356 On the Diatomacece. [Sess. 



ally take place, the contents of the cell would be completely 

 protected on all sides by its flinty case. The new cells thus 

 formed are, however, smaller than the old ones by double the 

 thickness of the connectives. It is thus evident that by this 

 process of multiplication a time would come when, by this 

 continual decrease in size, the frustule would cease to exist, 

 and this order of vegetable life would come to an end. Nature 

 has, however, provided a remedy, and another process of 

 reproduction takes place, by which a new frustule is formed 

 of the original size. Diatomists are not all agreed as to the 

 mode in which this reproduction takes place. Here again I 

 follow Dr P. Miguel, who has actually observed it in some 

 species. In multiplication by division the protoplasm has 

 increased in a direction parallel to the line which unites the 

 nuclei of several frustules — that is, supposing such a line to 

 be drawn. This has the effect of forcing the valves apart. 

 When, however, a certain stage in the decrease of the size of 

 the frustule has been reached, the protoplasm swells out, in a 

 direction perpendicular to this line, on both sides, until it 

 reaches the original size of, or perhaps larger than, the parent 

 frustule. The protoplasm as thus increased is a soft mass, 

 but it soon secretes to itself a silicious envelope, though it 

 requires to pass through two or three generations before the 

 exact form of the original frustule is attained. 



Different genera of diatoms have different modes of life. 

 Some are free, — that is, each new frustule after division 

 separates itself entirely from the parent frustule and leads an 

 independent life : others do not entirely separate themselves, 

 but remain attached by their external mucous membrane, and 

 thus forms chains, some in a direct line, others in zigzag. 

 Some are attached to higher alga?, and spread out in the form 

 of a fan, while others live in tubes attached to stones in 

 shallow waters round the sea coast. I have collected some 

 of these tubes, more than an inch in length, packed full of 

 living diatoms. 



Habitat. 



Where are diatoms to be found ? Some are peculiar to 

 fresh water, others to salt water, while others are to be found 

 only in brackish pools. In slowly running streams you will 



