PSOTOPHYTA. 127 



earth or the wet sides of watering-troughs, etc. A pecul- 

 iarity of these plants is their power of oscillating from 

 side to side, while at the same time they move forward. 

 In this manner they are enabled to travel considerable dis- 

 tances. 



223. In Eivularia the filaments are generally arranged 

 radially in little rounded masses. One of these (Eivularia 

 fluitans) is often very abundant in lakes and slow streams, 

 the little floating greenish balls being a millimetre or less 

 in diameter. Other species occur as green slimy masses, as 

 large as pin-heads, on the stones and stems of water-plants 

 in ponds and brooks. 



Practical Studies. — (a) Scrape off a little of the greenish slimy 

 matter from a damp wall, mounting it in water ; examine under a 

 high power. Some small blue-green or smoky-green cells will he 

 found belonging to the Blue-green Slimes (Chroococcus, etc.) ; of 

 these some will probably be found in process of fission. Larger 

 bright-green cells filled with granular protoplasm will also be found ; 

 these are a species of Protococcus (par. 236). 



(&) In midsummer look along the water-line of fresh-water lakes 

 and ponds for soft, amber-colored, rounded masses from the size of a 

 pea to that of a hickory-nut. By mounting a small slice of one of 

 these it will he seen under the microscope to be composed of myriads 

 of filaments of Nostoc similar to A, Fig. 61. Occasionally a filament 

 may be seen with a larger cell (a heterocyst), as in the figure. Its 

 function is not known. 



(c) Secure a handful of the dark-green filamentous growth which 

 is common on the wet sides of watering-troughs, and place it in a 

 dish of water. If an Osoillaria (Fig. 61, B), it will rapidly disperse it- 

 self, an hour being long enough to show quite a change in position. 

 Now mount a few filaments in water and examine under a high 

 power. They will be seen to sway from side to side, and to move 

 quite rapidly across the field of the microscope. 



(d) In midsummer scrape off one of the small jelly-like masses of 

 Eivularia, so common on the submerged stems of water-plants ; mount 

 in water, crushing or cutting the mass so as to show the individual 

 filaments. Each filament tapers from the centre of the mass outward, 

 and at its larger end there is generally a large cell (a heterocyst). 



Systematic Literature. — Wolle, Fresh-water Algse of the United 

 States, 335-335, Flora of Nebraska, 1. 15-25, pi. IS. 



