IV. FUCACE^.— Halidrys. 63 



scarcely two in which the vesicles are of the same size and shape. On a specimen 

 from Mr. Menzies, they are very small and spindle-shaped ; on Dr. Coulter's, some 

 are globose and some ellipsoid and ovoid ; and on Captain Beechey's, some are 

 pyriform and others spindle-shaped, and of large size. The only valid reason for 

 regarding P. Chamissoi as a species, is its habitat, should it really be, as is said, a 

 native of the Atlantic. 



In the Botany of Beechey's Voyage I distinguished a variety with leaves much 

 broader than usual, under the name of Macrocystis obtusa, but I have long ceased 

 to regard it as anything more than a form of P. Menziesii. At that time I had seen 

 but few and imperfect specimens of this plant, and was not aware how greatly it 

 varied in the shape and size of its leaves. 



III.— HALIDRYS. Lyngl. 



Root, a conical disc. Frond much divided, distichous, pinnatifid below, pinnated 

 above, without distinct leaves ; and forming its air-vessels and receptacles from 

 transformed portions of the upper branchlets. Branches alternate, the lowest 

 flattish or somewhat leaf -like, the upper narrow, repeatedly compound and sub-fili- 

 form. Air-vessels petiolate, siliquteform, acuminate, articulated, divided by 

 transverse septa into numerous loculi. Receptacles formed by transformation of 

 the terminal ramuli, pedicellate, lanceolate or pod-like, tuberculated, unarmed, of a 

 densely cellular substance ; having numerous pores, beneath which are placed the 

 spherical conceptacles (or spore-cavities). Spore-cavities containing both spores and 

 antheridia in the same loculus. Spores numerous, oblong, sub-sessile, having a 

 hyaline perispore. Antheridia on branching filaments, densely racemose. Parane- 

 mata simple or forked, clothing the walls of the conceptacle. 



The frond originates in an oblong, alternately-toothed root-leaf. As this increases 

 in size, the marginal dentations lengthen out into lateral lobes, and the leaf becomes 

 pinnatifid. Soon the uppermost lobes are found to elongate and become again 

 pinnatifid. Some of the lacinise are afterwards changed into articulated air-vessels, 

 and of course rendered abortive ; others become branches, margined with 

 similar air-vessels and ramuli ; and the apex of the developing lacinia is eventually 

 drawn out into a sub-filiform or compressed branch, which is repeatedly divided in 

 a pinnate manner. The fruit is formed by a change of the ultimate divisions of 

 the upper branches, and the receptacle, which is distinctly pedicellate, sometimes 

 springs from the rachis of the branch, and sometimes crowns a vesicular ramulus 

 or air-vessel. 



The genus contains but two known species, both of which come within the limits 

 of the North American flora, and one of them is peculiar to our shores. Both are 



