2] THE VARIOUS GROUPS OK PLANTS 47 



the ' algal ' element is enabled, by the protection afforded by the 

 fungal envelope, to live in dry and exposed situations where other- 

 wise it could not exist, while the Fungus derives food which results 

 from the photosynthetic activity of its partner in places where 

 otherwise none would be available. 



The ' algal ' elements in Lichens may be members of either the 

 Chlorophyceae or the Cyanophyceae and, like the Fungi concerned, 

 are usually definitely identifiable. With the varying combinations 

 involved, as well as differing heritages, habitats, and growth 

 tendencies, a vast array of different forms of Lichens result, though 

 their growth tends to be very slow. In size, Lichens vary from 

 minute to some which are whole metres in diameter. In colour 

 they may be of almost every conceivable shade, being often of 

 brilliant hue ; on the other hand a great many are a dull greenish- 

 grey, as a result of combination of the colours of the components. 

 The main groups of forms are (i) ' crustose ' (crustaceous), forming 

 a thin crust over (or sometimes mainly beneath the surface of) the 

 rock or other material on which they grow, (2) ' foliose ', being 

 more or less prostrate and fiat, with leaf-like lobes, and (3) ' fruticose ', 

 being upgrowing, branched, and often bush-like, or pendulous from 

 the branches of trees. A range of types is shown in Fig. 12. 



The nutrition of Lichens is primarily by the photosynthesis of 

 their ' algal ' components, in which connection the fungal element 

 may in a sense be considered parasitic. Vegetative reproduction 

 is by fragmentation of the plant body, especially when this becomes 

 old and decrepit and liable to break down, or by special structures 

 termed soredia, which are groups of fungal filaments interspersed 

 with ' algal ' cells. These soredia become separated from the 

 parent, being often produced in large numbers, and blow about 

 easily. Sexual reproduction is confined to the fungous partner and 

 follows its particular pattern, usually involving the formation of 

 spores in a special structure, following a fusion of gametes. On 

 germination of these spores, new lichen plants are formed only if 

 some fragment of the appropriate Alga or Schizophyte is present. 



Lichens occur plentifully in a great variety of habitats, usually 

 of dryish nature. Thus they favour the trunks and branches of 

 trees, exposed rocks, and bare ground provided the surface is stable. 

 In these and other situations they are to be found practically every- 

 where on land, though shunning large cities owing to their sensitivity 

 to fumes. Although particularly characteristic of high mountain 

 peaks and of arctic and antarctic regions, where they may dominate 



