582 MICROSCOPIC FORMS OF VEGETABLE LIFE THALLOPHYTES 



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

 multiplies itself by binary subdivision ; but the plan 011 which this 

 takes place is often peculiarly modified, so as to maintain the 

 symmetry characteristic of the tribe. In a cell of the simple 

 cylindrical form of those of Desniidiutn (fig. 440), little more is 

 necessary than the separation of the two halves at the sutural line, 

 and the formation of a partition between them by the infolding of 

 the primordial utricle ; in this manner, 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, how 

 ever, the increased development of the sides of the cells which re- 

 main 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. 436 ; Plate IX, fig. 2) the two halves of 

 the endochrome 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 hourglass-like contraction, which pro- 

 ceeds 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 segment 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, indicating the 

 formation of the sutural band ; but there is 110 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, 

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

 of the endochrome ; but gradually a transparent space is formed, 

 like that at the opposite extremity, by the retreat of the coloured 

 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 by the division of which it 

 originated. 



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

 in Staurastrum, the division taking place across the central con- 

 striction, and each half gradually acquiring the symmetry of the 

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

 consists of two lobes united together by a narrow r isthmus, the divi- 

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

 of the outer wall separate at the sutural line, a semi-globular protru- 

 sion of the endochrome is put forth from each half; these protru- 

 sions are separated from each other and from the two halves of the 

 original cell (which their interposition carries apart) by a narrow 

 neck ; and they progressively increase until they assume the appear- 

 ance of the half-segments of the original cell. In this state, there- 

 fore, 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 



