336 Pnh. Pnget Sotaid Biol. Sta. Vol. 2, No. 53 



branches of the axial filament, from which the first cortical cells were cut 

 off, have elongated and the adjoining cells have divided. The growing 

 region of the branch hair is thus moved outward, so that branches below 

 the growing point of the main axis have intercalary growth, while branches 

 above the growing point, as previously explained, develop basipetally 

 (Plate 62, Figs. 1 and 2). The dividing cortical cells extend outward 

 along the bases of the lateral branches, as well as around the axial cells, 

 so that when the axial cells have one layer of cortical cells covering their 

 lateral walls between the paired branches, there is a flat extension of 

 cortical cells surrounding and between the bases of the successive lateral 

 branches which is two or three cells broad. Division of cortical cells in 

 the plane of the lateral branches proceeds quite rapidly, the cells of the 

 axial filament elongate and divide, and there is thus formed a flat thallus 

 consisting of a filament of large cells bearing distichous branches, the whole 

 covered by a single cortical layer of very small cells. The lateral branch- 

 es increase in length by addition of cells at the intercalary growing point, 

 and the cortical growth spreads outward, keeping the branches covered al- 

 most to their growing points. 



At a distance 4 to 6 axial cells behind the point where complete corti- 

 cation is effected, the cortical cells have begun to divide parallel to the 

 surface of the thallus, thus forming the beginning of the ground tissue. 

 A few cells farther back, another parallel division has taken place, the inner 

 cell of the first division having greatly enlarged meanwhile, accompanied 

 by the rapid division of the meristematic outer cortical layer in a plane 

 perpendicular to the flat surface of the thallus, to keep pace with its rapi^ 

 increase in length and width. In a small pinnule well stained and clear- 

 ed one can focus through the successive layers of cells and make out their 

 size, shape and arrangement very distinctly. At this point all the cells 

 of the thallus still contain chromatophores, the small cortical cells of the 

 surface layer being densely crowded with them, while the rapidly en- 

 larging cells of the ground tissue still have a few scattered through the 

 layer of peripheral protoplasm. They are also present in the axial cells. 



As Jonsson (1901) says, there is no definite point at which primary 

 growth stops and secondary growth begins. The material used in this 

 study still bore assimilating hairs, though most of them had been shed. 

 Cortication was still in progress at the tips of some of the pinnules, while 

 the formation of the secondary assimilation system had already begun 

 farther back toward their bases. 



In focusing down through the superficial walls, the inner assimilation 

 system first appears as a series of long slender filaments lying alongside 

 the large cells of the axial filament. In cross section the axial filament 

 appears to be surrounded, or partly surrounded, by small cells (Plate 62, 

 Fig. 6). In longitudinal section, the filaments can be seen originating 



