LICHENS AND THE ROCK FLORA. 247 



These Parmelias and Physcias spread out in little 

 flat rosettes, and generally grow in a zonal or circular 

 manner. They consist of many rounded lobes, which 

 are, as a rule, whitish or brown on the under surface. 

 The prostrate habit is necessary as a protection against 

 wind. Their growth is very slow, about half an inch in 

 the year, though not so slow as is generally believed. 

 Even at the rate of half an inch in the year a patch 

 eight inches in diameter will be sixteen years old. 

 Pamnelia physodes, which is the regular greyish-white 

 lichen common on the bark of both small and large 

 tree branches, is the commonest of all lichens ; and 

 Parnielia saxatilis^ which often covers old walls and 

 dykes with hundreds of thousands of flat grey patches, is 

 almost as common. The lobes divide in a forked 

 {dichotomous) way and have a brown colour on the under- 

 side near the tip. Physcia parietina is very abundant 

 on rocks, walls, or palings near the sea. It is a con- 

 spicuous reddish-yellow, and can be distinguished by an 

 insect from a long distance. On reaching the patch the 

 insect visitor is at once attracted by the bright orange, 

 saucer-like cups (about i to i inch across) which stud 

 the surface, and crawls over them and covers itself with 

 spores. Other Parmelias and Physcias resemble P. 

 physodes and P. saxatilis in being grey, whilst others 

 again, are exceedingly small blackish or brownish patches 

 on the bark of trees. Many of them are very closely 

 pressed to the ground or to the bark, and are often difficult 

 to distinguish from the Lecanoras. Peltigera, and Gyro- 

 phora belong to the same tribe as Parmelias and Physcias. 

 The former has been already sufficiently described. 

 Gyrophora is generally found on mountain rock or 

 boulders in alpine districts, or upon seaside rocks and 

 stones. It is a mournful-looking plant, sometimes 



