MICROSCOPIC FORMS OF VEGETABLE LIFE. 



281 



this mode GJ propagation. It seems probable that, so long as the vege- 

 tating processes are in full activity, multiplication takes place in prefer- 

 ence by self-division; and that it is when 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 conditions favor a renewal of active 

 vegetation ( 234). 



280. Conjugation, so far as is at present known, takes place among 

 the ordinary Diatomacea3 almost exactly as among the Desmidiaceae; ex- 

 cept that it sometimes results in the production of two 'zygospores/ 

 insead of a single one. Thus in Surirella (Fig. 175), the valves of two 



Biddulphia pulchella : A, chain of cells 

 in different states; a, full size; b, elongation 

 preparatory to subdivision; c, formation of 

 two new cells; d, e, young cells; B. end-view; 

 c, side-view of a cell more highly magni- 

 fied. 



Conjugation of Epithemia turgida:*., front 

 view of single f rustule ; B, side view of the same ; 

 c, two frustules with their concave surfaces in 

 close apposition ; D, front view of one of the frus- 

 tules showing the separation of its valves along 

 the suture; E, F, side and front views after the 

 formation of the zygospores. 



free and adjacent frustules separate from each other at the sutures, and 

 the two endochromes (probably included in their primordial utricles) arc 

 discharged; these coalesce to form a single mass, which becomes inclosed 

 in a gelatinous envelope; and in due time this ' zygospore ' shapes itself 

 into a f rustule resembling that of its parent, but of larger size. t But in 

 Epithemia (Fig 168, A, B) the first Diatom in which the conjugating 

 process was observed by Mr. Thwaites 1 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 endochrome; 

 and thus two ' zygospores ' (E, F) are formed, which, as in the preceding 

 case, become invested with a gelatinous envelope, and gradually assume 



1 See " Annals of Natural History," Ser. 1, Vol. xx. (1847), pp. 9, 343; and 

 Ser. 2, Vol. i. (1848), p 161. 



