134 



Aquatic Organisms 



during the warmer portion of the year, are among the 

 commoner constituents of the plancton. 



There are a number of filamentous blue-greens that 

 are more permanently sessile, and whose colonies of 

 filaments assume more definite form. Rivularia is 

 typical of these. Rivularia grows in hemispherical 

 gelatinous lumps, attached to the leaves and stems of 

 submerged seed plants. In autumn it often fairly 

 smothers the beds of hornwort {Ceratophylktm) and 

 water fern (Marsilea) in rich shoals. Rivularia is 



Fig. 52. Colonies of Rivularia on a disintegrating 



Typha leaf. 



brownish in color, appearing dirty yellowish under the 

 microscope. Its tapering filaments are closely massed 

 together in the center of the rather solid gelatinous 

 lump. The differentiation of cells in the single filament 

 is shown in fig. SiH. Such filaments are placed side 

 by side, their basal heterocysts close together, their tips 

 diverging. As the mass grows to a size larger than a pea 

 it becomes softer in consistency, more loosely attached 

 to its support and hollow. Strikingly different in form 

 and habits is the raftlike Merismopcedia {fig. 53). It 

 is a flat colony of shining blue-green cells that divide in 

 two planes at right angles to each other, with striking 



