FRESH-WA/ER ALGJS. 265 



ducive 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 edge of the vessel. 



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

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

 tococcus cell seems to shun tne 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.* 



StephanospJwera pluvialis is another variety of fresh- 

 water alga3, first observed by Cohn. It consists of a 

 hyaline globe, containing eight green primordial cells, 

 arranged in a circle (see Plate 1, No. 24 d). The globe 

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

 the aid of projecting nagella, two of which are seen to pro- 

 ceed 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 microgonidia, 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 flagella, form a thicker membrane as at 

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

 the vessel. If tlie vessel be permitted to become thorough- 

 ly dry, and again water is poured into it, motile Stephan- 

 osphsera reappear : from which circumstances it is proba- 

 ble that the green globes are the resting spores of the 

 plant. When in its condition of greatest activity its divi- 

 sion 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. Conn, translated 

 ly . Buk, F.R.S. for the Ray Society 



