PHYCOPHYTA. 135 



Order 3. FSOTOCOCCOIDE^. The Green Slimes. 



236. Common Green Slime may be taken as the represen- 

 tative of this order. It consists of minute, globular, green 

 cells, and is to be found as a thin green layer on damp 

 walls and rocks and the sides of flower-pots in greenhouses 

 and conservatories, and in wet weather on wooden walks 

 and the roofs and sides of houses. Green. Slimes are com- 

 monly known under the name of Protococcus, although 

 species of other genera are more common. 



237. They reproduce asexually by fission, each cell divid- 

 ing into two, and also by the formation of zoospores which 

 swim about for a time, after which they form a cell-wall 

 and develop into new plants. The zoospores of some Green 

 Slimes unite sexually and produce resting spores. 



238. One kind of Green Slime (HEematococcus lacustris) 

 is the noted Eed-snow Plant, which in the high north lati- 

 tudes often covers the snow, giving 

 it a reddish color. It also occurs on 

 the mountain-tops in lower latitudes. 

 Although really a green plant, its 

 color is reddish in one of its stages. 



239. Eelated to the foregoing are 

 the curious little lunate plants (spe- 



^ ^ -^ FiQ. 66.— Green Sllmea, 



cies of Scenedesmus) which always lie magnified, a, Protococ- 



' ■' ous ; 6, Scenedesmus ; c, 



side by side in fours, and the some- Pediaatrum. 



what similar species of Pediastrum, consisting of a flat 



colony of 4 to 64 angular and loosely aggregated cells. 



240. The Water-net (Hydrodictyon) is one of the most 

 curious of the common plants of pools and slow streams in 

 midsummer. Well-grown specimens are from 30 to 30 

 centimetres long (8 to 12 inches), and consist of an actual 

 net made of cylindrical cells joined at their ends. The 



