INTRODUCTION. ‘ vii 
time, use what precaution you may, the cells will leak, more or 
less, in the majority of instances. No medium has yet given 
us absolute satisfaction, but all things being considered, we are 
most favourably disposed towards glycerine jelly, which is too 
dense to enter the cells, and does not alter the form more than 
glycerine would do. Wemay add that the slides should always 
lie flat in the cabinet, and not on edge, and they should not be 
left exposed to the light, or the green colour will soon be lost. 
Hints for cultivating in growing cells, or preservation in 
small aquaria, are scarcely necessary in these days of manipu- 
lation. Many forms of growing slides have been designed 
and described, each possessing some special advantage, and 
arrangements of common glass tumblers, communicating with 
each other by small syphons, or lax threads, will keep up a 
constant flow of water, sufficient to preserve Alge living and 
thriving throughout the year. The life-history, changes, and 
development of many species still require to be patiently inves- 
tigated, and for this purpose there are ample contrivances, all 
that is required being leisure, patience, and perseverance. 
It can scarcely be necessary to insist on the presence of fructi- 
fication, if a given species is to be accurately determined. This 
is especially the case in Gdogoniacee and Zygnemacee, but it 
applies also in a less degree to the majority of families. The 
great difficulty that is experienced in determining, with any 
approximation to accuracy, many of the species described by 
authors half a century ago, lies in the insufficiency of the ¢ha- 
racters of the fructification. A large number of species of 
Cdogonium, for instance, as recent as Hassall’s work, cannot be 
definitely placed on account of this insufficiency. This may at 
the same time serve as a caution to the inexperienced, that it 
will be worse than useless attempting to affix a name to speci- 
mens collected in a young or imperfect condition, at any rate in 
genera the fructification of which is known, and embodied in 
the generic and specific characters. 
Although it can hardly be considered essential to allude in 
this place to theories which have been propounded in connection 
with Fresh-Water Algw, we may make a passing allusion to 
the suggestion that sexually distinct filaments exist in the 
