A L than those of any other—yellow, brown, olive, red, purple, violet an< vai Sub^ 'iations of all these being known. They divisions, undoubtedly represent the lowest AlgalAlgae. life, and their distribution rivals that grade of the ofGreen They occur in the sea, in fresh water, on moist earth, on damp rocks, and on the bark of trees. Certain species are regularly found in the intercellular spaces of higher plants; such are species of JYostoc in the thallus of Anthoceros, the leaves of Azollcc, and the roots oi Cycads. Many of them enter into the structure of the lichen-thallus, as the so-called gonidia. It is remarkable that species belonging to the Oscillatoriacece are known to flourish in hot springs, the temperature of which rises as high as 85° C. The thallus may be unicellular or multicellular. When unicellular, it may consist of isolated cells, but more commonly the cells are held together in a common jelly {Chroococcacece) derived from the outer layers of the cell-wall. The multicellular species consist of filaments, branched or unbranched, which arise by the repeated division of the cells in parallel planes, no formation of mucilage occurring in the dividing walls. Such filaments may not give rise to mucilage on the lateral surface either, in which case they are said to be free ; when mucilage does occur on the lateral wall, it appears as the sheath surrounding either the single filament, or a sheaf of filaments of common origin. The mucilage may also form an embedding substance similar to that of Chroococcacece, in which the filaments lie parallel or radiate from a common centre {Rivulariacecc). The cells of the filament may be all alike, and growth may occur equally in all parts (Oscillatoriacece) ; or certain cells (heterocysts) may become marked off by their larger size and the transparency of their contents ; in which case growth may still be distributed equally throughout {Nostoc), or the filament may be attached where the heterocyst arises, and grow out at the opposite extremity into a fine is.x{llivulariacece). An African form (Camptothrix), devoid of heterocysts and hair-like at both extremities, has recently been described. Branching has been described as “false ” and “true.” The former arises when a filament in a sheath, either in consequence of growth in length beyond the capacity of the sheath to accommodate it, or because of the decay of a cell, becomes interrupted by breaking, and the free ends slip past one another. “ True ” branching arises only by the longitudinal division of a cell of a filament and the lateral outgrowth of one of the cells resulting from the division (Sirosiphonacece). The nature of the contents of the cells of Cyanophycece has given rise to considerable controversy. The cells are for the most part exceedingly minute, and are not easy to free from their colouring matters, so that investigation has been attended with great difficulty. Occupying as these Algse do perhaps the lowest grade of plant life, it is a matter of interest to ascertain whether a nucleus or chromatophore is differentiated in their cells, or whether the functions and properties of these bodies are diffused through the whole protoplast. It is certain that the centre of the cell, which is usually non-vacuolated, is occupied by protoplasm of different properties from the peripheral region ; and Fischer has further established the fact that the peripheral mass, which is a hollow sphere in spherical cells, and either a hollow cylinder or barrel-shaped body in filamentous forms, must be regarded as the single chromatophore of the Cyanophyceous cell. But whether the central mass is anything more than protoplasm laden with the products of assimilation still remains uncertain. Among other contents of the cell, fatty substances and tannin are known. A curious adaptation seems to occur in certain floating forms, in the presence of a gas-vacuole, which may be made to vary its volume with varying pressure. There is evidence that the dividing wall of filamentous forms is deeply pitted, as is found to be the case in Red Algae. Reproduction is chiefly effected by the vegetative method. Asexual reproductive cells are not infrequent, but sexual reproduction even in its initial stages is unknown. Nor is motility by means of cilia known in the group. In the unicellular forms, cell-division involves multiplication of the plant. In all the multicellular plants of this group which have been adequately investigated, vegetative multiplication by means of what are known as hormogonia has been found to occur. These are short segments of filaments consisting of a few cells which disengage themselves from the ambient jelly, if it be present, in virtue of a peculiar creeping movement which they possess at this stage. After a time they come to rest and give rise to new colonies. True reproduction of the asexual kind occurs, however, in the formation of sporangia, particularly in the Chamcesiphonacece. Here the contents of certain cells break up endogenously into a great number of spores, which are distributed as a fine dust. Resting spores are also known. In these cases, certain cells of a colony of unicellular plants or of the filaments of multicellular plants enlarge greatly and thicken their wall. When unfavourable external conditions supervene and the ordinary cells become atrophied, these cells persist and reproduce the plant with the return of more favourable conditions. The Oscillatoriacece are capable of a peculiar oscillatory movement, which has earned for them their name, and which enables them to move through con-
