THE FRESH-WATER ALGAE 125 



It should be borne in mind that among the plankton forms 

 there are many which will not develop in either. For these 

 the best solution has been found to be a solution made from the 

 organisms in the plankton itself. In this a perfectly normal de- 

 velopment may be obtained for many forms, though even in this 

 some fail of development. Bouillon, earth decoctions, moist, finely 

 pulverized earth, bits of bark and cubes of sterilized peat, all form 

 good substances for the ordinary cultivation of the unicellular 

 algae. The filamentous algae are far more difficult to cultivate. 

 Before satisfying one's self with the life history of any form, that 

 form should be maintained in culture for an extended period, 

 when observation can be made from time to time and the effect of 

 different conditions determined. 



An attempt has been made to give the principal genera of fresh- 

 water algae found in North America, but the list is by no means 

 complete. A very few genera of diatoms and desmids here cited 

 have not been found by the writer and no report of their occurrence 

 in North America could be obtained; but these groups are distrib- 

 uted so universally that they probably will be discovered in this 

 territory. 



KEY TO NORTH AMERICAN FRESH-WATER ALGAE 

 CLASS I. Bacillariaceae (Diatoms) 



Color yellow; plant a single cell, sometimes united into chains; membrane 

 silicified, with minute, definite markings. 



These are unicellular algae but, by means of a gelatinous substance, are 

 frequently held together in bands or masses. The membrane is silicified, 

 making it hard and rigid. It is always composed of two parts, valves, which 

 may be separated from each other and which are often compared to a box and 

 its overlapping cover; the side where the edges overlap is spoken of as the 

 girdle side, while the outer surface is referred to as the valve side; this and, 

 more rarely, the girdle side also are sculptured with fine striations, dots, 

 dotted lines, and grooves. Many have extending lengthwise a conspicuous 

 line, the raphe, which frequently bears at its middle and both ends rounded 

 portions called nodules. 



Reproduction is by auxospores, either sexual or asexual. The asexual 

 are formed by the contents of a cell collecting, throwing off the membrane, 

 and forming either one or two spores. The sexual auxospores are formed by 

 the throwing off of the membrane and the copulation of two cells in one of the 

 following ways: (a) Two cells divide, making two pairs of daughter-cells; each 

 individual of one pair fuses with one from the other pair, thus making two 

 spores. (6) Two cells unite to form one auxospore. (c) Two cells come 

 together but do not copulate; two auxospores are formed. 



