4.6 BRITISH- MARINE ALG. 
conditions of this plant are vastly different. Specimens which are met 
with in spring or early summer, whether growing’ or thrown ashore, are 
clothed throughout with tufts of delicate filaments, which fall away. as the 
plant arrives at maturity. Old fronds of this species are destitute of 
these filaments, and the branches are set on each side with spine-like 
ramuli, hence the specific name, that of aculeata. In perennial specimens, 
when new. branches shoot forth, they are always clothed with tufts of fine 
confervoid filaments, which are apparently a necessity in the progress of 
growth, and probably, as suggested by Dr. Harvey, perform in some way 
the office of leaves in higher plants. The root of this species is a small 
round disk, the stem short and cylindrical, and the branches long, nume- 
rous, repeatedly divided, and irregularly set with a second and third 
series of branches and branchlets. Plants from deep water are often over 
3ft. long. Smaller specimens, when carefully mounted, are extremely 
beautiful, especially when the marginal tufts of fibres are well displayed, 
for in drying they change from olive to a brilliant green colour. No time 
must be lost in putting this species under pressure, as decomposition 
takes place very soon after its removal from the water. Our illustration 
is from a slightly magnified portion of a fine specimen cast ashore at 
Brighton many years ago. OD. viridis is the most delicate, and in its 
summer dress the most attractive of its tribe. The whole plant is much 
more slender ; it is repeatedly and excessively branched, all the branches 
being set exactly opposite and gradually attenuated towards the tips, the 
terminal branches and ramuli being more and more slender and capillary 
or hair-like. The olive tint of this lovely species changes very quickly 
to a delicate green, and it closely adheres to paper in drying. D. viridis is 
a summer annual, and is usually found in rock pools between tide-marks ; 
sometimes on stones in the sea, and occasionally on the larger alge. 
In addition to the three species of Desmarestiew just described, there is 
another which, until very lately, has been considered a plant of extreme 
rarity. Its name is D. pinnatinervia, in reference to the wing-like 
nerve which traverses the frond in the manner of an obscure midrib. 
In outline and general appearance this rare alga bears some resemblance to 
a Punctaria, being unbranched and leaf-like, but having a short though 
distinct stem. It is taken off the Cornish coast, but on no other station 
on the English shores that I am aware of. 
The genus Arthrocladia is represented by one species, viz., A. villosa 
(Fig.53a). The name, Arthrocladia, signifies ‘‘ jointed branch,” the stem and 
branches being furnished at closeintervals with nodes or knob-like swellings, 
from which are produced whorls of delicate tufts of branched filaments, to 
which the specific term of villosa specially refers. The spores of this plant 
are produced in pod-like vessels, borne on the pretty tufted filaments which 
in early growth adorn the stem and branches. Fig. 53 a, represents a 
portion of the stem, magnified to show the situation and form of the whorls 
of branched filaments. At maturity the spores break through the mem, 
brane which incloses them, and, like the pencilled filaments on which the 
