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Mycologia 



tion of how much moisture the lichens of the rocks in the 

 desert may be able to obtain from upward passage through the 

 rocks. It is well known that rocks are more or less porous and 

 that the pores are larger toward the surface, porosity ranging 

 from less than one per cent, to thirty per cent, or more. C. R. 

 Van Hise states that water may rise 166 meters by capillarity, 

 that, after it has ascended as high as it can by capillarity, it will 

 still, through molecular attraction, creep along the walls of the 

 pores " from areas of greater to areas of less humidity," and 

 that there is no limit to such movement.* 



No reliable data are at hand regarding the porosity of the 

 tuffs and basalts of Tumamoc Hill and the amount of water that 

 reaches the surface through them. The basalt is more dense and 

 less porous than the tuff, and the latter may, like the soil, give 

 off moisture so rapidly as to become too dry to support lichens 

 very successfully through periods of prolonged drought. The 

 less porous basalt doubtless gives off water vapor coming up 

 from great depths very slowly, but perhaps in sufficient quantity 

 to keep the lichens growing on these rocks alive in the driest 

 times known in the desert; at least on surfaces often wet during 

 the rainy season, and especially on northward-facing exposures 

 where the effect of prolonged drought is felt least. Mr. Spald- 

 ing stated, in answer to inquiry, that the lichens seem to be quite 

 as numerous on large boulders as on the rock exposures in situ. 

 The boulders do not extend to great depth ; but if they extend 

 below the lower limit of evaporating surface in the soil in driest 

 times, the problem of distribution of lichens on them might not 

 differ materially from that of their distribution on other rocks. 

 This question of water supply for the lichens from the rocks 

 below them is well worth investigation at the Desert Laboratory. 

 It is certain that the supply obtained in this way is not alone 

 sufficient to sustain lichens, for observation proves that these 

 plants do not grow on desert rocks perennially dry. But the 

 moisture thus obtained may be sufficient to keep the lichens alive 

 during periods of extreme dryness, in situations where they may 

 obtain moisture otherwise during rains and wet seasons. 



* Van Hise, C. R. Treatise on Metamorphism. No. 753. Geol. Surv. 

 Mon. 47: 151. 1904. (House Documents, vol. 83.) 



