Notes on Florideas. By George Massee. 573 



or elliptical outline in section, whereas in Halymenici and numerous 

 other genera, the thallus is broad and membranaceous, owino- to 

 excessive development of the lateral branches. 



2. In the second type the substance of the thallus does not 

 depend on the presence of branches, but on a definite number of 

 pericentral cells or siphons, which are cut off from the axial cells by 

 curved tangential septa. These grow at the same rate as the axial 

 cells, so that the thallus consists of fascicles of superposed cells of 

 equal length. In some species a second set of irregular corticating 

 cells are present outside, and alternate with the siphons, consisting 

 of adpressed branches, originating at the nodes ; sometimes in Poly- 

 siphonia, from portions of protoplasm imprisoned by the apposition 

 of cellulose, during the thickening of the cell-wall, as described 

 above. The following genera illustrate this mode of thallus forma- 

 tion : Polysiplionia, Bostrychia, Dasya. "When flattened, as in 

 Odonthalia, the fascicled arrangement is masked, owing to subse- 



' quent growth, but the typical polysiphonous structure is clearly 

 seen in the younger portions. 



3. In a third type the thallus is typically flat and membra- 

 naceous, resulting from the branches of a filamentous thallus, as in 

 CaUUliamnion, being connected by a web or membrane of tissue. 

 In some species the connecting membrane is only one cell thick, in 

 others it is composed of several layers of cells, owing to subsequent 

 cell-division parallel to the surface. In most species the evidence 

 of their filamentous origin can still be traced in the so- 

 called " veins " in the membranaceous thallus, and microscopic 

 examination clearly shows that these veins give origin to the cells 

 which, by division, form the flattened portion. The arrangement 

 of the veins may be dichotomous and develope into a flabellate or 

 irregularly expanded thallus, as in Nitophyllum and Callophyllis ; 

 or pinnate, as in Delesseria, where the British species furnish a most 

 interesting sequence from the filamentous D. angustissima to 

 D. {Wormshioldia) sanguinea, where each portion of the thallus, 

 under favourable conditions, resembles an obovate or oblong leaf, 

 from four to seven inches long, with a strong midrib, giving off 

 secondary and tertiary veins corresponding to the cell traces. A 

 further proof of the statement as to the evolution of the membrana- 

 ceous genera mentioned, as well as others, from filamentous 

 ancestors, is the fact well known to algologists, that the form and 

 expansion of the thallus in this type is one of the most untrust- 

 worthy of characters ; a well-selected series of most species illus- 

 trating a transition from filamentous to broadly expanded forms ; 

 and further, it is by no means unusual to meet with the various 

 transitions on the same thallus, as shown in Harvey's figure of 

 Hxlymenia ligulata Ag. 



