306 ^ PLANT LIFE. 



often artificially introduced. Amongst other typical 

 undergrowth plants are the Guelder Rose, Dogwood, Elder, 

 and Holly, as well as many foreign species which are 

 chosen for ornamental purposes, such as Laburnum, 

 Lilac, Maple, and occasionally Rhododendron. Though 

 very often absent, there are interesting points about 

 this undergrowth, especially as regards what happens 

 at the edges of the wood, or after the wood has been 

 blown down. 



The Bark-flora is in Britain almost entirely Crypto- 

 gainic. In other countries, flowering plants continually 

 establish themselves in the forks of all old branches, or 

 even on horizontal boughs. There are no bark Orchids, 

 Aroids, or Figs, in British woods. Even the Mistletoe is 

 very rare. The only representative of these " Epi- 

 phytes " is an occasional Gooseberry Bush, which may be 

 discovered perched on an old tree, and developing 

 quite satisfactorily. 



On the other hand, there is an interesting series of 

 Lichens and Mosses. Those found on the youngest 

 twigs are generally lichens, especially Lecanora Subfusca^ 

 or some species allied to it. On rather older branches 

 various olive brown Physcias or grey Parmelias make 

 their appearance. Evernia prunastri and E. furfuracea 

 are also very common on the older trees. If these 

 latter are well grown, the bark of the trunk may be 

 nearly covered by a profusion of grey lichens, such as 

 the dark leaden-grey, white-dotted patches oi Pertusaria, 

 the leaf-like fronds belonging to Paimelia physodes^ and, 

 on very old trees, the greenish-grey Usneas and the 

 drooping olive-grey tangles of Alectoria flexuosa (see 

 p. 244). 



These lichens follow one another in a regular order, 

 and it is easy to trace the sequence by observing how, 



