" REINDEER MOSS'' LICHEN 



reindeer, which in winter clear away the snow with their horns 

 to browse on the lichen, and is not despised by mankind when 

 other food is scarce. In Britain the "reindeer-moss" (Fig. 39) 

 is seldom over three inches high, the mealy stalks growing in 

 branching tufts with fine drooping tips on which the fruits are 

 produced. 



The adaptations of lichens to their habitats offer an attrac- 

 tive and little-explored field for observational work. Apart 

 from heaths, which are characterised by Cladonias and similar 

 forms with erect assimilating branches, there are three chief 

 lichen habitats tree-bark, rocks, and moist and mossy places. 

 On trees the encrusting lichens follow the surface of the bark, 

 growing to keep pace with its extension and its cracking as 

 the trunk thickens year by year. On stones the lichens often 

 have the thallus sunk below the surface ; if growing on the sur- 

 face, it often shows furrows in which water can collect, and as 

 it grows it becomes raised into ridges or wart-like projections. 



Dry periods appear to be absolutely essential to the well- 

 being of most lichens, especially to those which grow on trees 

 and stones. These soon die and decay when kept moist arti- 

 ficially ; the alga cells grow and divide rapidly, but the fungus 

 tissue becomes waterlogged, and the lichen becomes deformed 

 and perishes. Lichens, like Peltigera, which grow in moist 

 places, can be cultivated when kept damp, though they also 

 retain the power to resist drought. 



It is obvious that the dryness of lichen habitats is due to 

 the nature of the substratum rather than to actual lack of rain. 

 Tree trunks, rocks, stones, and heathy soil, on which lichens 

 grow most luxuriantly, are not well adapted for retaining rain- 

 water, which either runs off quickly (tree trunks, rocks), or is 

 rapidly absorbed and sinks downwards (peaty or sandy soil). 

 Water is absorbed over the whole surface of the thallus ; the 

 rhizines, which are often poorly developed, serve mainly for 

 fixation, and, as we have seen (Usnea, etc.), the plant may be 

 attached at one point only. A dried-up and dead-looking lichen 

 can in a few minutes become saturated with falling rain or dew 

 the lichen's only sources of water and the surface layers become 

 transparent when wet, so that the underlying green alga cells can 



