BOG XEROPHYTES AND ACID SOILS. 117 



Later views. Friih and Schroter (1904 : 14) stated that the con- 

 trolling factors in low moor and high moor were the water-retaining 

 power of the peat, the low temperature, and the lack of oxygen, 

 which hindered the respiration of the roots and consequently all their 

 functions. These conditions make the absorption of water diffi- 

 cult, producing the physiologically dry soil of Schimper. The 

 xerophytic character of many species of the low moor, as shown in 

 the terete leaves of Cyperacece and Juncacecs, stomatal protection and 

 roll-leaves in Carex and Poacece, equitant leaves in Iris and Tofieldia, 

 waxy coating in Primula farinosa, small leaves in Lysimachia, 

 Epilobium, Veronica, and Centaurea, and marked cutinization in 

 Scirpus, are not entirely clear in their significance. They bear some 

 relation to the greater difficulty of absorption, but probably in spite 

 of this the higher water-balance is made possible through the ina- 

 bility of all of these plants to close their stomata. Similar xerophytic 

 characters in the high moor, such as the ericoid leaf of Calluna, 

 Empetrum, and Oxycoccus, the leathery leaf of species of Vaccinium, 

 etc., are due partly to the more difficult absorption of water and partly 

 to the evergreen nature of these plants. Many typical inhabitants 

 of high moors can grow likewise in habitats very dry physically, but 

 most of the species of low moor are strict hydrophytes and do not 

 thrive in dry soil. 



Clements (1905 : 126; 1907 : 169) first questioned the conclusion 

 of Schimper that bog xerophytes are due to the presence of humic 

 acids which inhibit absorption and aeration in the roots, and that 

 bogs and swamps are consequently physiologically dry. The fact 

 that weak solutions of organic acids usually increase transpiration was 

 regarded as making it improbable that small quantities of humic acids 

 should decrease absorption sufficiently to produce xerophytes in 

 ponds and bogs. Moreover, not a trace of acid was discovered in 

 many ponds and streams where Heleocharis, Scirpus, Juncus, etc., 

 grow. Plants with a characteristic hydrophytic structure through- 

 out are regularly found alongside of apparent xerophytes, and these 

 also show a striking contrast in size and vigor of growth where they 

 grow both upon dry gravel-banks and in the water, indicating that 

 the available water is much greater in the latter. The conclusion 

 was reached that the xerophytic features found in amphibious plants 

 are due to the persistence of stable structures, which were developed 

 when these plants were growing in xerophytic situations. 



Clements grew Ranunculus sceleratus (1905 : 120, 156) and Sag- 

 ittaria latifolia (1907 : 169) in various water-contents under control, 

 and found that the amphibious form in swamps was a hydrophyte, 

 as shown by the differences between it and artificially produced 

 xerophytic forms, in growth, number of stomata, and transpiration. 

 Folsom (1918:809) has later grown Ranunculus sceleratus under 

 control with similar results. 



