330 Pnh. Paget Sound Biol. Sta. Vol. 2, No. 53 



tJon he carried on this work, explains in a later article "the i^eculiar ap- 

 pearance of an inner assimilation tissue," assuming that lively respira- 

 tion goes on in the large protoi^lasm-filled cells of the central cylinder, 

 and that the CO^, formed ini this way is taken up immediately in sunlight 

 by the inner assimilation system, while the outer assimilation system 

 makes use of the COj absorbed from the surrounding water. This ar- 

 rangement he considers necessary because of a lack of either stomata or 

 intercellular spaces, as in higher plants, to facilitate gas exchange. 



Soderstrom also described and figured cross sections of the thallus 

 from near the holdfast, where "certain cells of the ground tissue" had 

 also become surrounded by a system of very small thin-walled cells which 

 had the same appearance as those of the inner assimilation system. Jonsson, 

 by means of diagrammatic longitudinal sections, proves that the central 

 cell of that group does not belong to the ground tissue, but is formed by 

 secondary branching of the lateral branches of the central axis ; these 

 secondary branches, arising from the lower side of the lateral branches, 

 grow downward toward the base of the plant between the cells of the 

 ground tissue, and become surrounded, like the cells from which they origi- 

 nate, by a tissue of small assimilation cells. 



Jonsson calls Soderstrom's inner assimilation tissue a "secondary as- 

 similation tissue" and describes a "tertiary assimilation tissue" in the 

 older parts of the thallus. This is formed by outgrowths from the sec- 

 ondary assimilation system which pierce the walls of the central cylinder 

 cells and form within the cell cavities a dense mass of hypha-like growths. 

 These divide by cross walls to form small, closely packed cells which 

 contain many chromatophores and therefore aid in assimilation. If Wille's 

 assumption as to the necessity for the secondary assimilation system be 

 correct, one wonders how the formation of this tertiary system can be 

 explained on the basis of function? 



The conducting hyphae, as described by Soderstrom, also originate 

 from the cells of the ground tissue. They seldom arise in young parts of 

 the thallus, but develop gradually, with the growth of the plant. In older 

 parts they are found penetrating everywhere between the cells of the 

 ground tissue, so that the latter may become widely separated. The hypha 

 first appears as a sac-like outgrowth from the side of a cell of the ground 

 tissue, is cut off from the parent cell by a cross wall, and elongates by 

 terminal growth. Cross walls are formed as growth progresses. It forces 

 its way among the cells of the ground tissue, twisting and turning, but 

 keeping its course steadily downward toward the base of the plant. These 

 hyphae branch repeatedly and twist and turn about themselves and about 

 the cells of the ground tissue until it is impossible to follow a single hypha 

 for any distance through the tangled mass. Since their growth is steadilv 



