IV. 
LAMINARIACE2E. 
83 
are tlie Arctic genera Agarum 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-. Many 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 Ecklonia are characteristic of warmer climates, and extend, as well 
as some species of Laminaria , into the tropical zone. 
SYNOPSIS OF THE NORTH AMERICAN GENERA. 
1. Frond having 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. 
IY. Alaria. Lamina traversed by a single rib. 
Y. Co st aria. Lamina traversed by several parallel ribs. 
** Lamina without midrib. 
YI. Laminaria. Lamina either simple or cloven. 
3. Frond flat, pierced, like a colander, with holes. 
YII. Agarum. Lamina midribbed. 
YIII. Thalassiopiiyllum. Lamina without midrib, spirally developed round a 
(branching) stipe. 
4. Frond cylindrical , tubular or bag-shaped. 
IX. Chorda. Frond filiform, septate within. 
