DESMIDIACEiE. 



[ 244 ] 



DESmDIACE^. 



the parent-cell acquires a constricted ap- 

 pearance iu the middle, probably not by 

 actual constriction, but by the two halves 

 retreating from each other, Avhile a new 

 hour-glass-shaped prolongation of the mem- 

 brane is formed in the middle. It appears 

 probable also that the primordial utricle 

 first becomes constricted, since specimens 

 are met with in which this appears divided 

 into two portions in the line of the division. 

 The constriction of the outer cell-wall at 

 length becomes complete the halves sepa- 

 rate and the truncate new end of each then 

 grows out so as to restore the symmetry of 

 the new frustule. In such forms as Desmi- 

 dium, Didymoprium, &c., the di\"ision takes 

 place in a manner apparently resembling 

 that occurring iu the tilamentoas Confervte. 

 Here there is no necessity for the subsequent 

 restoration of sjTnmetry, as in Closteriuin. In 

 those forms where pau's of globular or ellip- 

 tical or aug-ular lobes are united by a narrow 

 neck (bipartite forms), the process of divi- 

 sion is very curious, and displays itself very 

 clearly. To produce two new symmetrical 

 frustules out of one, it is evident that two 

 new half-frustules must be formed, as in 

 Closterium ; but in the present cases the 

 foundations of the new halves are laid, and 

 theii' development often far advanced, before 

 the division of the parent is completed. 

 The central region of the isthmus expands 

 and displays two globular enlargements, 

 separated from each other, and from each 

 half of the parent, by a neck. These two 

 enlargements are the rudiments of the new 

 *haK-frustules;' and they increase in size 

 (PI. 14. fig. 11), gradually pushing the halves 

 of the parent-cell apart, until they form two 

 complete half-frustules, back to back, con- 

 nected by a short neck, at which point they 

 are sooner or later detached from one an- 

 other. In Sjjhcevozosma the cells thus pro- 

 duced remain connected in rows in a gela- 

 tinous sheath ; and this mode of division is 

 well illustrated by the cells iu various stages 

 sometimes seen in such filaments ; in Euas- 

 trum, Cosmarium,Staur<istrum, &c.,the new 

 cells separate, the old half-frustules taking 

 away each their new halves as new bi- 

 partite individuals. The membrane of the 

 nascent halves is very delicate, and at first 

 devoid of the characteristic markings and 

 processes ; and it often happens that these 

 are not completely formed before the division 

 is complete. Archer has described some 

 monstrosities of these new halves. 



A second mode of reproduction has been 



described by Caspary and Braun, in Pedi- 

 astrum. The contents of the parent-cells 

 become retracted from their walls, and 

 the whole transformed into a number of 

 active ciliated zoospores, which are dis- 

 charged within a delicate sac from the 

 parent, and after some time come to rest 

 and arrange themselves within this sac (PI. 

 10. %. 11) into a colony having the regular 

 pattern of the species, each zoospore be- 

 coming one of the notched frustules of the 

 group (see Pediasteum). 



A third process, analogous to this, has 

 been observed by Priugsheim in the genus 

 Coelastrum, hkewise composed of grouped 

 families : here the contents of each cell are 

 divided into a number of portions, as if for 

 the formation of zoospores {still zoosjwyes), 

 but no motion takes place ; they acquire 

 cellulose coats, arrange themselves within 

 the parent according to the typical pattern ; 

 and then the wall of the parent-ceU splits 

 and peels off, leaving them as the founda- 

 tion of a new gxoup. Comiected with this, 

 is a phenomenon which has been observed 

 and figured in Closterium, by Focke, where 

 the entire green contents were only retracted 

 from the AvaUs, and broken up into a number 

 of green encysted globules (PI. 10. fig. .3B), 

 closely resembling the thick-walled resting- 

 spores or winter-spores of Volvox (PI. 7. 

 figs. 20, 34), &c. 



The fourth mode of reproduction is by 

 what is called Conjugation, where two cells 

 of a single filament, or of two separate fila- 

 ments, contract an organic union, their cavi- 

 ties becoming contimious, and their contents 

 becoming blended to form the substance of 

 a zygospore. These are at first cellulose 

 vesicles filled with green and graniUar con- 

 tents, then- starch being converted into 

 fat ; by degrees they become brown or red, 

 and the coats become thickened. In some 

 genera the coats remain smooth : iu others 

 they acquire a granular, tuberculated or 

 even a spinous surface (PL 14. fig. 12), 

 these spines being either simple or forked. 

 Bodies exactly resembling these are found 

 fossil in flint, and are regarded as of the 

 same nature bj- Ralfs and others ; Ehrenberg 

 described them as species of Xaxthidium. 

 Li germination, the contents of the 

 zygospore escape as a thin-walled vesicle, 

 and become divided into a more or less 

 numerous brood of secondary cells,resembling 

 those of the parents (PI. 10. fig. 3 A d.). 



The Desmidiaceae may be collected in the 

 same manner as is recommended for the 



