602 MICROSCOPIC POEMS OF VEGETABLE LIFE THALLOPHYTES 



endowed with a motion similar to that of the species which are 

 normally free. This circumstance has caused the abandonment 

 of Mr. W. Smith's proposal to assign a generic value to the condition 

 in which the frustule is possessed of this property without regard to its 

 form . Hence those genera are not now generally recognised which differ 

 only in being enclosed in a membranous frond, or in being stalked, 

 especially since frustules contained in a sheath, for example in Schizo- 

 nema, 1 have been seen to escape from it, and to be prevented from 

 returning again to it in company with the sister JVaviculce. Hence 

 the genera Schizotiema, Berkeleya, and Dickiea must be reunited to 

 Nawcula ; Cocconema, Endonema, and Colletonema to Cymbella ; and 

 Hotneocladia to Nitzschia. The singular phenomenon of movement 

 which may be observed in many genera of diatoms among which 

 the most singular is that presented by Bacillaria parudoxa (fig. 449), 

 in which the rod-like frustules are seen to be continually gliding one 

 along another, in a retrograde direction, before they become detached 

 is found to be in general a movement backwards and forwards in a 

 straight line so far as they meet with no impediment, while the 

 intervention of obstacles determines a passive change of direction. 

 The backward and forward movements of the Naviculce have been 

 already described ; in Sicrirella (fig. 453) and Campylodiscus 

 (fig. 454) the motion never proceeds further than a languid roll from 

 one side to the other ; and in Gomphonema (fig. 463), in which a 

 foramen fulfilling the nutritive office is found at the larger extremity 

 only, the movement (which is only seen when the frustule is separated 

 from its stipe) is a hardly perceptible advance in intermitted jerks 

 in the direction of the narrow end. The cause of this movement is 

 uncertain. It has been referred by different authors to the action 

 of endosmose and exosmose ; to cilia ; to the projection of pseudopode- 

 like masses of protoplasm through orifices in the raphe, or of a single 

 elongated protoplasmic thread ; but the most probable interpretation 

 attributes it to the action of the changes resulting from the nutrition 

 of the cell, which must necessarily absorb food in a liquid condition. 

 Taking account, therefore, of the relatively considerable quantity of 

 silex necessary to the organisation of the diatom cell in proportion 

 to its minute dimensions, and bearing in mind, at the same time, 

 the incalculably small traces of silex in solution in the water, it 

 may be understood how active must be the exchange from the 

 exterior to the interior of the cell, and vice versa, and hence how 

 such an exchange must determine a continual change of position 

 backwards and forwards, through the reaction exercised on the 

 delicate floating frustules. 



The principles upon which this interesting group should be classi- 

 fied cannot be properly determined until the history of the genera- 

 tive process of which nothing whatever is yet known in a large 

 proportion of diatoms, and but little in any of them shall have 

 been thoroughly followed out. The observations of Focke 2 render it 



1 See Castracane, ' Observations on the Genera Homeocladia and Schizonevia,' 

 in Atti dell' Accad. Pontif. del Nuovi Lincei, May 23, 1880. 



2 According to this observer (Ann. of Nat. Hist. 2nd series, vol. xv. 1855, p. 237) 

 Navicula bifrons forms, by the spontaneous fission of its internal substance, spherical 

 bodies, which, like gemmules, give rise to Surirella microcora. These by conjuga- 



