DICTYOTACE.E. 33 



habit there is considerable diversity among the plants of this 

 order. The simplest are flat, undivided expansions, consist- 

 ing of three strata of cells, of which the central one is colour- 

 less, the two superficial coloured. In others the frond is a 

 simple bag, closed at both ends. Others, of rather higher 

 type, are cylindrical and branching, the branches of some 

 being hollow, those of others solid. Some have flat fronds 

 pinnated or dichotomously divided ; in others the fronds take 

 a more or less perfectly fan-shaped outline. In the first of 

 these, the cells of which the structure is composed are ar- 

 ranged in parallel series; in the last they radiate fi^-om the 

 base of the frond as from a central point. One genus only 

 {Haliseris) is furnished with a distinct midrib, running 

 through a flat, membranaceous frond; and in no member of 

 the order is there a distinction into stems and leaves, but 

 whatever form the frond assumes in its first growth is re- 

 peated in its after developments. Many of them, and perhaps 

 all, at some period of their growth, are clothed with exceed- 

 ingly fine, articulated hairs, which often decompose the rays 

 of light, reflecting prismatic colours. These are most ob- 

 servable in Padina and Cutleria, but exist in Punctaria, in 

 Asperococcus, and other low forms. 



The fructiji cation appears under almost as many phases 

 as the organs of vegetation. In nearly all the spores are sim- 

 ple ; a single, elliptical or obovate embryonic mass being 

 contained in a hyaline perispore : but in Cntleria the peri- 

 spore contains eight sporules. The spores of some are scat- 

 tered singly ; of others, and these the greater number, they 

 are collected into spots or sari, which are round, oblong, or 

 linear, and either dotted irregularly over the whole surface, 

 or confined to a definite portion of it, or ranged in transverse, 

 horizontal or concentric bands. In some species both scat- 

 tered and aggregated spores occur on the same or on different 

 individuals. In most instances the spores are accompanied 

 by paranemata, sometimes few in number and of low organi- 

 zation as in Asperococcus and Piwctaria); sometimes as in 

 Stilophoi-a) constituting the larger portion of the fructification. 

 In a few (as Cntleria) antheridia sometimes occupy the place 

 of spores; and in others {Punctaria, Dictyota, &c.) gem- 

 mules (.?) of round shape and large size occur on the same 

 frond which produces true spores, or on diff"erent individuals 

 of the same species. Perhaps the so-called spores of Striaria 

 and Dictyosiphon are of this description. 



D 



