MICROSCOPIC FORMS OF VEGETABLE LIFE. 265 



263. When the single cell has come to its full maturity, it commonly 

 multiplies itself by Unary subdivision ; but the plan on which this takes 

 place is often peculiarly modified, so as to maintain the symmetry charac- 

 teristic of the tribe. In a cell of the simple cylindrical form of those of 

 Didymoprium (Fig. 160), little more is necessary than the separation of 

 the two halves at the sutural line, and the formation of a partition be- 

 tween them by the infolding of the primordial utricle ; and in this man- 

 ner, out of the lowest cell of the filament A, a double cell, B, is produced. 

 But it will be observed that each of the simple cells has a bifid wart-like 

 projection of the cellulose wall on either side, and that the half of this 

 projection, which has been appropriated by each of the two new cells, 

 is itself becoming bifid, though not symmetrically ; in process of time, 

 however, the increased development of the sides of the cells which remain 

 in contiguity with each other brings up the smaller projections to the 

 dimensions of the larger, and the symmetry of the cells is restored. In 

 Closterium (Fig. 156, D), the two halves of the eridochrome first retreat 

 from one another at the sutural line, and a constriction takes place 

 round the cellulose wall ; this constriction deepens until it becomes an 

 hour-glass contraction, which proceeds until the cellulose wall entirely 

 closes round the primordial utricle of the two segments ; in this state, 

 one half commonly remains passive, whilst the other has a motion from 

 side to side, which gradually becomes more active ; and at last one seg- 

 ment quits the other with a sort of jerk. At this time a constriction is 

 seen across the middle of the primordial utricle of each segment, indicat- 

 ing the formation of the sutural band ; but there is no division of the 

 cell-cavity, which is that belonging to one of the halves of the original 

 entire cell. The cyclosis, for some hours previously to subdivision, and 

 for a few hours afterwards, runs quite round the obtuse end a, of the 

 endochrome; but gradually a transparent space is formed, like that at 

 at the opposite extremity, by the retreat of the colored layer ; whilst, 

 at the same time, its obtuse form becomes changed to a more elongated 

 and contracted shape. Thus, in five or six hours after the separation, the 

 aspect of each extremity becomes the same, and each half resembles the 

 cell in whose self-division it originated. 



264. The process is seen to be performed after nearly the same method 

 in Staurastrum (Fig. 155, D, E); the division taking place across the cen- 

 tral constriction, and each half gradually acquiring the symmetry of the 

 original. In such forms as Cosmarium, however, in which the cell con- 

 sists of two lobes united together by a narrow isthmus (Fig. 158), the 

 division takes place after a different method ; for when the two halves of 

 the outer wall separate at the sutural line, a semiglobular protrusion of 

 the endochrome is put forth from each half ; these protrusions are sepa- 

 rated from one another, and from the two halves of the original cell 

 (which their interposition carries apart), by a narrow neck ; and they pro- 

 gressively increase until they assume the appearance of the half-segments 

 of the original cell. In this state, therefore, the plant consists of a row 

 of four segments, lying end to end, the two old ones forming the extremes, 

 and the two new ones (which do not usually acquire the full size or the 

 characteristic markings of the original before the division occurs) occu- 

 pying the intermediate place. At last the central fission becomes 

 complete, and two bipartite fronds are formed, each having one old and 

 one young segment : the young segment, however, soon acquires the full 

 size and characteristic aspect of the old one ; and the same process, the 

 whole of which may take place within twenty-four hours, is repeated ere 



