the reach of the botanist. The frequent occurrence of favourable aspects on shores composed of sandstone, or of clay-slate-and the colour of these rocks, render such shores the most prolific in species.

I shall now take a rapid survey of the vegetation which characterises what is termed the litoral zone,* or that belt of rock or shingle which extends from highwater to low-water mark. Within this space a large proportion of the sea-weeds of our latitude is produced; and the remainder, with the exception of a few stragglers that extend into deeper water, occur within the limit of two or, at most, of four fathoms beyond the lowest water of spring-tides.

Sea-weeds are usually classed by botanists in three great groups, each of which contains several families, which are again divided into genera; and these, in their turn, are composed of one or many species. The number of species as yet detected on the British coasts is about 370, and they are grouped into 105 genera. I cannot, in this place, enter into the niceties of classification to which botanists resort in working out the history of these plants, but must confine myself to the general features of the great groups, and their distribution. Taken in the order in which they present themselves to us on the shore, and limiting each by its most obvious character, that of colour, we may observe :—that the group of green sea-weeds (Chlorospermea) abound near high-water mark, and in shallow tide-pools within the tidal limit; that the olive-coloured (Melanospermea) cover all exposed rocks, feebly commencing at

* See Prof. Edward Forbes, in Geol. Surv. Memoirs.

CHLOROSPERMEE, OR GREEN SEA-WEEDS.

57

the margin of high-water, and increasing in luxuriance with increasing depth, through the whole belt of exposed rock ;-but that the majority of them cease to grow soon after they reach a depth which is never laid bare to the influence of the atmosphere :-and that the red sea-weeds (Rhodospermeæ) gradually increase in numbers, and in purity of colour, as they recede from highwater mark, or grow in places where they enjoy a perfect shade, or nearly total absence of light, and are never exposed to the air, or subjected to a violent change of temperature.

The green sea-weeds are the simplest in structure, and the least varied in species, on different coasts, and consequently the least interesting to the collector of specimens. With the exception of the beautiful genus Cladophora, which contains about twenty species, our British Chlorosperms are chiefly composed of Ulva and Enteromorpha, whose forms vary with so little order, that it becomes difficult, and, in some instances, hopeless, to attempt to classify the varieties. The Enteromorpho are the first to make their appearance about high-water mark, covering loose boulders or smooth rocks with a slippery vesture of bright green, or filling the shallow tide-pools with grassy fronds. These plants consist of tubular membranes, simple or branched, appearing to the naked eye like fine green silk, and shewing to the microscope a surface composed of minute cells, full of granules. The commonest species near high-water mark is E. compressa, which commences of a very stunted size, and with thread-like branches, if exposed to the air, and gradually acquires

58

CHLOROSPERMEÆ, OR GREEN SEA-WEEDS.

length and breadth as it grows in deeper water. When fully developed, it has a frond divided nearly to the root into many long, subsimple branches, which bear a second or third series, all of them much attenuated at their insertion, and more or less distended at the extremity. The diameter of the tube varies extremely, and the broader and simpler individuals are only to be known from E. intestinalis, by their being branched ; the tube in the latter species being absolutely simple. To the Enteromorpha succeed Ulva, distinguished from Enteromorphæ merely by being flat, instead of tubular. The beautiful lettuce-like plaited leaves found in tidepools, belong to plants of this genus, the commonest species of which is U. latissima. It has a very broad, more or less ovate, plaited leaf, of a brilliant green, and remarkably glossy, when in perfection reflecting glaucous tints, if seen through clear sea-water, and is certainly a very ornamental species. It is sometimes brought to table as a laver, or marine sauce, but it is much inferior in flavour to the purple laver (Porphyra laciniata), a plant of the same family, equally beautiful, equally common, and more generally collected for food. The purple laver grows on exposed rocks near lowwater mark, and though called purple, assumes at different seasons of the year different shades of colour, according to its age. In form it resembles the green laver (Ulva latissima), but is of a still more delicate substance, consisting of a perfectly transparent and very thin membrane, elegantly dotted with closely set grains, to which it owes its colour. When these grains are in perfection, they are of a dark violet-purple; and

GEOGRAPHICAL DISTRIBUTION OF ULVE.

59

this is the case in winter and early spring, when the plant is collected for table. Later in the year the fronds are of stunted size, and more or less olivaceous colour, and much less suitable for gathering. The plant appears to be of very rapid growth and decay, a few weeks sufficing for its full development. Like many fugitive plants, however, it is not confined to one season, but continues to develop throughout the year; but with this difference, that the plants developed in summer are very much smaller, more tenacious, and of a dull colour. These last are regarded by some authors as a different species, and called P. umbilicata.

There is a circumstance connected with the history of our common Ulvæ, Enteromorpha, and Porphyra, which deserves notice. Most of the species common to the European shores are found in all parts of the world to which a marine vegetation extends. In the cold waters of the Arctic sea, Ulva latissima, Enteromorpha compressa, and Porphyra laciniata, vegetate in abundance; and these same plants skirt the shores of tropical seas, and extend into the southern ocean as far as Cape Horn. Vegetation, at least with its most obvious features, ceases in the south at a much lower parallel than in the Arctic regions, and the shores of the Antarctic lands appear to be perfectly barren, producing not even an Ulva. But the fact of the great adaptability of plants of this family to different climates, is beautifully illustrated by the last land plant collected by the acute naturalist attached to our Antarctic expedition. The last plant that struggles with perpetual winter was gathered at Cock

60

GEOGRAPHICAL DISTRIBUTION

burn Island, 64° S. (a latitude no greater than that of Archangel, where the vine is said to ripen in the open

air), and this proved to be an Ulva (U. crispa*), identical with a small species which may often be seen in this country on old thatch, or on damp walls and rocks, forming extensive patches of small green leaves. It is not common to find marine plants with so wide a distribution; but a nearly equal extent of sea is characterized by another of the British Chlorosperms, of

ULVA CRISPA.

a much greater size and more complex structure. On most of the rocky coasts of Britain may be gathered, in tide-pools, or rocks near low-water mark, an alga of a bright green colour and spongy texture, cylindrical, and much branched, the branches dividing pretty regularly by repeated

forkings, and the whole

CODIUM TOMENTOSUM.

invested, when seen un

der water, with a downy coat of colourless filaments. The name of this plant is Codium tomentosum. Under the microscope it is found to be wholly composed of

*See "Flora Antarctica," vol. ii. p. 498. In the northern hemisphere, Ulva crispa extends to Spitzbergen, in lat. 80°.