506 



LIFE-HISTORIES 



lichens is by what are caUcd soredin,^ which are Uttle masses 

 of hyphte surroun(hng 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 mo.ss." Soredia arise through luxuri- 

 ant development of the inner cottony layer at certain points 

 where they i-upture the rind, and force their way to the surface 

 from which they eventuallj' 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 tliem clinging to the rocks of mountain peaks, 



Fiu. X'>4. — Ir-pland moss. f 'inss-sectioii of apothecium through thallus- 

 Iol)(\ showing the thalhis-rind (;', /•), the cottony interior mass of 

 hyphte (m, vij, among wliioh are green alg^, and tlae layer of spore-sacs 

 and paraphyses which form the hj-menium (h, h)\ somewhat diagram- 

 matic, Y- (Luersscn.) 



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

 the first plants to appear upon the newh' uncovered rock, thus 

 beginning that slow accmnulation of soil whiclr after many 

 centm'ics permits the growtli of liigher plants. For this 

 reason lichens have well l)een called Nature's jjioneers. Their 

 wonderful i)owei- of living upon the air, witli what the winds 

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

 beneficial co-operation between the alg;e and the fungi com- 

 posing the thallus. Eitliei- alone could not grow where both 

 together tlirive. Tlie algie of course are the food-making 

 members of the little community; while the fungus, living 

 upon the organic nraterials they provide, affords them pro- 

 tecti(in against too intense sunlight, soaks up the rain and 

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

 ' So-ro'di-uiii < Gr. -sonKs, a ticap. 



