HOW TO KNOW THE FRESH-WATER ALGAE 



272a Plants stout; walls thick and lamellate; chloroplast a dense reti- 

 culum with many pyrenoids 273 



272b Plants slender, very minute (not more than 5 ^ in diameter); wall 

 thin; cross wall at the base of a branch above the level of origin 

 from the main axis. Fig. 191 MICROTHAMNION 



Fig. 191. Microthamnion strictissimum Rab. 



This species and its relative, M. Kuetzingianum 

 Naeg. are easily overlooked in collections because 

 the attached filaments are so minute and the chloro- 

 plast often so nearly colorless. They occur on 

 larger filaments of other organisms but usually 

 break away and are found floating free at ma- 

 turity, intermingled with other algae. 



Figure 191 



273a Branching open and spreading; cells mostly cylindrical. Fig. 192. 

 CLADOPHORA 



Fig. 192. Cladophora spp. a, cell showing pari- 

 etal, net-like or discontinous chloroplast; b, 

 habit of branching. 



There are numerous species of this genus 

 both in fresh and in salt water. They are dif- 

 ferentiated by size, shape of cell, and plan of 

 branching. The habit of branching is variable 

 according to habitat and species definitions, 

 therefore, are poorly made. Some plants exist 

 over winter in lakes and become wave-washed 

 and lose their original appearance. Other spe- 

 cies in lakes become free-floating and by wave 

 action become rolled over and over with the 

 result that "Ciadophora-balls" are produced. 

 These appear as densely branched and en- 

 tangle 1 growths, with cells very irregular in 

 shape. Perhaps the most characteristic habitat 

 of Cladophora is on rocks in flowing water, es- 

 pecially on dams and waterfalls. 



120 



Figure 192 



