594 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



rather confirm these general inferences than present any new data 

 of knowledge concerning the diatoms. By his great courtesy we 

 have been favoured with a phototype plate prepared by Dr. Yan 

 Heurck from his own photo -micrographs, and the reader will be 

 enabled to study these in Plate XI, of which a full description is given 

 in the earlier part of this treatise, giving descriptions of the plates. 

 He has further enhanced the plate by giving in fig. 7 a photo-micro- 

 graph of Robert's nineteenth band. 



Diatoms, like other organisms already described, are reproduced 

 by conjugation, and multiply by autofission or division. Repro- 

 duction is necessary to every organism, while multiplication by 

 fission belongs only to certain organic types. In the early days of 

 the study of diatoms, it would appear that even that distinguished 

 observer William Smith had at least not a clear idea of the encyst- 

 ing of the frustule or 

 individual diatom, which 

 implies the existence of 

 the two valves and of 

 the double girdle or 

 zone or connecting ring 

 projecting from each 

 valve in a direction at 

 right angles to its plane. 

 Hence, instead of find- 

 ing, as a result of fission, 

 a progressive diminu- 

 tion of the diameter of 

 the frustules, Mr. Smith 

 speaks of their increase, 

 of which he is unable to 

 offer any explanation. 

 The fact that in Afelosira 

 siibflexilis (fig. 444, A) 

 and M. varians (fig. 

 Melosird varians. 444, B) large and small 

 . frustules are seen united 

 in rows, ought to be sufficient to show that they are dependent not 

 only on binary subdivision, but also 011 the special conditions of 

 evolution of the new frustule, by which it is able to increase 

 materially in size. This power of diatoms to expand their siliceous 

 coatings has therefore been denied by some, who are induced to 

 maintain this necessary consequence of the division of encysted 

 frustules, viz. the progressive decrease in size of the young 

 frustules, which would thus reach the smallest possible dimensions. 

 This has led Pfitzer L to imagine that when diatoms have reached 

 their smallest possible dimensions by repeated binary division, 

 the process of conjugation takes place between them, resulting 

 in the formation of an auxospore, capable of reproducing two 

 sporangial frustules of considerably larger- size, which would again 

 give rise, by fission, to a new series of diminishing frustules, 

 1 Untersuchungen iiber Bau it.. Entwickelung der Bacillarien, 8vo. Bonn, 1871. 



A FIG. 444. 



Melosira siibflexilis. 



