34 BRITISH MARINE ALG. 
“toothed rockweed,’’ is the most easily distinguished, and is the hand- 
somest of its tribe. Although very variable in length, breadth, and 
colour, its regularly toothed or serrated margin affords a ready means by 
which this species is recognised at once. Air-vessels are absent in this 
species, but it has a very distinct midrib. The frond is flat throughout, 
and the fruit is produced in receptacles which terminate the branches. 
Our illustration is from a very characteristic but barren frond of this 
elegant species ; the plant from which it was taken grew on the inside of 
the Plymouth Breakwater, and was upwards of 6ft. in length. F. cera- 
noides, or “‘horned-wrack,’’ is one of the rarities of this group, and is 
found most frequently in situations where a fresh water stream runs into 
the sea. Its substance is far less tough than that of the other Fuci, and 
tbe whole plant is thinner and more delicate, both in the growing state and 
after it is dried. The midrib is very narrow but distinct, and there are 
no air-vessels. The fruit is produced at the tips of dichotomous or forked 
branches, which are set alternately along the sides of the main stem. The 
most abundant species of this group of seaweeds, which may be termed 
par excellence the ‘‘ kelp-weed,’”’ since it is more extensively used in 
the manufacture of kelp and iodine, than all the other species of Fuci 
put together, is F. vesiculosus (Fig. 37). Itis extremely variable in size and 
appearance, so much so in fact, that some writers have constituted varieties 
to characterise peculiarities of form. Specimens growing in salt marshes 
and near high-water mark, where they are only occasionally covered by the 
BM Mt nny 
5 
Fig, 37. Fucus vesiculosus, Fie. 38. Halidrys siliquosa. 
sea, are very narrow in the frond, and often destitute of air-vessels ; while 
those which grow in rock pools, or where they are constantly within the 
influence of the tide, and frequently submerged, are provided with numerous 
air-vessels, which are set in pairs, one on each side of the midrib. Plants 
which vegetate in suck situations are often found from three to four feet 
