FRESH-WATER ALG^. 265 



to tlie manifestation of vital action in the motile zoospores, 

 and they always seek it, collecting themselves at the 

 surface of the water, and at the edges of the vessel. 



But in the act of propagation, on the contrary, and when 

 about to pass into the still condition, the motile Pro- 

 tococcus cell seems to shun the light ; at all events it 

 then seeks the bottom of the vessel, or that part of the 

 drop of water in which it may be placed, furthest from 

 the light. Too strong sunlight, as when it is concentrated 

 by a lens, at once kills the zoospores. A temperature of 

 undue elevation is injurious to the development of the 

 more vigorous vital activity, that is to say, for the forma- 

 tion of the zoospores ; whilst a more moderate warmth, 

 particularly that of the vernal sun, is singularly favour- 

 able to it. Frost destroys the motile, but not the still 

 zoospores.* 



Stephanosphcera pluvialis is another variety of fresh- 

 water algse, first observed by Cohn. It consists of a 

 hyaline globe, containing eight green primordial cells, 

 arranged in a circle (see Plate 1, ISTo. 24 d). The globe 

 rotates, somewhat in the same manner as the volvox, by 

 the aid of projecting cilise, two of which are seen to proceed 

 from each cell and pierce the transparent envelope. Every 

 cell divides first into two, then four, and lastly eight 

 young cells, each of which divides into a great number of 

 microQfonidia, and are seen to have a motion within the 

 globe, and ultimately escape from it. Under certain cir- 

 cumstances each of the eight cells is observed to move 

 about in the interior of the mother-cell ; eventually they 

 escape, lose their cilia, form a thicker membrane as at h, 

 for a time become motionless, and sink to the bottom of 

 the vessel. If the vessel be permitted to become thoroughly 

 dry, and again water is poured into it, motile Stephano- 

 sphsera reappear : from which circumstances it is probable 

 that the green globes are the resting spores of the plant. 

 When in its condition of greatest activity its division into 

 eight is perfected during the night, and early in the 

 morning the young family escapes from the cell, soon to 

 pass through similar changes. It is calculated that in 



* On the "Natural History of Protococcus pluvialis," by F. Cohn, translated 

 by G. Busk, F.R.S. for the Ray Society 



