IV. LAMINARIACEJ5. 83 



are the Arctic genera Agariim and Thalassiophyllum, both found within our limits 

 and described below. 



The Order contains some fifty species, about half of which are natives of the 

 western world, and the largest portion of these of the northern continent. They 

 are plants of deep water, rarely vegetating within tide-marks, or barely reaching 

 a few inches above low water mark, and characterise a broad zone of depth extend- 

 ing from low water to four or five fathoms below it ; while the larger species 

 straggle into deeper water, to an unknown distance from the surface. ]\Iany of 

 these probably first vegetate on detached masses of rock at a moderate depth, and 

 are afterwards drifted, carrying their rocky anchors with them, into the deeper sea. 

 They are mostly plants of high latitudes, to which the greater number are confined. 

 Macrocystis and EcUonia are characteristic of warmer climates, and extend, as well 

 as some species of Laminarla, into the tropical zone. 



SYNOPSIS OF THE NORTH AMERICAN GENERA. 



1. Frond hailing a stem, furnished with definite leaves. 



1. Macrocystis. Stem filiform, branched. Leaves simple, secund along the 



stem, each leaf rising from a stalked air-vessel. 



II. Nereocystis. Stem filiform, unbranched, bearing at its summit an air-vessel, 



from which many forked leaves spring. 



III. Lessonia. Stem dichotomous (or simple?). Leaves terminating the branches. 



Air-vessels none. 



2. Frond stipitate, the stipes expanding at the summit into a simple or cloven lamina. 



* Lamina midribbed. 



IV. Alaria. Lamina traversed by a single rib. 



V. CosTAEiA. Lamina traversed by several parallel ribs. 



* * Lamina without midrib. 



VI. Laiyhnaria. Lamina either simple or cloven. 



3. Frond flat, pierced, like a colander, with holes. 



VII. Agarum. Latnina midribbed. 



VIII. Thalassiophyllum. Lamina without midrib, spirally developed round a 



(branching) stipe. 



4. Frond cylindrical, tubular or hag-shaped. 



IX. Chorda. Frond filiform, septate within. 



m2 



