-SECT. n. D1ATOMACE&. 201 



mud at the bottom of the streams at West Point, in the 

 United States of North America. Its frustule or single 

 diatom is long, slender, and rectilinear, but being broader 

 at one end than at the other, by continued bisection and 

 adhering to one another they form a circular, spiral, or 

 flattened helical screw of several turns. The individual 

 frustules of some marine diatoms have a precisely similar 

 form, being rectilinear and broader at one end than the 

 other, but each frustule is attached by its narrow end 

 to the extremity of branching cellulose stems fixed to 

 sea- weeds or stones, and by a continuous subdivision of 

 which the stem does not partake, they are spread out 

 at their free ends like a fan. 



By continual bisection a diatom is propagated 

 through many generations, but at some stage or other, 

 owing to an unknown 

 cause, propagation by con- 

 jugation takes place. When 

 two frustules are near to 

 each other, two little swel- 

 lings arise in one, which 

 meet two little swellings in 

 the other opposite to it. 

 These soon unite and elon- 

 gate, the septum or division 

 between them is absorbed 



IT i i-i n , , Fig. 15. Meridion circulate. 



so that they form two tubes 



in which the endochrome of the two frustules becomes 

 mixed, and a spore is formed in each of the two con- 

 necting tubes, which increase in size and change in 

 form till they resemble in every respect the parent, 

 except in being much larger. As these young diatoms 

 swell, they split the two parent frustules, become free, 

 and lay the foundation of a twin series of generations. 

 In the Fragillaria only a single spore is formed. 5 



4 Mr. Berkeley's ' Cryptogamic Botany.' 



