INTRODUCTION. Xlll 



a considerable modification of our ideas concerning the genera- 

 tion and growth of certain life-germs. 



For the behoof of the uninitiated, a few words may here not 

 be out of place, on 



HOW TO FIND, HOW TO COLLECT, A.XD HOW TO PRESERVE 



Fresh-water Alyw. As a large majority of them, especially the 

 DES.AIIDS, are free-floating plants, it would be a waste of time to 

 seek them in rapid waters; they affect pools, sluggish streams 

 and ponds ; the latter afford the most satisfactory results to the 

 explorer, when the pond is a mile or more in length and is fed 

 by one or two creeks; the indentations on the margins of such 

 a pond and its tributaries, usually abound in water grasses and 

 mosses which shelter and support the floating Algae. 



The outfit need not consist of more than a nest of four or five 

 tin cans (tomato or fruit) one within the other for convenience of 

 carriage; ten or a dozen wide-mouthed vials, and a small ring- 

 net of fine muslin at the end of a rod about four feet in length. 

 Should a boat be needed it can usually be hired on the spot. 

 After selecting what seems to be a good locality, drag the net a- 

 few feet among the grasses and mosses, above indicated, allow 

 the bulk of the water to drain through the muslin, and then 

 empty the residue into one of the cans ; repeat this process as 

 often as may be desirable. Ten or fifteen minutes after the cans 

 have been filled, most of the surface water may be poured off, 

 and the remainder transferred to a glass vial, where the solid 

 contents will gradually sink, and the superfluous water can be 

 again poured off, and the vessel filled up with deposits from 

 other vials. In shallow places, what is known as s. vamp-moss 

 (SphaffHum^, bladderwort (Utricttlaria), water-milfoil ( Uyriophyl- 

 lum} or other finely cut-leaf water plants are likely to abound; 

 these should be lifted in the hand and the water drained, or 

 squeezed from them into a tin can to be subsequently treated as 

 already stated. A few drops of carbolic acid in each vial, just, 

 enough to make its presence perceptible, will preserve the con- 

 tents for months and even years from deterioration; the green 

 coloring matter (C/iIorophyl) may fade, but this in the case of the 

 DKSMIDS, is of little importance ; nevertheless, when practicable, 

 always examine the material when fresh. When dried on paper 

 for the herbarium, the specimens can still, after being moistened 

 with water, be microscopically examined, but not with the best 



