Ch.X1 THE LICHENS 461 



looser within; and it sends rhizoidal threads among the 

 inequalities of the substratum, to which it thus adheres. 

 In the thallus can be seen the masses of Green Alga, 

 around and against which the hyphse form a close mesh. 

 The reproduction in this form occurs chiefly through the 

 ascospores, formed in asci among paraphysesin the hymenium 

 of the apothecial cup, the entire arrangement being the 

 same as already described for the Discomycetes (page 450). 

 The Alga reproduces by division, keeping pace with the 

 growth of the mycelium. 



Among typical Lichens are the closely clinging or 

 crustose forms, often brightly colored, which occur upon 

 rocks, including boulders, in old pastures. Other kinds are 

 more leaf-like or foliose (Fig. 322), some of them, indeed 

 suggesting Liverworts (Bryophytes). Others branch freely, 

 almost like small shrubs, and are called fruticose, as in 

 case of the Reindeer Lichen and the Long Moss of northern 

 woods; while some are gelatinous or filamentous. The 

 color in the Tree Lichen is gray, but in some others the 

 Algse show through, making them greenish. Others con- 

 tain pigments which give them bright orange or red hues, 

 as in case of some kinds common on rocks in pastures. 

 They are mostly slow-growing and long-lived. The my- 

 celium of the Fungus, slightly gelatinous in its texture, has 

 high power of resistance to loss of water, with great capacity 

 to absorb it when offered. This feature, in conjunction with 

 the presence of photosynthetic Alga cells in the thallus, 

 permits the Lichens to grow in more barren and exposed 

 places than any other plants, even to bare rocks in alpine 

 and arctic regions, and in deserts. 



The Algse in the Tree Lichen are Protococcales, and 

 such Green Algse are the commonest in Lichens ; but Blue- 

 green Algse are frequent, and a few from other groups are 

 known to occur. All are kinds which exist by themselves 

 outside of the Fungus, though, almost without exception, 

 the Fungus is unable to exist without the Alga. This fact 



