1920 Pease; on Desmarestia 331 



downward, the lower parts of the thallus are made up almost entirely of 

 this tangled mass of hyphal threads, the original cells of the ground tissue 

 having been pushed widely apart by them. They continue down through 

 the base of the stipe and into the holdfast, where they spread radiallj^, and 

 form the main bulk of the disk. In the basal portions of the plant, where 

 they constitute most of the substance, their walls are much thickened, ac- 

 cording to Soderstrom, and their function is almost entirely mechanical. 



b. Methods 



The foregoing summary is intended to present in a concise form 

 what is already known of the structure and development of the thallus 

 in the genus Desmarestia. Practically all morjDhological work has been 

 done with D. aculeata as the basis for study. Since this was the first 

 member of the genus to be recognized and described by the early sys- 

 tematists, is the most widely distributed species, and seems to be at least 

 fairly common throughout its entire range ; it is not surprising that it has 

 received more attention than other species of the genus, and that its struc- 

 ture as worked out by Kiitzing (1843), Janczewski (1875), Reinke (1880), 

 and especially by Soderstrom (1889) and Jonsson (1901), should be con- 

 sidered as typical. But since the type of thallus in the two groups of the 

 genus is so entirely different it is natural to suppose that there would be at 

 least minor differences in the formation and development of the various 

 tissues, if not in, their structure. The writer has studied the structure of 

 I), aculeata (L.) Lamour., D. viridis (Muell.) Lamour. and D. media 

 (Ag.) Grev. among the filiform species; and D. ligulata (Lightf.) Lamour., 

 D. herbacea (Turn.) Lamour., D. latissima Setchell and Gardner in lit., 

 n, sp., and D. foliacea n. sp. among the ligulate species. The work done 

 on the filiform species was for the purpose of verification and comparison, 

 and will not be discussed in the present paper. Microphotographs of cross 

 and longitudinal sections through the mature part of the thallus of D. 

 aculeata are given (Plate 61, Figs. 6 and 7) so that the size, shape, and 

 distribution of cells may be compared with those of the ligulate species. 

 Reproducing material has not been collected in Puget Sound waters, and 

 the writer will not attempt to discuss reproduction. 



All material used was collected in the San Juan Islands in July 191G 

 and Julv 1918, and preserved in 2% formalin in sea-water. As wanted 

 for study since, the material lias been removed from formalin, cut into 

 pieces of suitable size, dehydrated by running up slowly through a close 

 series of alcohols, and imbedded in paraffin, using both xylol and cedar 

 oil as solvents. Sections were cut from 10 to 15 micra in thickness, and 

 stained with Bismarck brown. IMost of the material was stained after 

 sectioning, but toward the cud of the work a few slides were made by 



