1920 Pease; on Desmarestia 329 



filament is not developed equally on all sides, but extends chiefly laterally, 

 so as to form first a two-edged and then a flat or even leaf-like stem. In 

 this process of lateral extension, or widening of the stem, the lower por- 

 tions of the pinnae of the primary filament, being enclosed within the 

 cellular wings of the flattened branch, become the lateral nerves of the 

 frond." 



The outer cells remain small and are crowded with small lens-shaped 

 chromatophores, thus forming the principal assimilating region of the 

 plant. The cells immediately within the outer cortical layer are somewhat 

 larger, the chromatophores are less crowded, and the protoplast becomes 

 peripheral, having within it a single large vacuole in which the nucleus is 

 suspended by slender strands of cytoplasm. The cells rapidly increase in 

 size toward the center and the chromatophores rapidly decrease in number 

 and finally disappear, though the large cells of the ground tissue remain 

 nucleated after all trace of chromatophores has vanished. Thus it can be 

 seen that in the primary tissues only the three or four outermost layers of 

 cells of the thallus are engaged to any appreciable degree in photosynthetic 

 work. 



At the end of the period of elongation, then, which occurs early in 

 spring, the newly formed thallus consists of three primary tissue regions: 

 (1) The axial row of cells formed from the primary growing region, which 

 bears regular opposite distichous branches; (2) The cortical region, con- 

 sisting of two or three layers of small isodiametric cells densely filled with 

 chromatophores. The outermost layer remains permanently meristematic, 

 cutting off cells laterally to accommodate the increasing width and leno-th 

 cf the thallus, and dividing parallel to the surface to add to its thickness. 

 The inner cortical cells enlarge to form (3) the ground tissue, composed 

 of greatly enlarged cells with bluntly tapered ends in which the proto- 

 plasm is distributed as a very thin peripheral layer within which a few 

 scattered chromatophores may be imbedded. 



At about the close of the growing period, which is marked externally 

 by the shedding of the filamentous hairs and the complete cortication of 

 the thallus, the formation of secondary tissues begins. This includes the 

 "inner assimilation tissue" and the "conducting hyphae" which ramifv 

 through the intercellular spaces of the ground tissue. The tissue of small 

 cells which completely surrounds the original thallus cells of the axial 

 row, called by Soderstrom the inner assimilation system, is described 

 by him as consisting of several layers of extremely small, thin-walled, 

 endochrome-containing cells which originate by repeated oblique divisions 

 in all directions of the cells of the ground tissue which lie next to the 

 central cell. Soderstrom is at a loss to explain the presence of chroma- 

 tophores deep witliiii tlie thallus. T3nt Wilh' (ISP.)), under wliose direc- 



