INTRODUCTION. Vll 





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. We may add that the slides should always 

 \\eflat 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 Algse 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 (Edogoniacece and Zygnemacece, 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 cha- 

 racters of the fructification. A large number of species of 

 (Edogoniwn, 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 Alge, we may make a passing allusion to 

 the suggestion that sexually distinct filaments exist in the 



