422 Ecology 



and stones as a bluish-black stratum. Symploca muscorum is frequent on 

 mossy ground and Nostoc commune often occurs in quantity on the surface 

 of cultivated land after a period of damp weather. 



Various species of Stichococcus frequently form a thin green stratum on 

 walls, damp stones and wooden palings, and Ulothrix gequalis is frequent 

 on damp shady banks. Coccomyxa subellipsoidea occurs as a mucous green 

 stratum on damp rocks and stones, particularly on the softer sandstones. 

 Mesottenium macrococcum occurs on damp rocks and M. caldariorum some- 

 times forms a stratum on the ground under the shade of trees. Vaucheria 

 terrestris and F. hamata are common Algae on damp soil, forming bright 

 green felts similar in outward appearance to moss-protonema. 



Porphyridium cruentum, which is a primitive member of the Bangiales, is a familiar 

 object at the base of damp walls and on the damp flag-stones of cold greenhouses, old 

 churches, etc. 



II. ASSOCIATIONS OF IRRORATED ROCKS. 



There are many associations of Algae on rocks which are kept constantly 

 wet by trickling water. Such Algte are really submerged, but with the 

 maximum of aeration. It is in damp mountainous regions that algal 

 associations of this kind occur in abundance and they are especially well 

 marked in the mountainous areas of the British Islands where the rainfall 

 varies from 40 to upwards of 100 inches. The Myacophycese (or Blue-green 

 Algw) are for the most part dominant. Pure algal strata are not uncommon 

 and often cover several square yards of rock-surface, but mixed associations 

 with Bryophytes are more frequent. These associations are very widespread 

 in the damper parts of both temperate and tropical countries, and it is not 

 too much to state that a great deal of the colouration of the landscape in 

 damp temperate countries results from associations of Blue-green Algae. 

 The same is true of the damper parts of the tropics, but on the whole to 

 a less degree, not because of the fewer Blue-green Algae, but because their 

 colouration there enters into a competition with coloured foliage such as 

 does not occur in temperate climates. 



It is on the dripping rocks in the deep glens and gulleys of mountainous 

 areas that the Blue-green Algae are most abundant. Many species of Nostoc 

 are found among the wet mosses, certain of the larger species of Stigonema 

 occur in tufted masses, and Scutonema Myochrous and S. mirabile often form 

 thick mats with bristling upstanding branches. Many species of Phormidium 

 occur in quantity, sometimes in pure sheets covering many square yards, as 

 may be the case with Ph. purpurascens (vide W. & G. S. W., '01). Various 

 species of Oloeocapsa and Chroococcus are often found in pure gelatinous 

 masses, although they are frequently abundant among filamentous types. 



