systematic account of the genus Sir uvea . 271 
base of the stem was best seen. In S. tenuis they were hidden, 
and in S. pulcherrima they were not very clear owing to the 
fragments of shell, etc. to which they were attached. 
The Frond of S. plumosa is formed, according to Harvey, 
by the subdivision of the apical cell into a vertical series of 
cells, each of which produces from its shoulders two opposite 
branches, at first free and pectinate, then once and again pin- 
nulate, the pinnulae ‘ anastomosing ’ and producing the net- 
work. Agardh supposes that an apical cell is repeatedly 
cut off and a pair of branches formed below it each time. If 
this be so, and Harvey’s Fig. 4 makes it probable, then this 
species differs in the mode of development of its frond from 
that of vS. tenuis , which is described below. 
The filaments composing the frond of S. plumosa show stria- 
tion of their walls very well (Fig. 1 k ) ; it is seen equally well 
in the young stalk before it becomes too much thickened. 
Longitudinal and transverse striations are easily observable. 
The former appear to be more numerous in the outer layers of 
the cell wall, the transverse ones in the middle layers, and the 
longitudinal again in the innermost. Fig. 1 h shows these striae 
in one of the cells of the midrib. In some cases only the longi- 
tudinal striations are to be seen. Longitudinal and transverse 
striae of this kind are mentioned by Thuret 1 , and were held 
by him to be characteristic of the genus Conferva , but they 
have since been described in other genera, as mentioned above, 
in connection with the stalk. 
The structure of the frond of S. plumosa is very regular, 
the primary veins or pinnae being given off in opposite pairs 
from the midrib at an angle of about 6o° ; they remain 
parallel to one another for some distance and then each curves 
upwards and inwards, attaching itself by its apex to the lower 
side of the similarly curved pinna next above it. 
The frond is regularly crenate, its margin being formed by 
the curved ends of the pinnae, which produce no branches 
from their outer side where they form part of the margin. 
1 Annales des Sciences naturelles, Bot. ser. 3, Tome III (1845), p 274. 
