338 Pub. Puget Sotind Biol. Sta. Vol. 2, No. 53 



dent midrib which characterizes all the ligulate species. This would 

 account also for the increased thickness of the midrib toward the base 

 of the thallus (Plate 59, Fig. 4). 



In the short cylindrical stipe and the slender petiole-like bases of the 

 branches, very few ground tissue cells were laid down originally, and the 

 number is not increased from year to year. The greater part of the 

 tissue in these regions is made up of hyphal filaments which follow down 

 the main axis of the thallus or of its lateral branches. 



Jonsson (1901) was the first worker who correctly interpreted the 

 origin and structure of the secondary inner assimilation systems which 

 appear in the basal portions of the thallus of old D. aculeata plants. He 

 shows, by means of diagrammatic longitudinal sections, that the axial cell 

 row originates as a branch, growing down from the lower side of one of 

 the basal cells of a lateral branch of the main axial strand. This branch 

 elongates, growing downward among the cells of the ground tissue, and be 

 comes covered with laj^ers of small cells containing chromatophores, in th^ 

 same way as does the main axial strand. This same secondary develop- 

 ment occurs in all the ligulate species of the genus, but is carried out on a 

 much more extensive scale. 



It was brought out earlier in the discussion that while the lateral 

 branches of that portion of the axial filament above the intercalary grow- 

 ing point develop basipetally, the permanent branches below the growing 

 point increase in length by means of an intercalary growing point which 

 ir; formed after cortication begins, and which remains always at the edge 

 of the flat thallus. Just as the original cells of the main axis may by re- 

 peated division form 2, 3, or 4 cells, so the cells of the permanent branches 

 also increase in number by repeated divisions. This is especially noticeable 

 at the base of the branches, where division takes place so rapidly that a 

 series of short cells is produced. It will be remembered that originally 

 the lateral branches are inclined at an angle a little less than 90° from the 

 main axis. During this period of elongation and rapid cell division in the 

 lateral branch, the new cross walls formed by the dividing basal cells are 

 laid down parallel with the main axis, so that the basal portion of the 

 branch, as these cells elongate, stands squarely at right angles with the 

 main axis (Plate 59, Figs. 2, 3, 6 and Plate 62, Fig. 3). 



It is from the lower side of one of these basal cells that a lateral 

 branch grows out which ultimately becomes the axis of a secondary inner 

 assimilation system. This secondary branch may appear at any point along 

 the lower side of the cell, though it is usually at about the middle, giving 

 the branched cell the appearance of a T-tube. The branch elongates by 

 apical growth and cross walls are laid down back of the growing tip. As 

 the cells thus formed mature, they take on the appearance of typical sieve 



