120 PLANT LIFE. 



to the basal end of certain of the cells. These concentric 

 rings are the result of intercalary cell-division, and originate 

 as follows. The young alga, whether originating from a 

 sexually produced resting-spore or from an asexual zoo- 

 spore, becomes attached at one end to some solid body, 

 the free end elongating for some time, and eventually be- 

 coming divided into two cells by the formation of a trans- 

 verse septum. Immediately above the septum, that is, near 

 to the basal portion of the terminal cell, a narrow zone of 

 the cell-wall becomes very much thickened by the deposit of 

 cellulose, in the form of a ring projecting inwards into the 

 cavity of the cell. When a sufficient supply of cellulose has 

 accumulated, the outermost portion of the cell-wall splits 

 in a circular manner over the ring-like thickening, and the 

 ridge of cellulose elongates into a cell which is interposed 

 between the two previously existing ones. The two margins 

 of the outer cell-wall, caused by the splitting over the region 

 of the accumulated internal ring .of cellulose, are now 

 separated from each other by the length of the new cell, 

 resulting from the extension into a long, circular cell of the 

 thickened ring, and one split circular margin points to the 

 apex of the cell, and the other, that has been pushed upwards 

 by the formation of the cell, points to the base of the plant. 

 A transverse septum, formed immediately above the internal 

 thickened ring, separates the cavity of the new cell from 

 that of the terminal one. If the above account has been 

 understood, it will be seen that the diameter of the new 

 interposed cell is slightly less than that of the two original 

 cells, less in fact by the thickness of the outermost portion 

 of the outer cell-wall, that split to allow of the extension of 

 the thick ring of cellulose to form the new cell-wall. After 

 the new cell is fully grown a thick zone forms at its base, 



