OF THE DESMIDIE^. 11 



through the openings of the suture between the valves, they escape to a 

 distance and still keep up their active movement. 



In the genus Cosmarium this phenomenon is frequently and readily ob- 

 served. Mrs. Thomas, in her interesting obser\-ations on Cosmarium mar- 

 garitiferum (T. M. S. 1855, p. 33), has detailed the following appearances : — 

 " In each half (she wiites) the centre was occupied by a vesicle (as it appeared) 

 filled with moving granules, while smaller vesicles were at the four sides 

 (I. 2). The granules did not appear to circulate through the plant, but kept 

 to their own place, which was either a bag or cavity — I could not decide 

 which." In another example " the granules were swanning over the whole 

 plant." 



These peculiar movements of the granhles are not restricted to this tribe, 

 but are known to occur in many genera of Algae. Their piu'pose seems con- 

 nected with the reproductive process. Mrs. Thomas {he. cit.) refers to it as 

 in some way related with the formation of sporangia ; whilst Mr. Ralfs, who 

 speaks of the swarming particles as " zoospores," confesses himself perfectly 

 unacquainted with theii' subsequent histoiy, although he coincides with Pro- 

 fessor Harvey in regarding the phenomenon of swarming as a " strictly vege- 

 table pecuharity." 



Eepeoduction of Desmtdie^. — This function presents itself under two 

 phases, the end of one of which is to multiply or perpetuate the individual 

 plant, whilst that of the other is to reproduce the species. The fonner pur- 

 pose is attained by the process of fission, the latter by that of the development 

 of sporangia, and, it may be, by the swarming of zoospores. 



The act of self-division is frequently observed, and is in all respects the 

 same process as in the cells of other Algae, or indeed of any plant. Analogy, 

 and not, indeed, dii-ect obsei-vation, suggests as necessary the initiative action 

 of a nucleus to precede the constriction of the soft lining sac of the lorica, 

 i. e. of the primordial membrane, which is next followed by that of the harder 

 external coat. The proceeding is varied, in some non-essential particulars, by 

 the figiu'e of the fronds, and also by the cii'cumstance of its own completeness 

 or incompleteness. !Mr. Ralfs has well described the fission of Euastrmn (op. 

 cit. p. 4). The narrow connecting band between the two segments of the 

 frond lengthens and is " converted into two roundish hyaline lobules ; " these, 

 though at first veiy minute, increase rapidly in size, and exhibit from their 

 origin the deep constriction characteristic of the mature fronds. The advancing 

 growth of the interposed new fonnations necessarily pushes fiu'ther asunder the 

 original segments, which finally become disconnected, " each taking with it a 

 new segment to supply the place of that from which it has separated .... At 

 first the new portions are devoid of colour, and have much the appearance of 

 condensed gelatine ; but as they increase in size the internal fluid acquires a 

 green tint, which is at fii^st very faint, but soon becomes darker ; at length 

 it assumes a granular state. At the same time the new segments increase 

 in size and obtain their noiTaal figui'e ; the covering in some species shows 

 the presence of puncta or granules, and, as in Xanthidium and Staurastrumj 

 the spines and processes lastly make their appearance, beginning as mere 

 tubercles, and then lengthening until they attain their perfect fonn and size. 

 Complete separation, however, often occiu's before all these details of develop- 

 ment are complete (II. 11, 24, 26). This singular process is repeated again 

 and again, so that the older segments are united successively, as it were, with 

 many generations." Illustrations of this act are fiu-nished, in the case of two 

 species of Cosmarium, in the appended jDlates (I. 4 ; II. 26), to which the 

 above account will be found equally well to apply. 



^•In Sphcerozosma the same changes take place (I. 11), and are just as 



