430 



Ecoloyi/ 



M. staynorum ', Tribonema ujfine is not uncommon in peaty ponds and ditches. 

 Several species of Ophiocytiinn are very common in stagnant pools, especially 

 0. inajus and 0. parviditm. Species of Tribonema are not common in the 

 tropics and occur mostly in elevated areas, but the genus Ophiocytium is 

 quite as abundant as in any temperate region. Most genera of the Hetero- 

 kontse like shaded pools or those which are grass-grown at the margin. 

 Centritractus belonophorus and Botrydiopsis arrhiza occur as rare constituents 

 of the alga-flora of small ponds, although the former has been found in the 

 plankton of lakes. 



The Peridiniese of small ponds and ditches are not particularly numerous. 

 The commonest form in western Europe is most probably Peridinium cinctum 

 although several species of Gymnodinium are frequent. 



Fig. 268. Estimated relative importance of the several types of algal periodicity and the com- 

 position of the ala-flora at any time of the year in the waters of eastern Illinois. The 

 'irregulars' are not depicted. (After Transeau.) 



Fritsch ('06) suggested a division of small pieces of water into two series : 



a. Those containing Cladophoracea), in which the water is to a great 



extent fairly well aerated. 



b. Those from wliich Cladophoracea are absent, in which the aeration is 



much less. 



There is no doubt that the presence of perennial species of Cladophora 

 has not only a marked effect on the alga-flora of a pond but on the annual 

 cycle of that pond. There is, of course, always a struggle between Cladophora 

 and its epiphytes, and this is not much affected by the presence 'of other 

 Alga?. These epiphytes are, however, not confined to Cladophora and often 

 occur in abundance on FawcAma-filaments and on the leaves of aquatic 

 macrophytes. 



