Aquatic associations 423 



Many genera other than those mentioned are to be found, sometimes sparingly 

 and occasionally in abundance, giving an extremely varied character to these 

 Blue-green associations. 



Desmids are not uncommon on dripping rocks and some are particularly 

 characteristic of such habitats. They usually occur amongst the wet 

 Bryophytes, but they not infrequently form pure gelatinous masses. The 

 following are some of the typical wet-rock desmids : Mesotasnium chlamydo- 

 sporum, M. De Greyii, M. macrococcum, Cosmarium anceps, C. cymato- 

 pleurum var. tyrolicum, C. didi/mochondrum, 0. dovrense, C. Etchachanense, 

 G. Holmiense, C. microsphinctum, C. nasutum, C. pseudarctoum, C. speciosum, 

 C. sphalerostichum, C. siibexcavatum var. ordinatum, C. tumens, Staurastrum 

 Meriani and St. pileolatum. 



A few diatoms are also typically wet-rock types, and among them 

 Melosira arenaria and Navicula borealis deserve special mention. The 

 former often occurs as coarse mats on dripping sandstone rocks and the 

 latter, although abundant in various habitats in high latitudes or at altitudes 

 of over 1000 feet, is in some areas a conspicuous feature of the algal associa- 

 tions which occur among the mosses of wet rocks. Melosira Roeseana occurs 

 in similar habitats. 



In limestone areas certain of the Blue-green Alga? build up calcareous 

 deposits. Dichothrisc gypsophila is a notable example (consult p. 34 and 

 fig. 21). 



Mention should not be omitted of Hildenbrandtia rivularis, which on irrorated rocks 

 forms red incrustations of a very striking character. 



III. AQUATIC ASSOCIATIONS. 



The truly aquatic associations of freshwater Algae may be dealt with 

 under four headings : 



A. Swiftly running water. 



B. Bogs and swamps. 



C. Ponds and ditches. 



D. Pools and lakes. 



Unlike the formations of terrestrial plants, many aquatic associations 

 vary greatly from season to season. There is usually a marked periodicity, 

 a number of dominant forms succeeding one another in the course of twelve 

 months. Thus, such associations have their different phases and pass through 

 an annual cycle. 



In other cases the same association may occur all the year round, with 

 little variation for a number of years. This is well exemplified in Sphagnu/m- 

 bogs, possibly owing to the relatively uniform conditions which obtain in 



