118 PLANT-LIFE 



not necessitated. The formation of soredia has not 

 been observed in Physcia parietina>. 



There is some diversity of form among the Lichens. 

 We find species with flattened stems and erect branchings 

 growing on rocks on the seashore and on old trees. 

 The familiar " Old Man's Beard " Lichens grow on the 

 bark of trees in old woods; they are of two genera, 

 Usnea and Alectoria ; the former occur as upright tufts, 

 and the latter hang from branches, growing to a length 

 of nearly a foot. The Cluster Cup Lichens (Cladonias) 

 bear cup or trumpet-like parts, about an inch in height, 

 on which soredia and spores appear. Physcia is one of 

 many leaflike or foliaceous species. The crustaceous, 

 or encrusting, kinds form scales, crusts, or the appear- 

 ance of a stain, on rocks, bark, etc. ; they stick exceed- 

 ingly tenaciously to their substrata. There are also 

 gelatinous forms, such as Collema pulposum, in which 

 the Algal-partners are evenly distributed throughout the 

 thallus, and thus are not confined to any particular zone, 

 as is the case in the type we have just studied. 



Lichens, from a human view-point, are not utterly 

 useless. The Eeindeer-moss Lichen, to which the word 

 " moss " ought never to have been applied, serves as food 

 for reindeer. Iceland Moss (Cetraria islandica) makes 

 excellent jellies. Various species yield dyes of excellent 

 quality, but they are falling into disuse owing to being 

 superseded by aniline dyes. Yet in remote places, such 

 as the Highlands of Scotland, Lichens are still occasion- 

 ally used in preparing dyes. The colouring material 

 known as Archil, or Orchil, is made by treating pow- 

 dered Lichens with ammoniacal liquor. Cudbear, used 

 of old as a purple dye for woollen yarn, was prepared 



