Imagens das páginas
PDF
ePub

foliage altogether depends upon its supply: and if they be placed in even partial darkness the green quickly acquires a sickly yellowish hue, and finally becomes whitish. But with Algæ it is different. At enormous depths, to which the luminous rays, it is known, do not penetrate, species exist as fully coloured as those along the shore. They therefore, in this respect, either differ from all other plants (Fungi included), or perhaps, what are called the chemical rays, in which seem to reside the most active principles of solar light, may be those which cause colour among vegetables, and these may penetrate to depths to which luminous rays do not reach. But this is mere supposition. Lamouroux suggests that “the particles of light, or its elementary molecules combined, or mixed with the water," suffice for this purpose. However this may be, it is worth remarking that this property among Algæ, of producing vigorous growth and strong colour without the agency of light, affords another link between them and the animal kingdom, among the lower tribes of which, light is by no means essential to growth and the most brilliant colour.

There is this difference also in the distribution of colours among Algæ to what obtains among other plants. Among plants in general, nothing is so variable or uncertain as colour; far from serving as a mark to distinguish groups or genera, it does not even aspire to the rank of a specific character, and the utmost to which it can pretend is to separate one variety of a species from another. Among Algæ, on the contrary, it has been ascertained that the classes of colour enumerated above, are, to a great extent, indicative of structure, and consequently of natural affinity. Thus, the green species are of the simplest structure, and differ remarkably in their mode of propagation from either of the other tribes, their seeds being endowed at the period of germination with a sort of motion, which some have called voluntary, but which does not really possess that animal property. The olivaceous are the most perfect and compound, and reach the largest size; and the red form a group distinguished not less by the beauty and delicacy of their tissue, than by producing seeds under two forms, thus possessing what is called a double fructification. Hence, modern botanists, since the publication of Lamouroux's system, have, whatever their particular views of arrangement may be, almost invariably used colour as one of the principal characters on which their systematic arrangement is based, and to a great extent it may be safely trusted.

But the young student must be careful not to place too absolute dependance on this character, in referring plants which he may gather to their place in the system ; for some species, which in their healthy state are red, or of that class of colour, become, when growing under unfavourable circumstances, of an orange, yellowish, whitish, or greenish shade. Laurencia pinnatifida is particularly variable in this respect. When this species grows near low-water mark, it is of a fine, deep, purple-red; a little higher up it is dull purple-brown; higher still a pale brownish-red, and, at last, near high-water mark, it is often yellowish or greenish. The other species of Laurencia vary in similar but less striking degrees. Chondrus crispus too, when found in shallow water, is often of a bright herbaceous green; and Ceramium rubrum passes through every shade of red and yellow, and at last degenerates into a dirty white, before it ceases to grow. All these species vary in form and size, as they do in colour, and the various anomalous shapes that they assume are almost sure to deceive a young botanist into the belief that the varieties are so many different species.

Many species, whilst growing under the surface of the water, reflect colours which perish almost immediately after they are removed to the air. Of this class are several

spe

cies of Cystoseira, especially C. ericoides, which, though really of a greenish-olive, appears, when growing under water, to be clothed with the richest phosphoric greens and blues, changing momently, as the branches move to and fro in the water. Similar colours have been observed, though in a less striking degree, on some species of the red series. The genus Iridæa derives its name from this character, though our I. edulis is not remarkable in this respect. Miss Ball and Mr. W. Thompson have observed Chondrus crispus to be occasionally iridescent. At the Cape of Good Hope, Champia compressa, and an undescribed species of Chylocladin ( C. iridescens), present very brilliant rainbow colours. Miss Hutchins observed that Conferva Hutchinsiæ has changeable glaucous tints when fresh, and looks almost white when seen through the water. The cause of these brilliant colours has not been particularly sought after. One may naturally suppose

that they arise, like those of mother-of-pearl, from the finely striated or fibrous surface of the plant ; but no one has noticed what is the peculiarity in the epidermis of these plants, that other allied species, that are not iridescent, do not possess. The microscope, in careful hands, ought to solve this problem.

