568 Transactions of the Society. 



septum in the apical coll, are connected laterally by a protoplasmic 

 thread passing through the vertical septum. Each is the apical 

 cell of a new branch, which eventually, owing to the appearance 

 of a transverse septum, is cut up into a segment and an apical 

 cell. Each segment forms the basal cell of a new branch, and is 

 joined to three other cells by protoplasmic threads; to its sister 

 basal cell, laterally; to the cell below, from which it was seg- 

 mented, and to the cell above, which is the second cell of the new 

 branch. 



The wall in young cells consists at first of pure cellulose, and 

 remains as such until, owing to surface growth, the cell has in- 

 creased considerably in size. Surface growth is rarely uniform over 

 all points of a cell- wall, and as a rule is much more vigorous in 

 the direction of the axis of growth than transverse, so that a cell 

 originally presenting the appearance of a disc much broader than 

 long, becomes not unfrequently ten, or even twenty times as long as 

 broad. When pericentral cells are cut off from an axial cell by 

 vertical septa, they grow most in the same direction as the latter, 

 and usually at the same rate, thus giving origin to a stem com- 

 posed of fascicles of superposed cells of equal length, as in the genus 

 Polysiphonia ; but when branches spring from an axis their com- 

 ponent cells increase most in the direction taken by the new grow- 

 ing point, which may be at right angles to that of the parent stem. 

 Cortical cells, or those developed for the purpose of adding to the 

 substance of the axis, differ in origin from branches which form new 

 axes. The latter appear as protuberances before separation from 

 the mother-cell by a septum, while the first indication of cortical 

 cells is the presence of curved septa, cutting off portions of the 

 mother-cell, soon after its segmentation from the apical cell. This 

 mode of cortical cell development can be well studied in the genus 

 Polysiphonia. 



In some instances the cortication of the stem is due to adpressed 

 branches, as in the genus Ceramium, where the stem consists of a 

 single row of superposed cells. From the anterior end of each cell, 

 as in Batrachospermum, a whorl of branches originate, which 

 instead of developing in a normal manner, and leaving the stem at 

 an angle, remain adpressed to it, and by cell development cover it 

 more or less completely. In Batrachospermum the whorled 

 branches spread more or less at right angles to the stem, but the 

 secondary branch which springs from the basal cell of each of the 

 whorls of branches, grows downwards and is closely adpressed to 

 the stem. These corticating branches continue to grow downwards 

 until they reach the base of the stem or nearly so, where they act 

 as rhizoids, and assist in fixing the plant, so that towards the base 

 the stem of an old plant is densely corticated, whereas near the 

 growing point the adpressed branches may be seen starting from 



