IV. 
FUCACEflE. — Sargassum. 
57 
quite filiform, and their upper divisions are, in the majority of species, never 
winged. In a few species, the wing-like border is continued through all portions 
of the frond. The leaves which clothe the branches, the only leaves generally seen 
on full grown plants, are formed by dilatations of ultimate barren branchlets, and 
therefore arise in a manner the reverse of the primary leaves which spring from 
the root. The root-leaves, by losing their lamina, form the commencement of the 
filiform stem and branches ; and again, the barren apices of the stem and branches, 
by acquiring a lamina, become ordinary leaves. The branching throughout the 
frond, which at a hasty inspection seems to be alternate, or repeatedly pinnate, is 
in truth but a concealed form of dichotomous division, in which every alternate 
prong of the fork is stopped, while the twin prong is lengthened and again forked 
at its extremity. It is easy to see how an alternately pinnate frond, with a zigzag 
rachis, would result from the continual repetition of such a system of branching. 
In some species with zigzag stems and branches this mode of division is very 
evident throughout ; but in ordinary forms, as in our S. Montagnei (Plate I. f. A. 1.) 
the truly dichotomous division of the frond is only to be clearly perceived in the 
lesser fertile branches. If, however, these be carefully traced back to older portions, or 
the development of a young plant from its first leaf watched, the alternate sup- 
pression of parts will be very evident. From the same figure it may be seen, that 
the air-vessels are nothing but leaves in which the lamina has become inflated, 
while the apex of the midrib is prolonged into a mucro. In other species the trans- 
formation of the vesicated leaf is less complete, and then a wing-like border sur- 
rounds the inflated portion. These vesicles are usually placed between the ordinary 
leaves and the receptacles of fruit, and are, therefore, to be regarded as a form of 
bracts, or appendages to the inflorescence. They are most numerous in species 
which grow in shallow water, and serve to buoy up the branches. The receptacles 
of the fructification are, in like manner, but altered leaves ; and, as in flowering 
plants, they are the ultimate leaves. The frond which originated in a spore 
has passed through the various stages of its development, and at the end of its 
upward growth it again forms spores from which new plants may germinate. 
The number of species of the genus Sargassum is very considerable ; upwards of 
120 have been described, and probably many more remain uncharacterised in 
various herbaria. They are chiefly tropical and sub-tropical, and are found in the 
oceans of both the eastern and western hemispheres, but seem to be most numer- 
ous in the former. The following are all that I have been able to ascertain as 
natives of North America : — 
1. Sargassum vulgare , Ag. ; stem filiform, smooth or nearly so ; leaves linear or 
oblong-lanceolate, serrated, ribbed, brownish-olive, with evident glands ; air-vessels 
pointless, spherical, on compressed stalks which are as long as the air-vessel ; recep- 
tacles axillary, repeatedly forked, filiform, tuberculated, twice as short as the sub- 
tending leaf. — J. Ag. Sp. Alg. vol. 1, p. 342 ; Grev. Alg. Brit. t. 1 ; Harv. Phyc. 
Brit. t. 343. Fucus natans, Turn. Hist. Fuc. t. 46 (excl. vars.) E7ig. Bot. t. 2114. 
vol. in. art. 4. i 
