12 GENERAL HISTOEY OF THE INEUSOlilA. 



evident; but the cells continue linked together, and a filament is formed, which 

 elongates more and more rapidly as the joints increase in mmiber. This 

 continued miiltif)lication by division has its limits ; the segments gradually 

 enlarge whilst they divide, and at length the plant ceases to grow; the division 

 of the cells is no longer repeated ; the internal matter changes its appearance, 

 increases in density and acquires starch-granules, which soon become nume- 

 rous ; the reproductive gi*anules are perfected, and the individual perishes. 

 In a filament the two oldest segments are found at its opposite extremities ; 

 for so long as the joints divide, they are necessarily separated further and 

 further from each other. Whilst tins process is in progress, the filament in 

 Sjohcerozosma consists of segments of all sizes (1. 11) ; but after it has reached 

 matuiity there is httle inequahty between them, except in some of the last- 

 formed segments, which are permanently smaller. The case is the same with 

 those genera in which the separation of the cells is complete .... It is obvious 

 that the new portions must arise from the whole of the junction-margin of 

 the original valves ; consequently when the junction occupies only a part of 

 the breadth, the new portion will be naiTower than the old ; but when the 

 junction of the valves is as broad as the cell, the new poiiion will from the 

 beginning be of the same breadth," and will remain undistinguishable by its 

 size when fission is complete. 



Mr. Balfs goes on to say that, " when the ceU is oblong, or only rounded 

 at the extremities, the process, though similar, is less evident ; the cell at 

 first seems merely to elongate (II. 11), until it attains nearly twice its ori- 

 ginal length, when the division commences, and the rounding of the new ends 

 becomes apparent. The tapering cells present but little diiference, for the 

 separation takes place before the extremities are fully developed ; sometimes 

 these cells separate obhquely, as in Sjpirotce7iia.'^ 



The mode of self-division in Closterium has been illustrated by the Rev. 

 Mr. Osborne (J. M. 8. 1854, p. 57), from whose account we abstract the fol- 

 lowing particulars : — '^ I have (he says) watched for hours the process of 

 complete self-division ; one-half of the frond has remained passive, the other 

 has had a motion from side to side, as if moving on an axis at the point of 

 juncture ; the separation has become more and more ardent, the motion more 

 active, until at last, with a jerk, one segment leaves the other," each having 

 one extremity — the one newly formed along the line of junction of the two 

 segments — much more obtuse than the other. '^ The circulation of the con- 

 tained globules for some hours previons to subdivision, and for some few hours 

 afterwards, runs quite round the obtuse end of the endochrome." 



Previously to complete separation each segment begins to show a central 

 constriction of its endochrome, which in due time extends across the new 

 frond, and constitutes the median clear space or band. 



A true reproductive act is presented by the act of conjugation, or coupling 

 of two fronds, and by the resultant development of a sporangium (II. 6, 8 ; 

 XYI. 11, 12, 13, 14). This process consists in the apposition and subsequent 

 intercommunication of the cavities and contents of two cells, which may be 

 free, or otherwise, members of a chain or filament. It is an act not peculiar 

 to the Desmidieae, but common to them along with the Diatomeae and Con- 

 jugat£e. " In the family Conjugatae (says Mr. Ralfs) the cells conjugate 

 whilst still forming parts of a fidament ; but in the Desmidiese the filamentous 

 species almost invariably separate into single joints before their conjugation, 

 and in most of the species the valves of the cells become detached after they 

 are emptied of their contents." To bring about the necessary apposition, it 

 is usual for the conjugating cells to expand or bulge out on those sides which 

 are to come into union ; and whilst this is proceeding, the vesicles or globules 



