Subaerial associations 421 



British Islands, occurs almost entirely on the vertical faces of rocks facing 

 the 'drive' of the wind. This species ascends to 1200 feet in the Pennine 

 Chain. In the tropics many species of Trentepohlia are leaf- epiphytes. 



Belonging to the same family, the Trentepohliaceas, are the genera 

 Phycopeltis and Cephaleuros, most of the species of which are epiphyllous. 

 One species of Phycopeltis occurs in Europe, but all the known species of 

 Cephaleuros are tropical and one of them, Cephaleuros virescens, is a destruc- 

 tive parasite (consult p. 310). In some species of all the genera of the 

 Trentepohliacese the cells contain a quantity of the red pigment hoemato- 

 chrome and are thus able to live in situations where they are exposed to 

 light of strong intensity. 



Phyllosiphon (fig. 156) and Phytophysa (fig. 266), both of which belong 

 to the Phyllosiphonacese, have become partial parasites, the former in the 

 leaves of various genera of the Araceoe and the latter in the stems of Pilea, 

 a tropical genus of the Urticacea?. 



In damp tropical areas leaf- and bark-epiphytes are largely composed of 

 the Myxophycea\ On the bark of trees they often occur amongst a more- or 

 less prolific growth of Bryophytes. There are various associations in which 

 the following genera are abundantly represented : Hapalosiphon, Stigonema, 

 Scytonema, Schizothrix, Phormidium, Chroococcus and Gloeocapsa. Other 

 genera are also represented to a less degree. Fritsch ('07 B and '07 c) has 

 remarked upon the abundance of such associations in Ceylon, but it seems 

 that in certain of the mountainous islands of the West Indies these 

 Blue-green associations attain their maximum development (consult W. & 

 G. S. W., '94; '99) and amongst the tangled mass of threads many other 

 Algae also occur. In the island of Dominica various genera of desmids, 

 diatoms and of the Protococcales habitually live on trees ! 



One minute diatom Navicula contenta is chiefly found as an epiphyte 

 on the leaves of trees. 



5. Miscellaneous 'formations' and associations on rocks and on damp 

 ground. Among the Blue-green Alga? there are several more or less definite 

 subaerial formations. 



In the north-west of Scotland and in the Hebrides there is a Gloeocapsa 

 ma^ma-formation on the damp ground. It is sometimes sufficient in amount 

 to give a distinct reddish -brown colouration to wide areas. 



In West Africa Porphyrosiphon Notarisii may carpet the ground exten- 

 sively, covering wide areas with a reddish-brown felt. In the same part of 

 the world Scytonema Myochrous var. chorographicum is the cause of the 

 'pedras negras' of Angola (vide Welwitsch, '68; W. & G. S. W., '97). 



In temperate climates many kinds of subaerial Algoe occur on the ground 

 and on damp rocks and stones. Much the most frequent Blue-green Alga 

 is Phormidium autumn ale, which is found almost everywhere on damp ground 



