INTRODUCTION. 



must mention Dr. Horatio Wood's " Fresh-Water Alge of 

 North America," published in 1872. The period selected, 

 therefore, for a new work on the " British Fresh-Water Algae " 

 was in all respects opportune, but we are not expected to judge 

 whether good use has been made of the opportunity. 



The gradual accession to knowledge of the structure and 

 development of Algse, during the past century, would extend 

 this chapter far beyond the limits we have assigned to it, 

 whereas an intimation of the present position is generally indi- 

 cated under the several groups, in the body of the work, with 

 some reference to the sources of further information. 



Classification must, of course, depend very largely upon 

 investigation, and modifications necessarily follow new facts, so 

 that, whilst adopting suggestions made by authors of recent 

 monographs, our arrangement is not absolutely that of any 

 other work, although as far as possible the orders and larger 

 groups have not been disturbed. We will not venture to pre- 

 dict what will be the future of such families as the Palmellacece, 

 Protococcacece,and Chroococcacece during the next twenty years; 

 probably they will disappear altogether, becoming merged into 

 other groups of which most of them may be demonstrated to be 

 only imperfect conditions. At the present time we could follow 

 no other course than to include them, and leave the future to 

 take care of itself. We have preferred including too much to 

 excluding too much, in confidence that for this we should be 

 the more readily forgiven. 



The student will expect to find in this place some practical 

 hints to guide him in his pursuit, but these must be brief, as 

 experience, combined with a little personal contact with old 

 hands at field-work, will supersede a volume of precepts. 



The habitats attached to the various species will indicate 

 the kind of locality in which to search for them, but it may be 

 remarked that, generally, any slow stream, pond, or standing 

 pool will furnish something. Rapid currents will not repay the 

 search. Small pools on moors, amongst Sphagnum, will usually 

 prove prolific, and submerged plants, especially when dead or 

 dying, often serve as a habitat for attached species. Damp 

 rocks, down which the water trickles, old quarries, damp walls 



