168 



Volvocineae 



occurs in all the three known species of Sphserellu. The value of the haematochrome, 

 which is found mainly in the resting-stage and is especially ah accompaniment of 

 diminished vitality, probably lies in its greater stability than chlorophyll, and perhaps also 

 in its protective and heat-producing powers (Hazen, '99). 



The resting-cells develop motile zoogonidia when transferred from old foul water into 

 distilled water, and from long-continued darkness into light (Freund, '07). 



The only genera are Sphserella Sommerfeld, 1824 ( = Hsematococcus Agardh, 1828) l and 

 Stephanosph&ra Cohn, 1852, both of which are perhaps more confined to rain-water pools 

 than any other members of the Volvocineae. 



Note. It would seem most natural to place the genus Chlorogonium Ehrenberg, 1830 

 (inclus. Cercidium Dang., 1888) with the Sphserellaceae, since its chloroplast is often as 

 indefinite in its limitations as that of Sphserella, and is furnished with from 2 to 5 pyrenoids 

 (or more in Chi. euchlorum); moreover the protoplast contains a number (1216) of con- 

 tractile vacuoles. 



H 



Fig. 97. A F, Scourfieldia complanata G. S. West ; A C, three individuals after staining 

 with iodine; ch, chloroplast; H, nucleus; r, vacuoles (2?); A and B, seen from the front; 

 C, side view showing compression of cell ; D, side view showing resting position of cilia in 

 living cell ; E, diagram to show direction and nature of movements ; F, diagram to show 

 cross-section of cell and nature of chloroplast. G and H, Pteromonas Chodati Lenirn., front 

 and side views. I and J, Pteromonas angulosa (Carter) Lemm., front and side views. 

 A E, x2000; GJ, x 800. 



Family Volvocaceae. 



This is much the largest family of the Volvocinese, and includes both 

 unicellular and colonial forms. The most important unicellular genus is 

 Chlamydomonas. The colonies consist of motile coenobia of Chlamydomona- 

 dine cells, generally embedded in a mucilaginous envelope through which the 

 cilia project. The cells possess a distinct cell-wall, which in most cases is in 



1 Consult Hazen ('99) ; Wille ('03) ; and W. B. Grove, ' Sphaerella v. Mycosphaerella,' 

 Jmirn. Bot. 1912. 



