134 THE BOOK OF NATURE STUDY 



or tiles on which minute algae are growing we can readily observe 

 the production of a lichen. In nature, the algae which are present 

 in lichens are very widespread, so that the formation of a young 

 lichen thallus depends largely upon the existence of suitable 

 external conditions. (3) Many lichens are largely propagated 

 by means of small brood bodies (soredes) budded off by the 

 thallus at the surface. Each sorede consists of a few alga cells, 

 or only one, surrounded by a web of fungus threads. In some 

 cases the soredes are produced in definite clumps, but more 

 often they form a powdery layer sprinkled over the thallus. 

 They are readily carried by the wind, and on reaching a suitable 

 place can at once produce a lichen thallus. This is well shown 

 in the " trumpet " or " cup " lichens (Cladonias), the numerous 

 young plants seen in spring being produced from the soredes. 



Lichens do important pioneer work in nature. They help to 

 break up or " weather " the surfaces of hard rocks, and prepare 

 soil on which other plants can grow. In woods they contri- 

 bute to the formation of humus (" leaf-mould "), when lichen- 

 covered twigs break off and decay in the soil. Many lichens 

 are the hardiest and longest lived of all living creatures. As is 

 well known, the vital activities of all plants are absolutely 

 dependent on the presence of water, which is the most important 

 factor in their environment. Many plants, however, are able 

 to endure a prolonged period of drought and retain their vitality 

 in a dried-up condition, the processes of assimilation, respiration, 

 and growth being suspended. This capacity is especially marked 

 in lichens, which 'have an unusually strong hold on life, and can 

 withstand drought and extremes of temperature for practically 

 any length of time. Lichens are very long lived, and once estab- 

 lished they grow very slowly, as a rule ; plants have been 

 observed to increase very little in size during half a century. 

 They are, however, very sensitive with regard to the composition 

 of the air, and cannot grow in the impure atmosphere of towns 

 and their neighbourhood. 



In some lichens the plant body (thallus) is rather like that 

 of a liverwort (e.g. Pellia) at first glance, while others might be 

 mistaken for fungi especially as the spore fruits of lichens are 

 similar in structure to those of the cup fungi. 



