Description of the Landes of Acquitania. 103° 
- In describing the vegetation of these tracts, we must proceed by 
those divisions, which, in the vegetable kingdom, are marked out: 
by the habitats and localities of the plants themselves. 
The filamentous, laminar, and slightly arborescent plants of the 
ocean, differ materially from those inhabiting the downs, both in 
their structure and external characters, by which they are marked 
- out as forming a distinct division of vegetable productions; they 
obey, however, in their distribution, laws similar to those which re- 
gulate the dissemination of zrophytes, or terrestrial plants, which, 
by causes dependent on their organization, are strengthened against 
atmospheric vicissitudes ; or, on the contrary, are, by the possession. 
of a more subtle organization, capable of adaptation, without effort,» 
to the exigencies of climate and the caprices of seasons. Thus, as 
lichens and mosses, equally independent of elevation and latitude, 
are every where dispersed, requiring for their developement only a 
small number of circumstances ; so some hydrophytes re-produce 
themselves in distant countries, and fuci of the north of Scotland 
re-appear on the shore of Van Dieman’s Land. 
But in some species their localities become conditions of exis- 
tence ; some placing themselves on a spot which the tides cover, 
and leave bare every day, as others inhabit spots which the tide 
uncovers only in the syzygies or the equinoxes ; some prefer rough 
and agitated seas, others calm and tranquil spots ; some’ live and 
die in the space of a few hours or months, while others survive 
the tempests of many winters. 
But all have bands or zones of particular habitations in the dif- 
ferent depths of the ocean ; regions in which the column of sup- 
ported water, and the relative quantity of light and caloric are in 
harmony with the disposition of their organs. Plants will flourish 
in the centre of these zones, and perish towards its limits. The 
seeds which escape from them, appear also, by their specific gravi- 
ty, to place themselves in equilibrium with the column of water, 
which they displace, and to swim in the zone where the plants will 
alone vegetate. 
C. D’Orbigny, in the Memoires du Museum, Tom. VI. has enu- 
merated the marine plants growing on the shores of the Gulf of 
Gascony, and arranged them in their zones, drawn up to correspond 
with a table of the tides on those shores by Fleurian de Bellevue. 
From these documents, we find that there are 34 species of ulve 
growing between the first and sixth zone ; in the latter he only 
finds the U. tomentosa, which he considers to be a polypus: 63 
species of fucus, of which the F. pygneus and F’. amphibius alone 
belong to the first zone ; while the F. loreus, F. fibrosus, F. san- 
guineus, and F. coronopifolius, belong to the sixth zone: 29 spe- 
cies of Ceramium, of which none are found in the first zone, nor do 
they extend beyond the fifth, in which occurs the Ceramium cocci- 
neum and C. egagrophilum, the latter of which is rare. Two species 
of Diatoma, D. rigidum and D. fiecculosum, occur in the second. 
zone, in which also are found the Zostera marina and Z. medi-. 
terranea ; but the latter rarely, 
