450 PRACTICAL BOTANY 



for further details and description of other forms 

 reference is to be made to special treatises. 



When cell-division takes place in Diatoms, new 

 tests are formed, fitting within the old ones : specimens 

 dividing, or which have recently divided, should be 

 looked for, and examined minutely, under high powers, 

 in order to verify this point. 



As the tests once formed do not grow, it is obvious 

 that repeated division will lead to a diminution of size : 

 this is met by the formation of auxospores, which 

 may be observed in various Diatoms, especially in 

 summer. There are various details of the process 

 in different Diatoms, but the essential point is that the 

 tests are thrown off, and the contents elongate greatly, 

 and ultimately forming new tests, enter again on a 

 course of division. 



The tests are silicified, and by treatment with a little 

 potassium chlorate and nitric acid, and subsequent 

 ignition on platinum foil, skeletons of silica may be 

 prepared, which retain in minutest detail the configura- 

 tion and surface-markings of the tests of the Diatoms 

 so treated. 



