14 
NEW OR CRITICAL BRITISH ALG^E. 
father’s view that it is a variety of A. nodosum , the peculiar charac- 
teristics of which are due to its place of growth. More recently 
Dr. J. Reinke has (“ Algenfl.,” p. 34) expressed an almost similar 
opinion, suggesting that A. Alackaii is perhaps nothing more than 
the adventitious lateral branches of A. nodosum , which having 
become detached, continue to grow while in a free floating condi- 
tion. The descriptions and figures of the species as given in 
Turner’s “ Fuci,” t. 52, Sowerby’s “ English Botany,” t. 1927, and 
Harvey’s “ Phycologia,” t. 52, certainly favour this supposition, 
which is furthermore confirmed by authentic specimens of this 
species gathered by McCalla, Harvey, and Dr. Scouler, now in my 
Herbarium. 
It seems probable, therefore, that A. Mackaii may turn out to be 
nothing more than A. nodosum modified by its environment With 
regard to the plant found by Mr. Robertson, however, the position 
of the receptacles, which are usually borne on the extremities of 
the ordinary branches (see Plate 183, Figs. 1 and 2), and not on 
special lateral deciduous branches, as in A. nodosum , seems to me 
to preclude the idea of uniting it with that species. If, therefore, 
A. Mackaii on further investigation prove to be but a form of A. 
nodosum , I would raise the Arran plant, which I have named 
A. Mackaii f. Robertsoni (“ Journal of Bot.,” June, 1892) to 
specific rank under the name A. Robertsoni. 
In authentic specimens of A. Mackaii , from Roundstone Bay, 
the fronds are slender, nearly cylindrical, provided with air- 
vessels, and very irregularly branched, the receptacles stalked, 
pedulous, borne near the base of the frond. 
In the Arran plants, however, the fronds are compressed, desti- 
tute of air-vessels, more robust than the Irish specimens, irregu- 
larly branched below, and regularly dichotomously above, the 
receptacles are nearly all terminal, although not unfrequently a 
few are borne on short lateral branches, as in A. nodosum and 
A. Mackaii. 
So close a resemblance does the plant bear to Pelvetia canali- 
culata that it has more than once been suggested to me that 
perhaps it is a hybrid between that plant and A. nodosum , but 
beyond the inherent improbability of this theory the fact remains 
that Pelvetia and Ascophyllum produce their receptacles at different 
seasons of the year, and as Pelvetia is a monoecious genus, it is 
very improbable that it would cross with individuals of auother 
genus. The plants, moreover, grow side by side almost every- 
where around our coasts, and no trace of anything resembling a 
hybrid has up to the present been observed. It seems more 
reasonable to suppose that the species is of recent origin, and 
having sprung from A. nodosum still shows traces of its ancestry 
in the occasional lateral receptacles. 
The following is the diagnosis I propose for the new variety : — 
Fronds gregarious, growing in dense, nearly globular, un- 
attached masses, compressed, slender, 4-6 inches long, inch 
