line at the bottom of the talus. Conditions for growth 
here are more favorable, not only on account of soil 
accumulation, but on account of the partial shade, -again, 
the pioneer species consist of lichens, which become more 
abundant here than higher up on the more exposed portions 
of the slope. By far the most conspicuous is Parmelia 
oonspersa . which covers the top of nearly every fragment 
of rook (fig. 37). The humus derived from this speoies 
paves the way for fruticose forms, such as Cladonla rangi - 
ferina (L. ) Hoffm., Cladonia sylvatioa (L.) Rabenh. and 
Cladonia pyxidata (L.) Hofffti. Following the lichens or 
sometimes closely associated with them are three species 
of moss, namely, Thuidium abietinum (L.) Br. & Sch., Hed- 
wigia oiliata iShrh. and Ceratodon purpureus (L.) Bride. 
The two fern species already mentioned, Po'lypodium vulgare 
and Woodsia ilvensis , follow the lichens and mosses. They 
are very common along the bottom of the talus slope, send¬ 
ing their roots down between the slabs, an early stage 
of the mesophytic forest may then follow directly as shown 
in fig. 36, where the talus is fringed by white birch. The 
conspicuous ommission of the usual mat stage here is due 
to the ability of the white birch to invade tho talus be¬ 
fore almost any other woody form. Clumping specimens with 
several stems often occur on the bare talus, sending thoir 
