ALG^. 193 



to irrigate the meadows ; in the broad ocean, and the 

 shallow puddle left by the overflowing of a ditch : 

 the brackish water, where the tidal river meets the 

 sea ; salt-works and salt-pits ; even inland lakes, 

 which have a trace of salt in them, — each affords 

 a rich variety of characteristic Diatomaceae, varying 

 according to the chemical quality of the water. 

 They are to be frequently found also on rocks and 

 masses of stone, damp from overhanging trees, or 

 from the constant trickling of water." 



The Diatomaceae are now acknowledged to be true 

 one-celled algae, differing in many points from other 

 algae, but especially in the nature of the cell wall, 

 which is composed of silex or flint, secreted by a thin 

 membrane ; also by their mode of increase by cell 

 division ; and by the nature of their cell contents, 

 which contain a colouring matter not found in other 

 algae, and which is called diatomine. 



"The structure is most easily discerned in the 

 circular forms. In its earliest condition the cell has 

 the appearance of a double-convex lens, but, as 

 growth proceeds, the convex surfaces recede, and a 

 band is formed by the sides, as of a pill-box, which 

 gradually increases in breadth. If we take two pill- 

 boxes, minus the lids, which we call a and b, the 

 latter small enough to slide into the former, we have 

 a fairly correct idea of the diatom cell. When this 

 cell has attained its maximum development, the cell 

 contents begin to divide, one portion receding to one 

 end of the cell and the second to the other, and in a 

 short time a new end is formed to a, and we have a 

 cell in all respects like the first. The same occurs 



O 



