain species. Kihlman 5 and Gobel 6 point out that many 

 plants, though growing in soaking spots, are clothed with woolly hairs 

 (as is the case with species of Espeletia in Venezuela) or are protected 

 from rapid transpiration in some other way, because strong winds dry 

 the vegetation at a time when the activity of the root is checked by 

 coldness of soil. This well accounts for the xerophytic structure of 

 plants in the extreme north and high up on mountains, and it plays an 

 important part in the places under discussion. 



Another circumstance of potential significance is that in every wet, 

 badly-aerated soil, which is poor in oxygen, respiration is obstructed, 

 and consequently the root's functional activity is depressed. 7 According 

 to Freyberg, roots of marsh-plants consume less oxygen than do land- 

 plants in a given time, and in order to maintain the balance between 

 their working and that of epigeous parts the functional activity of the 

 latter must be depressed. The fact that many plants, such as Calluna 

 and species of Pinus, growing on heath and on other dry warm soils can 

 also grow on moors, is no longer incomprehensible when we remember 

 that heath often has an extremely ill aerated, periodically soaking, raw- 

 humus soil, which exemplifies the ' dry production of peat '. 



It must also be noted that peat has a great power of retaining 

 water* 



Many moors and heaths may become very dry in their upper layers 

 in summer. We can often walk dry-footed not only over moors clothed 

 with such marsh-plants as Scheuchzeria, Rhynchospora alba, and Carex 

 limosa, but can note that the bog-mosses are so dry as to crackle as we 

 tread on them. Many arctic swamps and moors often become completely 

 dried up. 



Livingston 9 arrives at the conclusion that at least in some bog- 

 waters there occur chemical substances ' which are not in direct relation 

 to the acidity of the water ', but which ' act on the vegetation ' : and 

 ' it is suggested that these substances may play an important role in the 

 inhibition, from bogs, of plants other than those exhibiting xerophytic 

 adaptations '. 



But the weightiest cause of the physiological dryness of the soil 

 probably lies in the presence of free humous acids and other dissolved 

 substances which chemically affect the roots. 10 Moor-soil is probably 



1 Johow, 1884. * Kihlman, 1890. ' Tschaplowitz, 1883. 



4 See p. 50. 8 Kihlman, 1890. Gobel, 1889-91. 



7 Transeau, 1903, 1905. * See pp. 47,61. * Livingston, 1904. 



10 See Weber, 1902, 1903 ; Schimper, 1898 ; Cowles, 1901 ; Bruncken, 1902 a ; 

 Friih und Schroter, 1904. 



O 2 



