While there are probably many habitats on earth which are surrounded by a fluid in more or less unidirectional motion, the peculiarities of the cross section of a stream, its seasonal habits and its linear but ramifying nature are unique. The algae which inhabit it find themselves provided advantages and disadvantages which are not shared in the lenitic habitat. In general, the unpredictability of flowing water presents a danger which few organisms have met to complete satisfaction. The evolution of river organisms, while successful in several isolated segments of the animal and plant kingdoms, has found relatively few organims susceptible of being molded to this difficult last. A general description of the stream habitat can be found in several recent works.