1920 Pease; on Desmarestia 337 



in the large cells of the ground tissue. When the filaments first begin 

 to elongate, cross walls are rather infrequent, the cells of the filaments 

 having a length of from 2 to 5 times their diameter (Plate 62, Figs. 3, 4, 5). 

 Intercalary cross walls soon appear, the filaments become more numerous 

 and begin to twist and turn about the central axis and about each other as 

 they force their way along the path of least resistance toward the base of 

 the pinnule. In this way a mass of small cells is formed about the axial 

 cells (Plate 59, Figs, 2-5). The filaments making up this assimilating 

 tissue elongate not by elongation of the individual cells, which remain 

 small and pratically isodiametric, but by apical growth, the end of the 

 filament elongating and forming cross walls. 



The "conducting hyphae" of Soderstrom originate in the same way as 

 the inner assimilation tissue, as outgrowths from the large cells of the 

 ground tissue. Soderstrom says that the cells of the ground tissue never 

 divide, but any increase in their number is by addition from the cortical 

 meristem. The writer has found all degrees, from the elongation of the 

 entire cell with formation of cross walls, forming a chain of several large 

 cells which gradually narrows to a small filament, down to small slender 

 outgrowths from the end or side of the ground tissue cell. The hyphae 

 vary exceedingly in diameter, and branch repeatedly, forming a tangled 

 network which binds the cells of the ground tissue firmly together, mak- 

 ing the lamina a tough pliable sheet. 



Inner assimilation tissue and conducting hyphae have the itavae. origin 

 and both develop in the same way. The only structural difference seems 

 to be that in the first hyphal filaments formed, which surround the axial 

 cell row, intercalary cross walls are formed which divide the filament into 

 small isodiametric cells, while in the hyphal filaments formed later, and 

 farther from the axial cell row, intercalary cross walls do not appear, 

 the filaments being made up of long cylindrical cells. Moreover,, the 

 major part of the conducting hyphae are developed immediately sur- 

 rounding the inner assimilation tissue. In D. aculeata, according to 

 Soderstrom, "the inner assimilation tissue consists of five or six layers 

 of very small, thin-walled, endochrome containing cells," while in D. ligu- 

 lata there are only one or two layers of small iso-diametric cells contain- 

 ing chromatophores, which are surrounded in turn by a zone of conducting 

 hyphae of varying M'idth, depending upon the age of the portion of thallus 

 examined, and the region of the plant body from which it is taken (Plate 

 59, Figs. 2-6). 



In the mature thallus the cells at tlic outer edge of the lamina are 

 younger, hence are smaller and have produced less of the hyphal growth. 

 The cells surrounding the axial row and its covering of small assimilat- 

 ing cells produce more of the hyphal growth than do the cells of the flat 

 lamina; thus causing a thickening of the thallus, and producing the evi- 



