342 Piih. Puget Sound Biol. Sta. Vol. 2, No. 53 



the thallus in some individuals testifies (Plate 60. figs. 5-8). The secondarj' 

 gro'vihs of sicvi; tubes, which in D. herhacae ramify among the conducting 

 hyphae, in this iy],ecies become invested with inner assimilation c<;]ls in 

 their older parts (Plate 60, fig. o), and are visible on the surface of the 

 lamina as the network of veins between the lateral branches of the midrib. 

 Toward the outer cdsce of the lamina, in the younger tissue, they are still 

 uninvested, and are not visible from the surface. 



The cells of the ground tissue are so firmly bound together by the 

 long brandling filaments of the conducting hyphae that in formalin material 

 the cortical layers, which become loosened and form large blisters, can be 

 stripped from both surfaces and an entire lateral branch will preserve its 

 original shape and texture, and the veining can be traced as easily as in a 

 skeleton leaf. In plate 62, figs. 8 and 9, is shown the method of branch- 

 ing of the conducting hyphae, while fig. 10 shows how firmly the cells of 

 the ground tissue are bound together by the interwoven meshes of the 

 hyphae. 



In studying sections of the stipe and holdfast of a young specimen, a 

 single longitudinal section plainly showed the axial cell row of the midrib 

 or "original thallus" extending through the base of the stipe to the bottom 

 of the holdfast (plate 60, fig. 9) ; and the covering of assimilating cells, 

 gradually becoming less and finally disappearing at the base of the holdfast. 

 This would seem to indicate that the plant originates as a simple monosi- 

 phonous filament which later develops branches and becomes corticated. 

 This is probably true of all members of the genus, as it is the condition 

 found at the thallus tips of all species at the beginning of the active growing 

 season. 



/. Desmarestia foUacea n. sp. 



(Plate 58. Figs. 5-10: Plate 61, Figs. 1-5) 



Except for its extreme thinness and delicacy of texture, this species 

 closely resembles a single lateral branch of D. latissima. In structure the 

 two are identical. Plate 61, fig. 1 shows a flat section through a younger 

 portion of the lamina, with the large cells of the ground tissue, the inter- 

 woven conducting hyphae, and the secondary growth of sieve tubes, not yet 

 surrounded by assimilating cells. The sieve tubes are easily distinguished 

 from the hyphae by the heavy callose rings formed on the cross walls. 

 In fig. 2, from an older part of the lamina, the sieve tubes are surrounded 

 by a layer of small assimilating cells. 



In fig. 3, a cross section through the edge of the lamina, the primary 

 growing region of the plant is shown, that is, the region in which new 

 ground tissues are added to older parts of the thallus, the result being an 

 increase in the width of the thallus. The increase in thickness of older 



