202 DIATOMACEsE. PART n. 



In Surirella arid Epithemia the manner of con- 

 jugation is somewhat different. In the former the 

 valves of two free adjacent fnistules separate from 

 each other at the suture or line of junction and the 

 two endochromes are discharged; they coalesce and 

 form a single mass, which becomes enclosed in a gela- 

 tinous envelope, and in time this mass shapes itself 

 into a frustule resembling that of its parent, but larger. 

 In Epithemia, however, the endochrome of each of the 

 conjugating frustules divides at the time of its dis- 

 charge into two halves ; each half of the one coalesces 

 with each half of the other, and two frustules are 

 formed which become invested with a gelatinous en- 

 velope and gradually assume the form and markings 

 of the parent frustules, but grow to a much larger size, 

 for the spore masses have the power of self-increase 

 up to the time that their envelopes are consolidated. 

 This double conjugation seems to be the ordinary type 

 of the process among the diatoms. 6 But these plants 

 multiply also by gonidia. It is thought probable that 

 as long as the vegetative processes are in full activity 

 diatoms multiply by self-bisection, but when a deficiency 

 of warmth, of moisture, or of some other condition, 

 gives a check to these, that they increase by gonidia, 

 some of which becoming encysted, possess a greater 

 power of resisting unfavourable circumstances, and 

 thus the species is maintained in a dormant state till 

 a change enables them to germinate. It is even 

 thought they may be the origin of distinct species. 



A peculiar spontaneous locomotion is exhibited by 

 some diatoms of a long narrow form, as the Naviculse, 

 which by a succession of jerks in the direction of their 

 length, go to a certain distance, and then return nearly 

 by the same path. The motion of the Bacillaria cur- 



6 ' Dr. Carpenter's ' Microscope.' 



