138 Bibliographical Notices. 



and processes make their appearance, beginning as mere tubercles, 

 and then lengthening until they attain their perfect form and size ; 

 but complete separation frequently occurs before the whole process 

 is completed. This singular process is repeated again and again, so 

 that the older segments are united successively, as it were, with many 

 generations. In Sphcerozosma the same changes take place, and are 

 just as evident, but the cells continue linked together, and a filament 

 is formed, which elongates more and more rapidly as the joints in- 

 crease in number. This continued multiplication 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 contains starch-granules which soon become numerous ; 

 the reproductive granules are perfected, and the individual perishes. 

 In a filament the two oldest segments are found at its opposite ex- 

 tremities ; for so long as the joints divide they are necessarily sepa- 

 rated further and further from each other. Whilst this process is 

 in progress the filament in SphtErozosma consists of segments of all 

 sizes ; but after it has reached maturity there is little inequality be- 

 tween 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. I admit that the di- 

 vision of the cells just described apparently differs greatly from that 

 in other simple Algae ; but I believe that the process in all is essen- 

 tially the same, and that whatever differences exist are modifications 

 necessarily resulting from the different forms of the cells. In the 

 examples already given the cell itself consists of two distinct por- 

 tions, having a constriction between them ; hence each of the new- 

 formed portions is similarly distinct from the older one which forms 

 it and to which it is united. 



" In order fully to elucidate the subject, cells may be distributed 

 into three principal kinds, distinguished by their form : 



" 1st. Bipartite cells, already described, and more or less con- 

 stricted at the middle ; 



" 2nd. Cells globose or rounded at the ends, or having the extre- 

 mities attenuated ; 



" 3rd. Cylindrical cells. 



" Bipartite cells belong only to the Desmidiece; cells globose or 

 roundish at the ends are seen in the Nostocs and Palmellece; atte- 

 nuated cells in the J)esmidie(B ; and cylindrical ones in the Conjugate, 

 Tiresias, &c. 



" 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 

 narrower than the old ; but when the junction of the valves is as 

 broad as the cell, the new portion will from the beginning be of the 

 same breadth. From this important fact, we may explain the differ- 

 ent sorts of division. Since in the two latter kinds of cell the valves 

 are united by their entire breadth, the new portions cannot be di- 

 stinguished by their size, we must therefore have recourse to other 



