200 Transactions of the Society . 



develope into cortical cells (fig. 4) ; thus the tetragonidia are 

 imbedded in the substance of the thallus, being in communication, 

 by means of a neck of protoplasm, with their mother-cell on the 

 axial side, and with the two cortical cells on their peripheral side, 

 these last being daughter-cells of the tetragonidium. These out- 

 growths from the tetragonidia originate by the method of cell- 

 division known as gemmation, appearing as minute papillae, which 

 increase in size and become constricted at the point of attach- 

 ment with the mother cell. The cortical cells and tetragonidia 

 that originate from an axial cell are also the results of gemmation. 

 It will thus be seen that the increase in size of a Polysiphonia 

 is the result of two distinct methods of cell-formation; the axial 

 row of cells, by which the plant increases in length, being the 

 result of fission or segmentation of the apical cell, whereas all 

 increase in thickness is due to gemmation from the axial cells. 

 The manner in which the continuity of protoplasm between adjacent 

 cells is kept up varies with the age of the cells. When a segment 

 is cut off from the apical cell, the partition wall is formed gra- 

 dually (fig. 5), but before reaching the centre its growth ceases, 

 so that a circular opening is left through which the contracted 

 neck of protoplasm passes unbroken. This opening increases in 

 size with the growth of the cell, being much larger in old than 

 in young cells. After the openings have reached a certain size, 

 they are closed by the growth of a cellulose plate, to the margin of 

 which the protoplasmic sac or " primordial utricle " is attached ; 

 this plate is perforated with minute holes through which slender 

 threads of protoplasm pass. The attachment of this plate to the 

 margin of the original large opening is not very firm and it can 

 be readily removed, when it presents the appearance of a flat or 

 convex disk (fig. 6). 



The first increase in size of the cells is in a radial direction, 

 the stem attaining its fuU thickness at a short distance behind 

 the growing point (fig. 10) ; afterwards the cells rapidly increase 

 in length by acropetal and basipetal growth, the rate of each being 

 shown by the relative length of the cell above and below the neck 

 of protoplasm connecting it with the axial cell, which may be 

 looked upon as a fixed point (fig. 11). After the cell has reached 

 its full development, the protoplasm disappears, leaving the empty 

 contracted protoplasmic sac ; the cell- wall at the same time increases 

 in thickness, stratification being in most species very distinct 

 (figs. 8, 9). 



The method of branching is dichotomous, segments being cut 

 off from the apical cell by inclined septa (fig. 14), the axis of 

 growth of the branch being at right angles to the septum which 

 cuts off the first cell of the branch from the apical cell. 



