THE MICROSCOPE. 151 



" Their living masses," says the Rev. W. Smith, " present 

 themselves as coloured fringes attached to larger plants, or form- 

 ing a covering to stones or rocks in cushion-like tufts (or spread 

 over the surface as delicate velvet), or depositing themselves as 

 a filmy stratum on mud, or intermixed with the scum of living 

 or decayed vegetation floating on the surface of the water. Their 

 color is usually a yellowish-brown of a greater or less inten- 

 sity, varying from a light chestnut in individual specimens to 

 a shade almost approaching black in the aggregated masses. 

 Their presence may often be detected without the aid of a mi- 

 croscope, by the absence in many species, of the fibrous tenac- 

 ity which distinguishes other plants. When removed from 

 their natural position they become distributed through the 

 water, and are held in suspension by it, only subsiding after 

 some little time has elapsed." 



The Diatomacese have been divided into two classes, by Herr 

 Johann Nave, viz. : — the free species, or those which have an 

 independent existence ; and the stipate, or such as are attached 

 to other objects, generally Alga 3 , by means of a stalk. There is 

 however a third division comprising the frondose species, or 

 those in which numerous individual frustules are enclosed, 

 held together by a gelatinous mass, and in appearance not un- 

 like sea-weed. 



The free growing species are found entangled with alga? and 

 mosses, or below the surface of the water, wherever the soil, 

 stone, or a fallen leaf is stained with a yellowish brown hue. 

 In rapid streams and mountain torrents they become scattered 

 and numbers of them remain suspended in the foam and may 

 be easily gathered. Also when the sun is shining they liberate 

 oxygen and rise to the surface with the gas. Whenever these 

 minute bubbles are seen to be coloured brown, it may be taken 

 as a sure sign that Diatoms are present. 



Certain stipate Diatoms are exactly similar to others of the 

 free kind so far as their frustules are concerned, and their sti- 

 pate existence forms a convenient basis for distinguishing the 

 families. 



As examples of the very close approximation in the form 

 of the frustule may be mentioned, the Cymbella and Cocconema; 

 Sphenella, and Gomphonema, which are respectively separated 

 solely on account of this difference. 



