CONJUGATION OF DIATOMAOE^. 



285 



and markings of the frustules, such as many consider sufficient 

 to establish a diversity of species, have their origin in this mode 

 of propagation. It is probable that, so long as the vegetating 

 processes are in full activity, multiplication takes place in pre- 

 ference by self-division ; and that it is v^hen deficiency of warmth, 

 of moisture, or of some other condition, gives a check to these, 

 that the formation of encysted gonidia, having a greater power 

 of resisting unfavorable influences, will take place ; whereby the 

 species is maintained in a dormant state, until the external con- 

 ditions are favorable to a renewal of active vegetation (§ 156), 



178. The process of " conjugation," or true Generation, has 

 been observed to take place among the ordinary Diatomacese, 

 almost exactly as among the Desmidiacese. Thus in Surirella 

 (Fig. 88), the valves of two free and adjacent frustules separate 

 from each other at the sutures, and the two endoehromes (proba- 

 bly included in their primordial utricle) are discharged ; these 

 coalesce, and form a single sporangial mass, which becomes 

 enclosed in a gelatinous envelope; and in due time this mass 

 shapes itself into a frustule resembling that of its parent, but of 

 larger size. In Epithemia (Fig. 82, a, e), however, — the first 

 Diatom in which the conjugating process was observed, by Mr. 

 Thwaites, — ^the endochrome of each of the conjugating frustules 

 (c, d) appears to divide at the time of its discharge, into two 

 halves; each half coalesces 

 with half of the other endo- 

 chrome ; and thus two spo- 

 rangial frustules (e, f) are 

 formed (as in Closterium linea- 

 tum, § 169, note), which, as in 

 the preceding case, become 

 invested with a gelatinous 

 envelope, and gradually as- 

 sume the form and markings 

 of the parent frustules, but 

 grow to a very much larger 

 size, the sporangial masses 

 having obviously a power of 

 self-increase up to the time 

 when their envelopes are con- 

 solidated. This double conju- 

 gation seems to be the ordi- 

 nary type of the process 

 among the Diatoms. A cu- 

 rious departure from the 

 usual plan is observed in 

 some of the filamentous spe- 

 cies; for their component 

 cells, instead of conjugating 

 with those of another filament (as is the case with the filamen- 



FlG. 82. 



Conjugation of Epithemia turgida-.—K, front 

 view of single fmslule; b, side view of the 

 same ; c, two frustules with their concave sur- 

 faces in close apposition ; d, front view of one 

 of the frustules, showing the separation of its 

 valves along the suture ; e, f, .side and front 

 views after the formation of the sporangia. 



