1 96 DIA TO MA CE^E. PART n. 



little tufts ; others form a dirty cloud upon the stems 

 and leaves of aquatic plants. They have been found in 

 a fossil state in flint, their spores have been discovered 

 in the grey chalk at Folkestone, and the cells of various 

 species of Clostermm and Euastrum are imbedded in the 

 marls of the United States of North America. 



The Diatomacese, or Brittleworts, are unicellular mi- 

 croscopic plants so numerous that there is hardly a spot 

 on the face of the earth, from Spitzbergen to Victoria 

 Land, where they may not be found. They abound in 

 the ocean, in still and running fresh water, and even on 

 the surface of the bare ground. They extend in latitude 

 beyond the limits of all other plants, and can endure 

 extremes of temperature, being able to exist in thermal 

 springs, and in the pancake ice in the south polar lati- 

 tudes. Though much too small to be visible to the 

 naked eye, they occur in such countless myriads as to 

 stain the berg and pancake ice wherever they are washed 

 by the swell of the sea ; and when enclosed in the con- 

 gealing surface of the water they impart to the brash 

 and pancake ice a pale ochreous colour. 



Although the diatoms have a vast variety of forms, 

 they all consist of a simple primordial cell whose ex- 

 ternal coat of cellulose is so deeply interpenetrated with 

 silex that it is indestructible, a structure which con- 

 stitutes the peculiar characteristic of the tribe. This 

 primordial cell, as in other plants, contains organiz- 

 able liquid or protoplasm, through which golden-brown 

 granules are pretty regularly distributed, except in the 

 centre, where they are collected into a nucleus. Round 

 this nucleus they commonly form a ring from which 

 radiating lines of granules diverge to the interior wall 

 of the cell. In each of these there is a double current 

 of granules, similar to the circulation in the Desmidiacese ; 

 it was discovered by Professor Smith in some of the 

 comparatively large diatoms. At times oil globules are 



