WOODS AND FORESTS. 307 



for instance, a Parmelia will overgrow a Lecanora, and 

 occupy its place. They occur generally on the drier 

 and more exposed part of the twigs and branches. 

 On the sheltered side, there is often a continuous green 

 cushion of moss. The Mosses show a similar suc- 

 cession, and appear to grow best along the course which 

 is followed by the rain-water when trickling down the 

 stem. In very wet places the Alga, Pleurococcus, forms 

 a continuous green coating, which often wholly covers 

 the wettest side of the tree, though absent on that 

 which is exposed. Round the base of the trunk there 

 are very often colonies of the grey Trumpet-licliens 

 i^Cladonia) or a dense moss-carpet. Even ferns may be 

 sometimes seen on its branches in especially favourable 

 spots. Thus the bark flora is a very considerable one, 

 and is by no means without importance (see p. 309). 



The Male-fern, the Bracken (in pine woods), the Oak 

 and Beech ferns, and others, are the most characteristic 

 Wood-floor plants in Britain (see pp. 271, 287). Of 

 the flowers, Dog's Mercury, Stachys silvatica, Garlic, 

 Hyacinth, Foxglove, Campion, Luzula, Woodruff and En- 

 chanter's Nightshade are amongst the most abundant. 

 They are especially suited to the loose, light, leaf-mould, 

 in which their rhizomes can grow without difficulty, 

 and they appear in orderly succession one after another 

 all through the season (see pp. 17, 65). There are 

 also special wood grasses, such as Broinus asper, Brachy- 

 podium and Melica. But very often the surface, 

 especially in winter and spring, is matted over by a 

 profusion of mosses such as the feathery Hypnums and 

 Hylocomiuins, or it is covered by tufts of Biyurn or 

 Polytrichtuji. The covering shows in fact an extra- 

 ordinary variety of vegetation ; it varies from month to 

 month, so that many different plants are able to produce 



