THE ALG.E 



229 



VaucTieria has three methods of reproduction yegetatiye, 

 by asejtuaLspores (zoospores), and by sexuaLspoges (oospores). 



One plant may use vegetative reproduction at one period 

 of growth, asexual spore reproduction at another, and sex 

 spore reproduction at another, but two methods are rarely 

 used at the same time. 



215. Other green algae. Although 

 in inland waters green algae are more 

 abundant than all others, only two 

 other genera will be mentioned in 

 this connection. Sphcerella, a unicel- 

 lular form somewhat like Pleurococcus, 

 is frequently found in stagnant water. 

 It sometimes grows so luxuriantly in 

 barnyard and roadside pools as to 

 give the water a bright green appear- 

 ance, and its resting spores may im- 

 part a dark red color to drying pools 

 in which the plants have flourished. 

 Chara, or stonewort (fig. 179), is a 

 complex alga that is found in great 

 abundance on the bottoms of shallow 

 lakes and streams throughout the 

 continent. It has a heavy coating of 

 calcareous material, which, as the 

 plant dies, falls to the bottom of the 

 pond or stream. Chara grows in such 

 luxuriance that its deposits eventually 



form deep layers of this calcareous material, or marl, as it is 

 called. Marl has been found of great value as one of the mate- 

 rials used in the manufacture of cement, and not a few of the 

 lakes in which Chara grows are dredged to secure the marl 

 deposits for this important manufacture. 



216. Algae and water supply. Many of our large cities have 

 found it advisable to adapt or construct reservoirs for water 

 supply. These are open pools, lakes, tanks, etc., and they are 



FIG. 179. The stonewort alga 

 (Chara) 



A, a slightly magnified piece of 

 a plant, showing the general 

 appearance ; -B, a more highly 

 magnified illustration showing 

 the oogonium (o) and the an- 

 theridium (a), by means of 

 which reproduction takes place 



