182 FRESH-WATER ALG^ OF THE UNITED STATES. 



(more frequently yellowish green); cell membrane rather 

 thick, colorless. 



Diameter of cells variable, 5-15 /*. 



This may be termed the conservatory form. It is com- 

 mon on flower pots, walls, and ground floors of green-houses. 



Porphyridium cruenta is merely another form of the same 

 polymorphous, arrested, growth. Compare Porphyridium,^ 

 page 194. 



The change of color is due to abnormal conditions rather 

 than to a normal characteristic. 



Plate CLXII, fig. 5. The usual condition of single cells ; 

 sometimes 2-4 are seen in a family. 



II. Aquatic Forms. 



e.--Var. INFUSIONUM. Chlorococcum infusionum, Bab. 



Cells spherical, very various in size; mostly 5-15 ^ in 

 diameter, but larger ones are intermingled ; the largest 

 measuring 75-100 //. 



Sometimes very abundant in early Spring in meadow 

 pools, largely intermingled with filaments of Ulothrix; evi- 

 dently the two are closely related. 



Syn. Protococcus Meneghinii, Kg.; Ewibryospliaera Meneghinii, 



Trevis ; Lepraria infusionum, Schrank. 



/ 

 Plate CLXI, figs. 2, 3, a range of sizes from the smaller to 



the largest cells. Normally the smaller cells greatly out- 

 number the larger sizes. 



/,--Var. HUMICOLA. Cystococcus humicola, Naeg. 



Stratum effused dark green ; cells globose, variable in 

 size, sometimes many united in families, involved in a com- 

 mon hyaline tegument ; cell membrane thin ; cell contents 

 at first homogeneous, later granular. 



A form recognized as belonging to this variety occurred 

 in cask of rain water around the water line. Diameter of 

 larger cells, 16-18 //. General appearance very near Plate 

 CLXI, fig. 1, P. viridis. 



#.--Var. BOTRYOIDES. Chlorococcus botryoides, Bab. 



Cells spherical, irregularly clustered, somewhat grape - 

 like. Cell membrane thin, hyaline, homogeneous, green; 

 with age often brownish ; cells average 12 yw. 



Not rare in pools. Not a normal form merely an adven- 

 titious condition. 



