506 | LIFE-HISTORIES 
lichens is by what are called soredia,: which are little masses 
of hyphe surrounding a colony of alge. Fig. 336, II shows the 
soredium of a lichen known as beard-moss (Fig. 336, I) 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 (7, r), the cottony interior mass of 
hyphe (m, m), among which are green alg, and the layer of spore-sacs 
and paraphyses whick form the hymenium (h, h); somewhat diagram- 
matic, 22. (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 winds 
and rain may bring them. is clearly the result of a mutually 
beneficial co-operation between the algze and the fungi com- 
posing the thallus. Either alone could not grow.where both 
together thrive. The alge 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, 
‘ 1$o-re’di-um < Gr. soros, a heap. 
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