506 LIFE-HISTORIES 



lichens is by what are called soredia, ' which are little masses 

 of hj'phie surrounding a colony of algae. Fig. 336, II shows the 

 soredium of a lichen known as beard-moss (Fig. 336, 1) nearly 

 related to the " Iceland moss." Soredia arise through luxuri- 

 ant development of the inner cottony layer at certain points 

 where they rupture the rind, and force their way to the surface 

 from which they eventually separate. Then being carried by 

 the wind to some favorable spot each grows into a new com- 

 pound thallus. The formation of soredia makes it possible for 

 lichens to gain a foothold where no other living thing could 

 grow. We find them clinging to the rocks of mountain peaks, 



Fig. .334. — Iceland moss. Cross-section of apothecium through thallus- 

 lobe, showing the thallus-rind (r, r). the cottony interior mass of 

 hyphse (w, ?n), among which are green algie, and the layer of spore-sacs 

 and paraphyses which form the hjTnenium {h, h); somewhat diagram- 

 matic, Y'. (Luerssen.) 



or in arctic regions, or deserts. After a land-slide lichens are 

 the first plants to appear upon the newly uncovered rock, thus 

 beginning that slow accumulation of soil which after many 

 centuries permits the growth of higher plants. For this 

 reason lichens have well been called Nature's pioneers. Their 

 wonderful power of living upon the air, with what the ^^^nds 

 and rain may bring them, is clearly the result of a mutually 

 beneficial co-operation between the algae and the fungi com- 

 posing the thallus. Either alone could not grow where both 

 together thrive. The algae of course are the food-making 

 members of the little community; while the fungus, living 

 upon the organic materials they provide, affords them pro- 

 tection against too intense sunlight, soaks up the rain and 

 dew and retains it sponge-like for a considerable time; and, 

 ' So-re'di-um < Gr. soros, a heap. 



