454 SYSTEMATIC BOTANY. 



tube, ultimately resolved into discoid fragments, which, when free, be- 

 come globular. The gelatinous " core," the vital part of the structure, is 

 capable of a peculiar movement, which causes the free portions or ex- 

 tremities of the filaments to vibrate like a pendulum, or with a slightly 

 vermiform oscillation, whence the name of the Order. Reproduction by 

 spores unknown. 



Our knowledge of the essential characters of this Order is imperfect ; 

 and the only mode of reproduction known is by simple division of the 

 central substance of the filaments, the portions slitting out of the ends of 

 the sheaths and secreting a new coat of their own. Their peculiar 

 oscillating motion is one of the marvels of Vegetable Physiology ; they 

 appear to be totally destitute of cilia. Their movements, and the nature 

 of their central substance (apparently devoid of starch, and coloured by 

 different matters besides chlorophyll), seem to indicate a relation between 

 Oscillatoriacese and Diatomaceaa, which would connect the latter with 

 Confervoids. They occur in water, fresh and salt, and on damp earth 

 everywhere. Genera : Oscillator ia, Bosc ; Microcoleiis, Desmaz. j Calo- 

 thrix, Agh. ; Rivularia, Roth. 



DIATOMACE^E. 



Class Algae, Endl. All Algales, Lindl 



Diagnosis. (Fig. 503, B, C, page 436.) Microscopic unicellular 

 plants, occurring isolated or in groups of definite form, usually 

 surrounded by a gelatinous investment, the cells exhibiting more 

 or less regular geometrical outlines, and enclosed by a membrane 

 striated or granular, either simply tough and continuous, or im- 

 pregnated with silex and separable into valves. Reproduction by 

 spores formed after conjugation of the cells (cT), by zoospores formed 

 from the cell- contents, and by division. 



ILLUSTRATIVE SUBORDERS. 



Subord. 1. DESMIDIE^] (fig. 503, B). Cell-membrane without silica, con- 

 taining chlorophyll and starch. Closterium, Nitzsch; Oosmarium, 

 Menegh. ; Euastrum, JShr. ; Pediastrum, Meyen ; Desmidium, Agh. 



Subord. 2. PIATOME.E (fig. 503, C). Cell-membrane impregnated with 

 silex, valmdar, containing a brown colour ing -matter. Eunotia, Ehr. ; 

 Diatoma, DC. ; Navicula, Sory, Isthinia, Agh. ; Melosira, Agh. 



Affinities, &c. These organisms were formerly included among Infu- 

 sorial Animalcules j but the vegetable character is very strongly marked 

 in Desmidieae ; and the reproduction by conjugation, characteristic of 

 certain tribes of Confervoids, occurs not only in Desmidiece, but in Dia- 

 tomece, which in respect to general organization cannot well be separated 

 from the Desmidiece, although the nature of the cell-contents has more of 

 the character of what we are accustomed to regard as animal substance 

 The Diatomece are also remarkable for the way in which they divide by 

 segmentation into a number of distinct frustules, each of which grows 

 into a perfect plant. 