There are other species which really change colour shortly after their removal from the water, as the various kinds of Sporochnoidea, which pass rapidly from a clear olive to a verdigris-green. But this is the effect of death and incipient decomposition, for with the colour they lose their crispness, become flaccid, and emit an offensive odour, and, as has been observed by botanists, possess the remarkable property of changing the colour of other small filiform Algæ with which they may come in contact. No doubt this is owing to the development of some active chemical agent, (perhaps Bromine). Professor Mertens, in describing the circumstance as occurring with Desmarestia ligulata and D. aculeata, says, that these species remain unaltered while they cause decay around them. But this I have not found to be the case. The Desmarestia always loses its rigidity, and its original olive is changed to verdigris before it possesses any destructive power. The Fucoideæ become black on exposure to the air. The Laminariee, on the contrary, first become green and finally white, under similar circumstances. Many of the Florideæ are much brightened in colour after having been cast upon the beach, especially if exposed to rain and sunshine. Amongst those of our own shores, Plocamium coccineum and Dasya coccinea are conspicuous in this respect. Both are, originally, of a dull, deep pink, but when thrown up and a short time exposed, become of a very rich scarletcrimson. But Gelidium cartilagineum, so common at the Cape of Good Hope, often presents the most splendid gradation of colour in a single specimen, from dull purplish-pink (its original dye) through scarlet, orange, yellow, and verdigris-green to white; to which colour all the red and green species may be bleached after long exposure.

Among the more delicate tribes several are instantly altered by being plunged into fresh water. Nitophyllum versicolor, as Mrs. Griffiths has observed, is remarkable in this respect; its full pink being instantly changed to a bright orange. Delesseria hypoglossum and ruscifolia have the same peculiarity, as have many of the Callithamnia and Griffithsiæ, and other delicate Floridee. All these changes are accompanied by decomposition. In the case of Griffithsiæ* especially, shortly after the change, the colouring matter of the joints is abundantly discharged with a crackling noise through the ruptured membrane, staining with a beautiful carmine colour, the water or the paper to which the specimen has been removed. No doubt a fine

* See an excellent description of this in a paper by Dr. Drummond, of Belfast, in 'Mag. Nat. Hist.' vol. ii. p. 121.

lake could be prepared either from G. setacea or multifida, could they be procured in sufficient quantity. Paper stained by them retains its colour after many years in the herbarium. At the Cape of Good Hope there is a species of Callithamnion (C. purpuriferum), which, when growing, is of a dull, deep, greyish-brown, with but a slightly reddish hue; but the moment it is placed in fresh water it discharges an abundance of fine, brilliant, purple powder, and almost immediately becomes flaccid and putrid. Many of the Polysiphonee also, which are, when growing, of a brown colour, become, in fresh water, purple or pink. Heat converts the colour of most species to green. If any of the Fucoideæ be plunged in boiling water they rapidly assume a bright green, but, on removal, revert to their original olive, and finally to black. The colours of Floridee may be more permanently changed, and also to green, by similar treatment. Dictyotee perhaps are less affected by fresh water, either cold or hot, than any others. Some of them are nearly unchanged; others assume more or less

shade. Most Algæ are, at some period of their growth, found attached to other substances by means of a root, or at least a hold-fast. It has been doubted whether, as no distinct vessels of absorption have been discovered, they receive any nourishment through this organ, but the question is by no means settled. Thus much is at least certain ; they appear to be as much influenced by the soil in which they grow as other plants are, for different species of rock afford different kinds in greater perfection, and a large number of those that are parasitical confine themselves to particular species. This selection of habitat would seem to prove that the root is not so sluggish an organ as it has been supposed to be. It does not, however, present much modification, and rarely attains a large size. The usual form is that of a hard, callous disk; sometimes this is

of a green

« AnteriorContinuar »