CHAPTER VII 



LICHENS 



LICHENS are plants which show great diversity in form, but 

 never have distinct stems and leaves like the mosses and leafy 

 liverworts. They are found in various habitats, but are especi- 

 ally abundant on heathy soil, on tree trunks, and on rocks and 

 walls, while a few grow in moist places among mosses and liver- 

 worts. 



Lichens form a quite exceptional group of plants with many 

 peculiar features. A lichen is a compound organism, consisting 

 of a fungus individual and numerous alga individuals. The fungus, 

 composed of branching and interlacing threads, has grown around 

 the algae, and enclosed them in a sort of nest. The result is that 

 the lichen can grow in places which would be quite unsuitable 

 for the independent existence of either the fungus or the algae 

 of which it is composed. Algae grow in water or in moist places, 

 and very few can live without a regular and abundant supply of 

 water, while (apart from the leathery and cakey bracket fungi) 

 fungi are, beyond most other plants, sensitive to cold and 

 drought. Yet lichens can thrive in the bleakest positions and 

 in the most severe climates, as on bare mountain rocks, where 

 at different times they may get no water for weeks on end, or 

 may be soaked with rain and mist for equally long periods, and 

 where they are exposed to the greatest extremes of heat and 

 of cold. In a typical lichen, like, for instance, Peltigera canina, 

 the fungus provides the organs of fixation ; protects the alga 

 cells, especially from drought, and causes them to be spread 

 out so as to catch the light ; absorbs water, with dissolved salts, 

 and air containing carbon dioxide ; and it alone produces the 

 spore fruits. The alga absorbs sunlight, and from the carbon 



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