1920 Pease: on Desmarestia 339 



tabes, their cross walls give the callose reaction, and protoplasmic con- 

 nections through the sieve plates can be demonstrated. As the branch de- 

 velops it becomes invested with a layer of assimilating cells in its older 

 parts, and as it grows downward through the ground tissue of the lamina 

 it may give off branches which in turn acquire an investment of assimilat- 

 ing cells (Plate 59, Figs. 2, 3, 6, and Plate 62, Fig. 7). 



It is obvious that these secondary assimilation systems must originate 

 in younger parts of the thallus, before the cells which bear them have be- 

 come closely invested with assimilating cells and several layers of closely 

 interwoven conducting hyphae. The entire mature thallus is penetrated by 

 these branching systems of sieve tubes surrounded by their assimilating 

 cells, and all in connection, through the lateral branches of the midribs of 

 the pinnae, with the larger branches, and so with the main axis of the com- 

 pound thallus. 



Soderstrom developed the idea of two secondary tissues originating 

 in the cells of the groimd tissue. The foregoing discussion shows that the 

 axial cells of the "original thallus" also give rise to a secondary tissue in 

 exactly the same manner. There are thus two systems of secondary tissues, 

 one developing from the original thallus, and the other from the ground 

 tissue which surrounds it. 



The holdfast in D. aculenta, according to Soderstrom, is made up in 

 the central part mainly of a mass of interwoven and outspreading hyphae, 

 which function more as mechanical support than for conducting, while the 

 tissue of the peripheral parts is made up of rectangular cells, lying in 

 regular parallel rows. In T>. ligulata the central tissue of the holdfast is 

 a mass of filamentous hyphae branching and running in all directions be- 

 tween the few cells of the original ground tissue, which have become widely 

 separated by this secondary growth. These h3'phae originate not only in 

 the ground tissue among which they lie, but descend along the midrib of 

 the stipe from the ground tissue cells of the lower parts of the frond. 

 Through all this central tissue ramify numerous branching sieve tubes 

 surrounded by assimilating cells, the sieve tubes originating as secondary 

 outgrowths from the lowest branches of the main axis, which naturally 

 forms the central structure of the holdfast, as it does of all other parts of 

 the frond. 



In the cortical region of the holdfast, the new cells which are cut off 

 from the inner side of the outermost! layer do not enlarge to form ground 

 tissue, but remain of practically the same diameter as the mother cell, al- 

 though they may elongate somewhat perpendicular to the plane of division. 

 Since several of these cells may be cut off during one season's growth, the 

 resulting tissue has the appearance of a mass of closely ]iackcd, roughly 

 parallel filaments. 



