26 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. - 
corresponding to this drier substratum, not only at station III but at 
other points, the Acarosporas are well represented upon it. 
Of atmospheric conditions likely to influence the distribution of lichens, 
relative humidity and air movements are undoubtedly the most important 
factors. The relative humidity is known to be as low as 8 per cent of 
saturation at the Desert Laboratory at times of special dryness in summer, 
and it varies from this to a high percentage during the rainy seasons. 
The drying winds from the east, south, and west, day after day, doubtless 
interfere to some extent with the development of lichens on these three 
sides of rock exposures, accentuating the effect of direct sunlight, thus 
leaving the northward-facing ledges by far the best habitats. 
It has been shown by Spalding (1906) that certain desert seed-plants 
absorb more or less water through their leaves and young shoots, some of 
them as much as 1g per cent of their weight, and he has also found that 
some species absorb a very small amount of water-vapor from a nearly 
saturated atmosphere, through their leaves and twigs. It is thought that 
this absorption of water and water-vapor through subaerial parts may 
be of some slight advantage. Jumelle (1892) has experimented with 
lichens in somewhat similar fashion. His experiments show that lichens, 
compared with seed-plants, contain very little water at any time, but 
that the former are much more able to absorb water or water-vapor 
in the proportions needed than are the latter, according to the results 
obtained by Spalding. 
Aside from the gelatinous species, lichens need but little water and 
are able to obtain all that is required through the general surface, instead | 
of through specialized organs, as roots. It may be assumed from these 
investigations, until otherwise proven, that lichens are able to absorb 
at least a large proportion of the moisture needed directly from water- 
vapor of the atmosphere and from water falling upon them in the form 
of rain. As far as appears, however, the statements of Jumelle do not 
rest upon experiments made on any desert lichens, and similar investiga- 
tions of some of the lichens about the Desert Laboratory would be cer- 
tain to yield instructive results. 
Experiments such as those made by the authors referred to, performed 
upon the crustose lichens of the desert in the driest condition in nature 
and giving the relation between fresh and dry weight, would give data 
regarding the amount of moisture retained in lichen thalli during the 
driest times. Accompanying this should go observations regarding the 
length of time that these lichens may be kept dry and then resume active 
respiration and assimilation on the return of favorable conditions. Since 
lichens have no special storage organs, nor organs of absorption extending 
any considerable distance into the substratum, the retreat of the evap- 
orating surface into the soil leaves any lichens growing upon exposed soil 
entirely in an air-dry environment, and under such conditions the ability 
to absorb water-vapor from the atmosphere and to quickly take up water 
when there are light rains would be a distinct advantage. 
It is still a question to what extent the rock-inhabiting lichens of the 
desert may be able to obtain moisture from the rocks on which they grow. 
The basalt is, in general, more dense and less porous than the tuff, and 
it seems likely that the latter may give off moisture so rapidly as to be- 
come too dry to support lichens very successfully through periods of 
prolonged drought. The less porous basalt doubtless gives off water- 
vapor very slowly, and this would be more favorable to the growth of 
