This species is usually found in small pools, crevices in rocks hold- 

 ing rain water, and in concrete basins of various kinds. A familiar 

 habitat is the bird-bath in gardens. Because intermittent drying of 

 the habitat keeps this plant in a dormant condition most of the time, 

 it usually appears as a brick-red, slimy or granular encrustment on 

 the substrate. 



Mich., Wis. 



STEPHANOSPHAERA Cohn 1852, p. 77 



A colony of 4-8 ovoid cells with branched ( both lateral and polar ) 

 protoplasmic extensions, arranged in a median circumferential band 

 within an oblate-spheroid colonial mucilage. Cells free from, and 

 some distance from, one another, not connected by protoplasmic 

 extensions. Flagella 2; pigment-spot lateral, near the anterior end of 

 cell; chloroplast parietal and usually dense, with 2 or more pyre- 

 noids. 



Stephanosphaera pluvialis Cohn 1852, p. 77 



PI. 46, Fig. 26 



Characteristics as described for the genus. Cells 7-12.5/x in diam- 

 eter; colony up to 60/a in diameter. Reproduction by isogametes. 

 Zygote up to 28ix in diameter. 



Michigan. 



ORDER TETRASPORALES^ 



Plants belonging to this order are essentially volvocaceous, but 

 they are nonmotile in the vegetative condition. Some forms are free- 

 floating, but many are sedentary. The volvocaceous characters are 

 pseudocilia (non-functioning flagella), the pigment-spot, and the 

 type of chloroplast. All of the features may not be exhibited by all 

 members. Most forms are colonial, with the cells embedded in 

 copious mucilage, but there are unicellular genera. It is easy to 

 confuse some of the small free-floating colonies with certain plants 

 belonging to the Chlorococcales. Differentiation must be made by 

 considering a combination of characteristics and the type of repro- 

 duction used. The Tetrasporales are able to reproduce freely by cell 

 division in the vegetative state, but the Chlorococcales are all in- 

 capable of this method of reproduction. In general, the chloroplast 

 in the Tetrasporales ( Asterococcus excepted ) , is a parietal cup ( see 

 Elakatothrix, however), with a single pyrenoid. In the Chlorococ- 

 cales it is nearly always a parietal plate or sheet, not a Chlamydo- 

 monas-\ike cup. 



'Some authors, Fritsch (1935), Pascher (1927), for example, treat the Tetrasporales as 

 a suborder of the Volvocales. 



[81] 



