Fink : Composition of a Desert Lichen Flora 99 



Relations to Moisture and Air Movements 



It still remains to discuss several factors which influence lichen 

 distribution in the area under consideration and at the same time 

 to state several further problems that may well receive attention 

 at some future time. Regarding the atmospheric conditions 

 likely to influence lichen distribution, relative humidity and air 

 movements are doubtless the most important factors. The rela- 

 tive humidity is known to be as low as eight per cent, of satu- 

 ration about the Desert Laboratory, at times of special dryness 

 in summer, and it varies from this to a high relative humidity 

 during the rainy seasons. The ordinary winds blow from the 

 east in the morning, later from the south, and by the middle 

 of the afternoon from the west, while the gales may come from 

 any direction. The drying winds from the east, south and 

 west, day after day, doubtless interfere somewhat with the devel- 

 opment of lichens on these three sides of outcrops of rocks, 

 accentuating the effect of direct sunlight, thus leaving the north- 

 ward-facing ledges by far the best habitats for lichens. 



Mr. V. M. Spalding has shown certain desert seed-plants to 

 absorb more or less water through their leaves and young shoots, 

 some of them as much as nineteen per cent, of their weight,* 

 and has also found that certain species of desert seed-plants 

 absorb a very small amount of water vapor from a nearly satu- 

 rated atmosphere, through their leaves and twigs.f He thinks 

 that this absorption of water and water vapor through the leaves 

 and twigs may be of some slight advantage. 



H. Jumelle has experimented with lichens in somewhat similar 

 fashion, in order to ascertain the amount of dryness of lichens 

 in their habitats. :[: He collected several widely different species 

 from trees and rocks, weighed them, placed them in a desiccator 

 and weighed them again after drying. Jumelle's first experi- 

 ments were performed upon lichens taken at a time when quite 

 dry and supposed to be in a latent condition, and he found the 



* Spalding, V. M. Biological Relations of Desert Shrubs. — II. Absorption 

 of Water by Leaves. Bot. Gaz. 41: 262-282. Ap 1906. 



t Spalding, V. M. Absorption of Atmospheric Moisture by Desert Shrubs. 

 Bull. Torr. Club 33: 367-375. Jl 1906. 



$ Jumelle, H. Recherches Physiologiques sur les Lichens. Rev. Gen. Bot. 

 4: 115. Mr 1892. 



