94 P. C. Miller et al. 



lack roots and, except for genera such as Polytrichum and Pogonatum, 

 most mosses lack the functional equivalent. The moisture supply in the 

 soil is largely unavailable, and moss tissue water contents can vary 

 widely. The movement of liquid water occurs primarily as a result of cap- 

 illarity on the outside of the plant (Bowen 1931, 1933, Magdefrau 1937, 

 Anderson and Bourdeau 1955), and is probably limited to within 1 to 2 

 cm above the water surface (Anderson and Bourdeau 1955). Water ab- 

 sorption from the capillary stream can take place at the leaf bases, leaf 

 traces, and especially at the thin-walled cells at the plant tip (Bowen 

 1933). Rapid absorption is aided by the lack of cuticle in most species 

 (Vaizey 1887, Czapek 1899, Kressin 1935, Patterson 1943), a feature 

 which also facilitates desiccation. Water vapor is absorbed less than li- 

 quid water. In a saturated atmosphere mosses generally only reach water 

 contents (g water gdw'') of 30 to 60%. At 95% relative humidity, water 

 contents of 50% were reported (Patterson 1943), whereas the water con- 

 tent of mosses placed in liquid water for a few minutes may be 300 to 

 700% (Muller 1909). Anderson and Bourdeau (1955) found that both 

 Atrichum and Polytrichum failed to become turgid in relative humidities 

 of up to 100% and that turgid mosses wilted at 95% relative humidity. 

 However, the mosses became turgid minutes after liquid water was added. 

 It appears, therefore, that mosses less than 1 to 2 cm high may acquire 

 water through capillary movement from the soil surface or through the 

 addition of liquid water in the form of rain, dew or fog. Taller mosses 

 must rely almost solely on aerial transport of liquid water in the form of 

 rain, dew or fog for active growth and photosynthesis. Possible excep- 

 tions are the Polytrichaceous species which, especially at relatively high 

 humidities, may maintain turgidity through the transport of water from 

 the underlying organic or soil layer, within and along the stem. In the 

 tundra at Barrow, moss shoots were usually less than 2 cm in height 

 above the soil surface. The frequent saturation of the soils and the high 

 incidence of rain, fog and dew suggested that both capillary movement 

 of water from the soil surface and the application of liquid water directly 

 to the moss surface were important in supplying moisture to the moss. 

 Transpiration rates in mosses are potentially much higher than in 

 vascular plants because of low resistance to water loss. Calliergon sar- 

 mentosum is a mesic to hydric species and showed little resistance to des- 

 iccation. Pogonatum alpinum occurred in xeric to mesic locations and 

 showed numerous xerophytic adaptations, including a well-developed 

 cuticle, the abihty to roll the tissue margins over the photosynthetic 

 lamellae during desiccation, and the ability to fold the leaf tissue against 

 the stem. Oechel and Sveinbjornsson (1978) showed that with water con- 

 tents of 400%, a temperature of 24 °C, relative humidity of 20%, and 

 wind speed of 1.7 m s"', Calliergon lost 0.23 g H2O gdw"' min"' and Po- 

 gonatum only lost 0.03 g H2O gdw"' min"'. Both air resistances and tissue 



