230 PHYSIOLOGY 



a. Gelatinous Lichens. The algal constituent of these lichens is 

 some member of the Myxophyceae and is provided with thick gelatinous 

 walls which have great power of imbibition and swell up enormously in 

 damp surroundings, becoming reservoirs of water. Species of Collema, for 

 instance, when thoroughly wet, weigh thirty-five times more than when 

 dry^. There are no interstices in the thallus and frequently no cortex in 

 these lichens, but the gelatinous substance itself forms on drying an outer 

 skin that checks evaporation so that water is retained within the thallus 

 for a longer period than in non-gelatinous forms. They probably always 

 retain some amount of moisture, as they share with gelatinous algae the 

 power of revival after long desiccation. 



Gelatinous lichens are entirely dependent on a surface supply of water: 

 their hyphae — or rhizinae when present — rarely penetrate the substratum. 



b. Crustaceous non-gelatinous Lichens. The lichens with this 

 type of thallus are in intimate contact with the substratum whether it be 

 soil, rock, tree or dead wood. The hyphae on the under surface of the 

 thallus function primarily as hold-fasts, but if water be retained in the 

 substratum, the lichen will undoubtedly benefit, and water, to some extent, 

 will be absorbed by the walls of the hyphae or will be drawn up by capillary 

 attraction. In any case, it could only be surface water that would be avail- 

 able, as lichens have no means of tapping any deeper sources of supply. 



Lichens are, however, largely independent of the substratum for their 

 supply of water. Sievers^ who gave attention to the subject, found that 

 though some few crustaceous lichens took up water from below, most of 

 them absorbed the necessary moisture on the surface or at the edges of the 

 thallus or areolae, where the tissue is looser and more permeable. The 

 swollen gelatinous walls of the hyphae forming the upper layers of such 

 lichens are admirably adapted for the reception and storage of water, 

 though, according to ZukaP,. less hygroscopic generally than in the larger 

 forms. Beckmann^ proved this power of absorption, possessed by the upper 

 cortex, by placing a crustaceous lichen, Haematomma sp., in a damp 

 chamber : he found after a while that water had been taken up by the cortex 

 and by the gonidial zone, while the lower medullary hyphae had remained dry. 



Herre^ has recorded an astonishing abundance of lichens from the desert 

 of Reno, Nevada, and these are mostly crustaceous forms, belonging to 

 a limited number of species. The yearly rainfall of the region is only about 

 eight or ten inches, and occurs diiring the winter months, chiefly as snow. 

 It is during that period that active vegetation goes on; but the plants still 

 manage to exist during the long arid summer, when their only possible 

 water supply is that obtained from the moisture of the atmosphere during 

 the night, or from the surface deposit of dews. 



1 Jumelle 1892. ^ Sievers 1908. ' Zukal 1895. ^ Beckmann 1907. ' Herre 1911^. 



