THE DIATOMS 29 



by a brown pigment, so that living Diatoms usually 

 appear brown, but the pigment masks the green chloro- 

 phyll, which is occasionally seen in dead specimens. 

 Oil globules also occur in the cells. 



The cell- wall of a Diatom is of peculiar interest: it 

 consists of two siliceous (flinty) valves, or frustules, 

 which fit one over the other like the lid on a box. The 

 manner in which they fit may be well illustrated by the 

 two halves of those telescopic baskets used for carrying 

 luggage. It is remarkable that a cell so minute should 

 be able to elaborate so wonderful a structure for its 

 protection. And no less remarkable than the existence 

 of a cell-wall which is practically imperishable is the 

 marvellous way in which the valves are marked with 

 ribs, pits, dots, etc., constituting a natural artistry 

 inimitable in its extreme fineness by any engraver or 

 sculptor. In some species there are pores through 

 which the cell forces a gelatinous secretion. These 

 siliceous frustules are not destroyed even by intense 

 heat, and the microscopist may clean them by boiling 

 them in nitric acid without impairing their form or 

 beauty. Some forms make valuable test objects for 

 microscope lenses. 



Now, how do Diatoms multiply ? In the first 

 place by longitudinal division. It will be obvious to 

 the thoughtful reader that the cell is " cabined, cribb'd, 

 confined " within its decidedly inelastic cell-wall, which 

 will not expand with growth. When, therefore, the cell 

 is up against the housing problem on account of its 

 increased proportions, it must necessarily divide. What 

 happens is this: The growing cell pushes its valves 

 slightly apart, division is effected parallel with the 



