THE ALG^E. 81 



References for Reading. Carpenter's "The Microscope," pp. 479- 

 485 ; Goebel's " Classification of Plants," pp. 34-38 ; Bennett and 

 Murray's "Cryptogamic Botany," pp. 292-295; Parker's "Elementary 

 Biology," pp. 267-270 ; Sachs' " Physiology of Plants," pp. 728-730 ; 

 Vines' " Text-Book of Botany," pp. 237, 238. 



The Protococcaceae. A large number of primordial 

 plants are included in this class ; but, inasmuch as their 

 life histories are only imperfectly known, it is not certain 

 that plants described under different names are not dif- 

 ferent forms of the same plant. In some plants no alter- 

 nation of generations from gametophyte to sporophyte is 

 known ; in other cases such an alternation is distinct. 



Among the most common plants are the different forms 

 of Protococcus. The commonest of these is Protococcus 

 viridis. Protococcus pluvialis is a form containing enough 

 of a red pigment to give a decided red spot to each cell; 

 it is found in stagnant rain water. Protococcus nivalis, 

 or red snow, is the interesting plant that sometimes coats 

 the snow with red in a single night 

 in the higher latitudes and on the 

 mountains. 



Other common forms are Chloro- 

 coccus, occurring in small aggre- 

 gations of cells, each of which is 

 surrounded by a comparatively thick 

 envelope of a gelatinous nature , Ra- FlG . 38. _ scenedesmm ob- 

 vhidium, occurring as long, slightly tys ' us Me > T - ( x 400. ) 



e (From Bennett and Mur- 



curved cells pointed at each end, ray .) 



either singly or in small groups, with FlG - 39 - - Kaphidium fai- 



6 J catum Ktz. ( X 800. ) 



the Cells Crossing One another at (From Bennett and Mur- 



their middle portions ; Scenedesmus, ray ^ 

 occurring as crescent-shaped cells, usually in groups of four, 

 not crossing one another. Tetraspora, as the name im- 

 plies, occurs in groups of four cells, which closely resemble 

 the nonmotile cells of Protococcus, but are imbedded in 



CLARK'S BOT. 6 



