DIATOMACE^E 



605 



with each act of binary division of the frustule, increases in breadth, 

 while the frustules themselves remain coherent, so that a beautiful 

 fan-like arrangement is produced (fig. 450). A splitting away of a 

 few frustules seems occasionally to take place, from one side or the 

 other, before the elongation of the stipe; so that the entire plant 

 presents us with a more or less complete flabella or fan upon the 

 summit of the branches, with imperfect flabella 1 or single frustules 

 irregularlv scattered throughout the entire length of the footstalk. 

 This beautiful plant is marine, and is attached to seaweeds and 

 zoophytes. 



In the next family, that of Fragilariete, the frustules are of the 

 same breadth at each end, so that if they unite into a filament they 

 form a straight band. In 

 some genera they are 

 smooth, in others trans- 

 versely striated, with a 

 central nodule ; when stria 1 

 are present, they run across 

 the valves without inter 

 ruption. To this family 

 belongs the genus Diatoma. 

 which gives its name to the 

 entire group, that name 

 (which means cutting 

 through) being suggested 

 by the curious habit of the 

 genus, in which the frus- 

 tules, after division, sepa- 

 rate from each other along 

 their lines of junction, but 

 remain connected at their 

 angles, so as to form zigzag 

 chains (fig. 451). The 

 valves of Diatoma, when 



FIG. 452. 



turned sideways (a), are 



Put. 451. 



FIG. 451. Diatoma vulgare: a, side view of 



frustule; b, frustule undergoing division, 

 seen, to be strongly marked FIG. 452. Grammatophoraserpentina: a, front 



by transverse stria 3 , which tiustuie , o, o, front 



. IP iincl euci views ot divided frustule ; c. frustule 



extend into the front View. about to undergo division; d, frustule com- 

 The proportion between the pletely divided, 

 length and the breadth of 



each valve is found to vary so considerably that, if the extreme forms 

 only were compared, there would seem adequate ground for regarding 

 them as belonging to different species. The genus inhabits fresh 

 water, preferring gently running streams, in which it is sometimes 

 very abundant. The genus Fragilaria is nearly allied to Diatoma, the 

 difference between them consisting chiefly in the mode of adhesion of 

 the frustules, which in Fragilaria form long, straight filaments with 

 parallel sides; the filaments, however, as the name of the genus 

 implies, very readily break up into their component frustules, often 

 separating at the slightest touch. Its various species are very 

 common in pools and ditches. This family is connected with the 



