19--^] Setchell-Gardner: Melanophyceae 559 



33. Desmarestia Lamour. 



Fronds attachod by a solid, parenchymatous disk, exceedingly 

 variable in size, with a longer or shorter, usually cylindrical stipe, 

 more or less profusely branched, wholly or in part terete or slightly 

 flattened or decidedly complanate and ligulate to broadly expanded 

 and membranaceous, with or without a midrib ; branching distichous 

 and either wholly opposite or alternate, or both opposite and alternate ; 

 fronds composed of a single fundamental axial filament of cylindrical 

 cells and more or less profusely branched, colored and projecting 

 beyond the surface at the apices and along the margin and in the 

 juvenile stage, early deciduous, with trichothallic growth, sur- 

 rounded in the frond by one or more layers of large colorless cells 

 enclosed by a cortical tissue of small assimilating cells ; reproduction 

 b}' zoosporangia composed of slightly transformed surface cells, 

 inconspicuous. 



Lamouroux, Essai, 1813, p. 23. 



The genus Desmarestia may be divided into three groups, some- 

 times reckoned as distinct genera, or at least it may be divided into 

 two genera. The cylindrical or at most only slightly flattened species 

 with mostly opposite branching, constitute Dichloria, while the flat- 

 tened forms are retained under the name of Desmarestia, or the sub- 

 genus Eudesmarestia, with branching mostly alternate. The Dichloria 

 group has the hairs scattered as in Phaeiirus, but unlike that 

 genus, soon loses them. In the Eudesmarestia group, the hairs are 

 branched and are the very tips of incipient' branches, some, or even 

 many, of which do not develop into permanent structures. In 

 Phaeiirus the hairs are scattered, but are more permanent and per- 

 sistent for a longer time than in the Dichloria group. In Arthrocladia, 

 the hairs are in whorls, branched, persistent, and possibly represent 

 the tips of incipient branches. One well-known characteristic of the 

 species of Desmarestia may well be emphasized and that is their habit 

 of turning verdigris green on drying. This color is associated also with 

 their power to bleach other algae in contact with them. The change 

 of color is followed, sooner or later, by decomposition. This happens 

 quickly in the more slender species of the Vw'ides-iieciion and fairly 

 early in the members of the JFZ^er&aceae-section, but affects only the 

 younger and more delicate portions of the members of the Aculeatae- 

 section. 



