LICHENS. 128 



walls, old posts and palings, on thatch and on the ground. 

 Wherever they are found they may be accepted as certificates 

 of the purity of the air. Formerly considered as a distinct 

 type, they are now held by the advanced school of cryptogam ic 

 botanists as commensals^ or partnerships formed between a 

 fungus and an alga. They are usually thin crusts, consisting 

 of an upper and a lower epidermis, formed of closely crowded 

 cells, and to the lower layer rootlike filaments are attached. 

 Between these layers are two differing elements ; a loose 

 stratum of green cells (gonidia\ which are said to be algce, and 

 below these a layer of fungoid threads. The contention of 

 the new school is that these alga; have been captured by a 

 fungus and held in bondage, being forced to elaborate starch 

 by means of their chlorophyll from the inorganic material 

 obtained by the rootlike filaments, which starch the fungus is 

 able to feed upon. Some of the green cells are pushed out 

 from time to time invested with a few wisps of fungus-threads, 

 and so reproduce the partnership. It is but right to add that 

 some good authorities on this branch of botany decline to 

 accept these views, and still regard lichens as independent 

 organisms and not partnerships. 



The species are very numerous, but their identification is 

 not easy, and requires serious application. The two figured 

 are exceedingly common in some districts. Various species of 

 Cup-moss (Cladonia) will be met on heaths, sandy hedge-banks, 

 etc. They have a flat crust-like base, from which arise pale 

 grey tubes or cups, bearing at their tips the bright scarlet, 

 pinky-brown, or even black fruits. A more common form in 

 woods and on banks is Cladonia pyxidata, with the tube 

 greatly increasing in width upwards. Cladonia rangiferina is 

 the well-known Reindeer-moss, of inestimable value in extreme 

 Northern latitudes as the food of the useful animal whose 

 name it bears ; it may be found in abundance in this country on 

 heaths and hillsides covering the ground beneath the heather. 



