262 W. A. CANNON 



can withstand considerable loss of water, as, for example, the 

 vegetative organs of some flowering plants which were not killed 

 by a loss of 90%. Quoting Schroder, Pfeffer says that the suc- 

 culent Sedum elegans survived a water loss of 90%, but Parie- 

 taria arhona succumbed from a loss of 70 to 76%, and was partly 

 killed when the loss was 50%. The "lace lichen," Ramulina 

 reticulata, air-dry, according to Peirce, can absorb so much water 

 that its wet weight is 2.04 times its air-dry weight.^ At the end 

 of a long dry summer, Peirce^ found, by drying the lichen in an 

 oven until constant weight was achieved, that there had been 

 13.9% water in the air-dry condition. Although following des- 

 iccation the plant naturally would not recover, it is not unlikely 

 that it could withstand a greater water loss than that experienced 

 by it as a result of the suimner's drought. At least such is true 

 of certain liverw^orts as established by Humphrey^ who found 

 that certain species, air-dry, could yet lose from 4.57 to 12.3% of 

 water and survive. Such forms, therefore, have a considerable, 

 although varying, margin of safety which would stand them in 

 good stead in unusually dry seasons. 



In comparing the leading climatic features of the region in- 

 habited by the liverworts studied by Humphrey with the region, 

 to be characterized directly, in southern Arizona where the liver- 

 wort, of which the present note especially treats, is indigenous, we 

 find certain points of agreement, although in other points, which 

 are definitive, they are unlike. For example, in both regions, 

 the rainfall is periodic, with a season of drought between the sea- 

 sons of rain. In California there is no regular rainy season in 

 summer. The California summers, however, in regions near the 

 sea, are visited by fogs, or dew, which probably serve to mitigate 

 to a degree the severity of the drought. The dry seasons in Ari- 

 zona, especially the foresummer, are without dew or fog, and, 



^ On the mode of dessimination and on the reticulation of Ramulina reticulata. 

 Bot. Gaz. 25. 1898. 



* The nature of the association of alga and fungus in lichens. Proceed. Calif. 

 Acad. Sci. 3d Ser. Botany, 1: 7. 1899. 



^ Studies in the physiology and morphology of some California Hepaticae. 

 Proceed. Washington Acad. Sci. 10. 1908. 



