OF THE FREE DIATOMACE^. 17 



The free growing members of the family (or those 

 which are in no sense parasitic) are found entangled 

 among the tufts of filamentous Algae, Oscillatoriae, Mosses, 

 &c. ; or we see them below the surface of the w T ater, 

 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 Naviculaceae and 

 Nitzschiae, 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 Algse, or Mosses, must be 

 gathered with the latter, care being taken to drain as little 

 moisture as possible from the tuft, lest the Diatomaceae 

 escape with it. Let the whole mass be carefully laid in 

 oiled paper, and on afterwards washing it in clean water, 

 the lesser Algae 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 

 Oscillatoriae, because the latter are so fragile, that it is 

 almost impossible to prevent numerous fragments of their 

 filaments from being mingled with the Diatomaceae. To 



