DIATOMACE.E. 75 



in the divided groups the cells are situated in two 

 alternate rows. 



The spores of many of the Desmidiacese are spi- 

 nous, and they are often found fossil in flint (PL V. 

 fig. 15). To detect them in this substance, thin slips 

 of flint may be examined under a half-inch power ; 

 or the chips of flint may be cemented to a slide with 

 balsam, and ground down on a hone. 



The Desmidiacese must be mounted in the moist 

 state : the smaller ones will keep well in chloride of 

 calcium \ but the larger ones are injured both by 

 that liquid and by glycerine. The remarks made upon 

 mounting, at page 15, are especially applicable to these 

 delicate organisms. 



DIATOMA'CE^E, or Siliceous Algae. The members 

 of this family are singly very minute ; but when ex- 

 isting in large numbers, as they are often found at 

 the bottom of ditches and ponds, on the submerged 

 stems of water-plants, or upon damp ground, they 

 form yellowish-brown evident masses or strata. They 

 occur both in sea- and in fresh water. They usually 

 consist, like the Desmidiacese, of single cells, which 

 are called frustules. But they are especially charac- 

 terized by the cell-walls being imbued with silica or 

 flint, so that if the frustules be heated to redness upon 

 the point of a knife or a slip of platinum-foil, which 

 destroys the organic part of the cells, the coat of 

 silica remains, exhibiting the perfect form of the 

 original cells or frustules. The form of the frustules 

 is very different in the various genera and species, as 

 represented in PL V. tigs. 22, 23, 27, 30, 31, and 

 PL VI. figs. 16, 17, 23; and it will be noticed that, 

 in the figures, two views are given of each frustule, 

 / indicating the front view, and s the side view. In 

 all the front views, as in PL V. fig. 22, one or more 

 lines will be observed running longitudinally down 

 the middle of the frustules, and corresponding to the 

 indications of division existing in the cells of the 



H2 



