2l8 



THALLOPHYTES. 



motion, interrupted only by certain periods of repose ; the motion, as is usually the 

 case with swarm-spores, being caused by two cilia. They are, ho\^'ever, distinguished 

 from swarm-spores by the cells — which either live isolated (Chlamydomonas, Chlamydo- 

 coccus), or are united into angular and tabular (Gonium) or spherical families 

 (Volvox, Stephanosphaera, Pandorina) — being surrounded, while in motion, by a mem- 

 brane of cellulose, through which the cilia project free into the water, and produce by 

 their vibrations the rotating and progressive movement of the single cells or of the whole 

 family. This hyaline envelope of cellulose lies either in close contact with the green 

 primordial cell (Chlamydomonas), or is separated from it by a colourless space (water?), 

 from which fine threads of protoplasm run from one to the other, as in Stephanosphaera 

 (Fig. 155, FII). As an example of the history of development, we may choose Stepha- 

 nosphara plwvialis (after Cohn and Wichura in Leopold. Akad. vol. XXVI, p. i). This 

 Alga occurs occasionally in rain-water in the hollows of large stones. When fully 

 mature, Stephanosphaera (Fig. 155, X, XI} is a hyahne ball ('envelope-cell') in which, 



Fig. 153. — Stephanosphc^ra pht-jialis (after Cohn and Wichura). 



standing vertically to its horizontal diameter, lie eight (or more) chlorophyll-green 

 primordial cells; these are fusiform (Fig. 155, XI), and attached to an equator of the 

 envelope-cell at both their ends by branched threads of protoplasm. These primordial 

 cells, derived from one mother- cell, form a family which rotates on the axis at right 

 angles to the plane passing through them all. Out of each cell of a family of this 

 kind is produced, so long as the conditions of vegetation (light, warmth, and water) are 

 favourable, a new family which begins to be formed in the evening and is matured 

 during the night. Each cell divides in succession into two, four, or eight cells, lying 

 in the same plane and forming a disc divided into eight parts ; they secrete a common 

 envelope and develope their cilia. The cells separate from one another and their 

 common envelope detaching itself become spherical ; and thus eventually eight young 

 families are found moving in circles within their mother-cell, until they are freed 



Berichte der schles. Ges. 1856. — Cohn tind Wichura, Ueber Stephanosphcera pluvialis: Nova Acta Acad, 

 nat. curios, vol. XXVI. p. i. [Quart. Joum. Micr. Sc. 1858, p. 131.] — Pringsheim, Ueber Paarung der 

 Schwarmsporen in Monatsber. der Berliner Akad. Oct. 1869. — De Bary, Bot. Zeitg. 1858, Supple- 

 ment, p. 73. [See also on Volvox, Carter, Ann. Nat. Hist. 1839, vol. III. pp. 1-20. — Williamson 

 in Phil. Soc. Manch. IX. p. 321 ; Trans. Micr. Soc. 1853, p. 45 ; Busk, Trans. Micr. Soc. 1853, p. 31. 

 —On Stephanosphoera : Cohn, Ann. Nat. Hist. 2nd ser. X. pp.321, 401; Archer, Quart. Journ. 

 Micr. Sc. 1865, pp. 116, 185.— On Protococcus: Cohn, Ray Soc, Bot. and Phys, Mem. 1852.— On 

 Pandorina, Henfrey, Trans. Micr. Soc. 1856, p. 49.] 



