S28 Pub. Puget Sound Biol. Sta. Vol. 2, No. 58 



The newest part of the thallus is a single row of articulate cells, lying 

 just below the growing region, which bears opposite distichous branches 

 at frequent intervals. According to Soderstrom, each original cell of the 

 nionosiphonous thallus below the growing region bears opposite branches, 

 which must therefore stand very close together. This is not the case for 

 long, however, as observation will show that at a short distance below the 

 growing region the segments bearing branches are separated by a distance 

 the length of three cells, showing that the original segment cell, through 

 successive divisions, has given rise to four cells. Soderstrom says that 

 no further division of the segment cells takes place, only an elongation to 

 keep up with the growth of the thallus, as repeated observation shows 

 branches arising from every fourth cell. 



According to observations made by Reinke (1880) and confirmed by 

 Soderstrom, the cortical cells arise as outgrowths from the axils of op- 

 posite lateral branches of the monosiphonous thallus at some distance be- 

 low the growing region. By repeated transverse divisions these outgrowths, 

 instead of developing as hair branches, form filaments which are applied 

 to the axial cell. These filaments, encircling the axial cell, meet and fuse, 

 and by repeated divisions form a layer of small cells surrounding the 

 original single cell row of the thallus. To the writer it would seem that 

 the idea expressed by Jonsson is more nearly correct, namely, that from 

 the bases of opposite lateral hairs cells are cut off which, by repeated di- 

 visions, formi a girdle about the central cell. Repeated examination of the 

 growing tips of three different species, both in cross section and from the 

 surface, gave no indication of the development of filaments which were 

 applied to the axial cell. 



When the axial cell has become completely enclosed, the cells of the 

 investing layer, or cortex, divide tangentially, forming a surrounding 

 tissue two cells thick. The inner layer, immediately surrounding the cen- 

 tral axis, is the beginning of the ground tissue, and from it, later, through 

 secondary growth, develops the "inner assimilation tissue." The outer 

 layer remains permanently meristematic, cutting off by repeated tangential 

 divisions cells which are added to the ground tissue, and dividing radially 

 to accommodate the increasing size of the thallus. According to Jonsson 

 the cells of the ground tissue never increase their number by division, the 

 increase is always by addition from the outer meristematic layer. The 

 cells of the ground tissue become very greatly enlarged in length and diam- 

 eter, are smaller at the extremities, with their ends lapping past each other, 

 and in older parts of the frond tlie walls are considerably thickened. 



According to Harvey (1858), the manner of growth in D. ligulata, 

 which is typical of the ligulate forms, is precisely similar to that in the 

 filiform species, "except that the new cellular integument to the primary 



