268 Murray and Boodle . — A structural and 
In 5 . plumosa (Fig. i a) the stalk is at first club-shaped 
with a smooth and delicate wall in which, as well as in the 
nature of its contents, it very closely resembles some species 
of Valonia (Fig. 1 b). At a later stage it becomes annularly 
corrugated below, while the apex remains smooth and obtuse. 
In this condition it increases in length, and ultimately the 
upper part becomes prolonged into a slender corrugated 
filament, from the apex of which a cell is cut off which, by 
subdivision, produces a series of ten or twelve cells one above 
the other, which, by their branching, give rise to the whole 
of the frond. This statement is derived from Harvey’s 
description 1 2 , which we have been unable to verify as regards 
the first stages in the formation of the frond, because the 
specimens of S. plumosa accessible to us do not include any 
of the exact age required. The mature stalk tapers slightly 
at both ends, and is corrugated throughout (Fig. i c ). By 
making a longitudinal section of a well-developed frondless 
stalk, we came to the same conclusion as Harvey and Agardh, 
viz. that the cavity of the stalk is not interrupted by any 
transverse walls. The outer wall is so much thickened, and 
in the older specimens encrusted with a Melobesia in addition, 
that without making a section one would probably be unable 
to detect septa if they were present. The constrictions are 
only inflexions of the membrane. The wall is formed of a 
great number of layers, and when cut or otherwise roughly 
treated the inner layers tend to break up into fibrils, as 
observed by Agardh 2 in the nearly related genera Apjohnia 
and Chamaedoris . A similar fibrose structure is described by 
us in the present volume (p. 171 ), ’ m a paper on Spongocladia ; 
it is of course connected with the striations seen in surface view 
in all these cases. The older stalks are incrusted in various de- 
grees with calcareous algae, chiefly a Melobesia , which Harvey 
refers to as a thin coating of calcareous matter, making it one 
of the characters of the genus. According to Leitgeb 3 the 
1 Phyc. Austr. pi. 32. 2 Monogr. Siphon, p. 107. 
3 Quoted from Bot. Zeit. 1888, No. 14; Sitzb. Kais. Akad. d. Wiss. in Wien, 
Bd. 96. 
