336 
POPULAIi SCIENCE EEVIEW. 
Indeed_, some have the power of increasing beyond this, and 
then they have had the prefix of ^^Macro^'’ (from fioKpog, long) 
given them ; as Macrogonidia/^ Thus it will be seen that the 
size of a cell alone cannot be made use of to distinguish species. 
These cells again may change colour, according to the sea- 
son of the year, the moisture, temperature, &c. In winter 
they change more or less to red or orange ; as the functions of 
the cell become active with returning warmth, the colour 
changes again to green ; frequently at this time both colours 
may be observed in the same cell. A well-known example 
may be cited in the red snow vegetable cell found by 
arctic and mountain travellers on the snow, having a form 
similar to that shown at fig. 12, Plate XI Y., of which, or of a 
kindred growth, it is the winter state. This feature, therefore, 
cannot be made use of as a specific character. 
Again their external casing, or cell-wall, may soften 
down and disappear, leaving the contents of the cell soft 
and without protection ; under which state it may possess 
the curious property of altering its form, and moving 
about slowly, giving it the appearance of an Amoeba,^^ 
which is one of the simplest forms of animal life, or, as 
these are called, of the Protozoa.’’^ This power of motion, 
&c., is not confined to the algge, but has been observed by 
Hofmeister, Beale, myself, and others, in the higher orders 
of plants. This is not the place to discuss the nature and 
cause of this motion. It is sufficient for our present purpose 
to show the variety of conditions which are natural to vege- 
table cells at different stages of their existence. However, 
the cell- wall does not always set free the contents by dissolving*, 
but a portion of it splits off like a little lid, or into two equal 
halves. 
This slight outline will help to show the many circumstances 
which may combine to render the appearances of the progeny of 
a given cell so dissimilar, that at first sight each variety might be 
considered as belonging to a totally different group ; indeed, 
the older algologists “actually did so, taking at one time size ; 
at another time the mode of division ; at a third colour, or 
form, as a specific or even generic sign. Frequently their 
description was taken from portions of masses which had been 
dried upon paper, and transmitted by post. Thus descriptions 
became confused, ill-defined, and the positions of the species 
and genera constantly changed by each successive writer.. 
Only a few algologists ventured to study the hfe-history of a 
small number — for instance, Cohn^s paper on the Protococcus 
pluvialis and Yolvox. These efforts were attended with ex- 
cellent results, and no doubt they have stimulated others ta 
follow the same plan, and have given us a history (or a portion 
at least) of one cell, which included in its phases many forms 
