IV. FUCACEiE.— Saegassibi. 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 bi'anches 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 — /. Ag. Sp. Alg. vol. 1, p. 342 ; Grev. Alg. Brit. t. 1 ; Harv. Pliyc. 

 Brit. t. 343. Fucus natans. Turn. Hist. Fuc. t. 46 (excl. vars.) Eng. Bot.t. 2114. 



VOL. III. ART. 4, I 



