OF THE FREE DIATOMACEAC. 
17 
The free growing members of the family (or those 
which are in no sense parasitic) are found entangled 
among the tufts of filamentous Alg®, Oscillatoriae, Mosses, 
&c. ; or we see them below the surface of the water, 
wherever the soil, or a stone, or fallen leaf, is stained with 
a yellowish brown hue. In the latter case the colour is 
almost invariably due to multitudes of Naviculacese and 
Nitzschise, genera which usually prefer shallow spots, only 
a few inches deep, though occasionally they occur in 
considerable depths, as for example in Alpine lakes. In 
swiftly flowing streams they become more scattered, and 
numbers of them remain suspended in the foam, consequent 
on the water beating violently against stones and other 
obstacles, and thus may be easily collected without any 
admixture of sand and mud. In like manner they often 
rise to the surface with the bubbles of gas, which are 
disengaged from water-plants under the influence of the 
sun’s rays. Whenever these foam bubbles are seen to be 
tinged with a brown colour, the collector knows at once, 
that they are charged with numerous specimens of the 
plants he is in search of, and he has only to skim them oft 
into a wide-mouthed bottle to be sure of ample materials 
for study on his return home. Those individuals which are 
caught among the filaments of Algas, or Mosses, must be 
gathered with the latter, care being taken to drain as little 
moisture as possible from the tuft, lest the Diatomaceaa 
escape with it. Let the whole mass be carefully laid in 
oiled paper, and on afterwards washing it in clean water, 
the lesser Algse will be disengaged from their temporary 
nidus, and after a while sink to the bottom of the vessel, 
when the superfluous water may be poured off. If it is 
desired to dry the sediment at once, this can be managed 
by filtering it through some fitting material. 
It is less easy, however, to separate them from the 
Oscillatorias, because the latter are so fragile, that it is 
almost impossible to prevent numerous fragments of their 
filaments from being mingled with the Diatomaceas. To 
