2 
rOPULAR SCIENCE REVIEW. 
and as most of them are of aquatic habits, exceedingly suscep- 
tible of any change, whether in the amount of light or in the 
quality of the water, the trouble of keeping them under con- 
stant observation has done much to retard our knowledge of 
them, and to render the number of our observers small. Add 
to this that their outward forms change considerably, if not in 
some instances completely, and that the changes once induced 
may continue as long as the same external conditions which 
produced the changes remain. 
To this point I have already called attention in a former 
paper, and its importance in this study is of great moment. 
Formerly the old observers were content to consider each form 
as the permanent condition ; and in works of not many years 
since it may be read how this species of Protococcus or that 
of Oscillatoria, &c. were sent dried by post. This was un- 
avoidable in the infancy of the study, but now it is indefensible ; 
and nothing must be considered but as provisionally placed 
whose whole mode of growth has not been well observed. 
After these remarks, let us now pass on to the consideration 
of those phenomena which attend the propagation of the fresh- 
water algse, or, as they are also called, “confervoid algse.” And 
in examining these, we shall meet with some very interesting 
changes. But first of all it may be as well, for the benefit of 
those who have not as yet made themselves acquainted with the 
plants in question, to give an outline description of the forms 
in question. 
They may be formed of one or of many cells, but in the latter 
case there is generally very little difference between each. Some- 
times, indeed very commonly, the cells are joined end to end; 
each cell, like other vegetable cells, is composed of a cell- wall, or 
transparent external firm coat (fig. 1, a ), and of the cell-contents , 
or soft-coloured matter, generally of a green colour, and hence 
called u chlorophyl ” (fig. 1, b). Now the cell-wall has no active 
vitality, this is possessed by the cell-contents ; but as these in 
general would be injured readily, the cell-wall is for their pro- 
tection, and to increase their solidity. The green cell-contents 
are disposed in different manners in different plants, and in 
different times in the same plant. Very commonly there is a 
colourless transparent body near the centre of the green mass* 
and this is called the nucleus (fig. 1, c; fig. 9, a; fig. 12, a). 
This was supposed to be the most active vital part of the cell, 
and as it is frequently the first to be divided in the process of 
cell-division, it was considered of essential importance. How- 
ever, inasmuch as in some cells it is not found at all, and as in 
them cell-division goes on all the same, some doubt must be 
entertained in regarding it the essential part of the cell. 
Although the arrangement of the cell-contents is variable. 
