358 Annals New York Academy of Sciences 



vicinity. These include massive sheets which cover rocks in the current and 

 may extend partly into the current as in Phormidium spp.; parenchymatous 

 or pseudoparenchymatous collections of cells; soft, gelatinous masses that 

 move slightly in the current (some diatoms such as Gomphonema olivaceum 

 (Lyngb.), Kiitz. (figure 2); and firmer, spherical, or hemispheric masses which 

 are frequently gelatinous as in Rivularia spp. (figure 3). The gelatin in these 

 types serves to lubricate the alga-current interface and to reduce friction and 

 injury to the plant but it also serves to separate adjacent trichomes or filaments 

 and to keep, in many algae, a rather precise spatial relationship between fila- 

 ments as they lie in their intercellular material (figure 3). 



When fresh water algae, generally, are compared and contrasted with marine 

 algae, the essential absence from the former of massive plant bodies, leathery 

 and foliose types which are so common in the marine Rhodophyta and Phaeo- 

 phyta is noteworthy. Although the Phaeophyta have proven generally un- 

 successful in fresh water and would not really be expected to produce such 

 plant forms in any event in fresh water, the same is not true of the Rhodophyta 

 or of the Chlorophyta. Nevertheless, the latter groups are not represented 

 in fresh water by forms more massive than Tuomeya, Lemanea, Chaetophora, 

 or Monostroma. 



The evolution of fresh water algae has thus been successful largely for the 

 smaller, more delicate forms which are characteristic of standing water rather 

 than of currents. If we suppose that the rather specialized current algae have 

 evolved at least in part from their fresh water relatives that are tolerant of 

 standing water, it must be granted that their form has not been greatly modified 

 by the change in habitat. 



References 



Blum, J. L. 19vS4. Two winter diatom communities of Michigan streams. Pap. Mich. 



Acad. Sci. Arts, Lett. 39: 3-7. 

 Blum, J. L. 1956. The ecology of river algae. Bot. Rev. 22: 291-341. 

 Cedergren, G. R. 1938. Reofila eller det rinnande vattnets algsamhallen. Svensk. Bot. 



Tidskr. 32: 362-373. 

 Gessner, F. 1955. Hydrobotanik. Vol. 1. VEB Deutscher Verlag der Wissenschaften. 



Berlin. 

 Lastochkin, D. 1945. Achievements in Soviet hydrobiology of continental waters. Ed. 



G. E. Hutchinson. Ecology. 26: 320-331. 

 PiCKEN, L. E. R. 1936. Mechanical factors in the distribution of a blue-green alga, Rivularia 



haematites. New Phytol. 35: 221-228. 

 Steeman-Nielsen, E. 1947. Photosynthesis of aquatic plants with special reference to 



the carbon sources. Dansk Botan. Arkiv. 12: 1-71. 

 Wallner, J. 1934. Beitrag zur Kenntnis der Vaucheria-Tuffe. Zentr. Bakteriol. Parasi- 



tenk. 2(90): 150. 

 Wehrle, E. 1942. Algen in Gebirgsbachen am Sudostrande des Schwarzwaldes. Beitr. 



Naturk. Forsch. Oberrheingebiet. 7: 128-286. PI. 1-3. 

 Whitford, L. a. 1960. The current effect and growth of fresh-water algae. Trans. Am. 



Microscop. Soc. 79: 302-309. 



