DESMIDIACEiE. 



[ 196 ] 



DESMIDIACE^. 



first becomes constricted, since specimens 

 are met with in which this appears divided 

 into two portions in the hne of the division. 

 The constriction of the outer cell-wall at 

 length becomes complete, the halves sepa- 

 rate, and the truncated 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 division takes 

 place in a manner apparently resembling 

 that occurring in the filamentous Confervae. 

 Here there is no necessity for the subsequent 

 restoration of symmetry, as in Closterium. 

 In those forms where hairs, globular or ellip- 

 tical, or angular lobes are united by a narrow 

 neck (bipartite forms), the process of divi- 

 sion is very curious, and displays itself very 

 clearly. To jiroduce 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 

 their development often far advanced, before 

 the division of the parent is completed. The 

 central region of the isthmus expands and 

 displaystwo 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 'half-frustules,' 

 and they increase in size (PL 10. fig. 11), 

 gradually pushing the halves of the parent- 

 cell apart, until they form two complete half- 

 frustules, back to back, connected by a short 

 neck, at which point they are sooner or later 

 detached from one another. In Sjjhcerozosma 

 the cells thus produced remain connected in 

 rows in a gelatinous sheath, and this mode 

 of division is well illustrated by the cells in va- 

 rious stages sometimes seen in such filaments; 

 in Euastrum, Cosmarium, Staurastrum, &c., 

 the new cells separate, the old ' half-frus- 

 tules ' taking away each their new ' halves ' 

 as new bipartite 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. 



A second mode of reproduction has been 

 described by Caspary, and more fully by A. 

 Braun, in Pediastrum. The contents of the 

 parent-cells become retracted from their 

 walls, and the whole transformed into a 

 number of active ciliated zoospores, which 

 are discharged within a delicate sac from the 

 parent, and after some time come to rest 

 and arrange themselves within this sac (PI. 6. 

 fig. 11) into a colony having the regular 



pattern of the species, each zoospore be- 

 coming one of the notched frustules of the 

 group (see Pediastrum). 



A third process, analogous to this, has 

 been observed by Pringsheim in the genus 

 Coelastrum (Nageli), likewise composed of 

 grouped families : here the contents of each 

 cell are divided into a number of portions, 

 as if for the formation of zoospores, 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- cell splits and peels 

 off", leaving them as the foundation of a new 

 group. This process bears the same relation 

 to the preceding as the formation of the 

 small resting-spores (without conjugation) 

 does to that of zoospores in (Edogonium, &c., 

 or the winter-spores to the moving young in 

 Volvox. Connected with this is a phaeno- 

 menon which has been observed and figured 

 in Closterium by Focke, where the entire 

 green contents were wholly retracted from 

 the walls, and broken up into a number of 

 green encysted globules (PI . 6. fig. 3 B), closely 

 resembling the above-named resting-spores or 

 winter-spores of FoZyoa?(Pl. 3. figs. 26,34), &c. 

 The fourth mode of reproduction is by 

 what is called conjugation, where two parent- 

 cells contract an organic union, their cavities 

 becoming continuous, and their contents be- 

 coming blended to form the substance of a 

 spore. The details of this process will be 

 found under Conjugation, and also under 

 Closterium and other genera of this fa- 

 mily; here we have merely to add some 

 observations respecting the sporanges or 

 spores, whichever they may be, formed after 

 conjugation. These are at first cellulose 

 vesicles filled with green and granular con- 

 tents ; by degrees the latter become brown 

 or red, and the coats become thickened. In 

 some genera the coats remain smooth, in 

 others they acquire a granular, tuberculated 

 or even spinous surface (PI. 10. 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 by Ralfs and others ; Ehrenberg 

 described them as species of Xanthidium.) 

 The ultimate history of the sporanges is at 

 present obscure. In regard to those of Clo- 

 sterium, some information exists; both Jenner 

 and Focke describe and figure a globular 

 gelatinous mass, ap])arently produced from 

 a sporange, in which were imbedded a num- 

 ber of minute frustules(P1.6.fig.3A,c?). Per- 

 haps the conditions may vary here, as they 