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siderable distances. It is not clear how the movement is effected though it has frequently been the subject of careful investigation! With the Cyanophycece must be included, as their nearest allies, the Bacteriacece. Notwithstanding the absence of chlorophyll! and the consequent parasitic or saprophytic habit, Bacteriaceoe agree in so many morphological features with Cyanophycece that the affinity can hardly be doubted. They are, however, not further considered here, since they are separately dealt with in this work. (See Bacteriology.) 2. (Jhlorophyceze.1—This group includes those Algae in which the green colouring matter, chlorophyll, is not accompanied by a second colouring matter, as it is in other groups. It consists of three subdivisions -.—Conjugatce, Euchlorophycece, and Characece. Of these the first and last are relatively small and sharply-defined families, distinguished from the second family, which forms the bulk of the group, by characters so diverse that their inclusion with them in one larger group can only be justified on the ground of convenience. Euchlorophyceoe are made up in their turn of three series of families:—Protococcales, Confervales, and Siphonalcs. Chlorophycece include both marine and freshwater plants. Conjugatce, Protococcales, and Characece are exclusively freshwater ; Confervales and Siphonales are both freshwater and marine, but the latter group attains its greatest development in the sea. Some Chlorophycece are terrestrial in habit, usually growing on a damp substratum, however. Trentepohlia grows on rocks and can survive considerable desiccation. Phycopeltis grows on the surface of leaves, Phyllobium and Phyllosiphon in their tissues. Gomontia is a shell-boring Alga, Dermatophyton grows on the carapace of the tortoise, and Trichophilus in the hairs of the sloth. Certain Protococcales and Confervales exist as the gonidia of the lichenthallus. The thallus is of more varied structure in this group than in any other. In the simplest case it may consist of a single cell, which may remain free during the whole or the greater part of its existence, or be loosely aggregated together within a common mucilage, or be held together by the adhesion of the cell-walls at the surface of contact. These aggregations or colonies, as they are termed, may assume the form of a plate, a ring, a solid sphere, a hollow sphere, a perforate sphere, a closed net, or a simple or branched filament. It is not easy in all cases to draw a distinction between a colony of plants and a multicellular individual. In a Volvox sphere, for example, there is a marked protoplasmic continuity between all the cells of the colony. The Ulyacece, the thallus of which consists of laminae one or more cells thick, or hollow tubes, probably represent a still more advanced stage in the passage of a colony into a multicellular plant. Here there is some amount of localization of growth and distinction of parts. It is only in such cases as Volvox and Ulvacece that there is any pretension to the formation of a true parenchyma within the limits of the Chlorophycece. In the whole series of the Confervales, the thallus consists of filaments branched or unbranched, attached at one extremity, and growing almost wholly at the free end. The branches end in fine hairs in Chcetophoracece. In Coleochcetacece the branches are often welded into a plate, simulating a parenchyma. In all Conjugatce and most Protococcales, and in the bulk of the Confervales, the thallus consists of a cell or cells, the protoplast of which contains a single nucleus. In Hydrodictyacece, Cladophoracece, Sphceropleacecc, and Gomontiacece this is no longer the case. Instead of a single relatively large nucleus, each cell is found to contain many small nuclei, and is spoken of as a ccenocyte. This character becomes still more pronounced in the large group of the Siphonales. Valoniacece and Dasycladaceae are partially septate, but elsewhere no cellulose partitions occur, and the thallus is more or less the continuous tube from which the group is named. Yet the Siphonaceous Algsemay assume great variety of form and reach a high degree of difi'erentiation. Protosiphon and Botrydium, on the one hand, are minute vesicles attached to muddy surfaces by rhizoids ; Caulerpa, on the other, presents a remarkable instance of the way in which much the same external morphology as that of cormophytes has been reached by a totally different internal structure. Many Siphonales are encrusted with lime like Corallina among Red Algae. Penicillus is brush-like, Halimeda and Cymopolia are jointed, Acetabularia has much the same external form as an expanded Coprinus, Neomeris simulates the fertile shoot of Equisetum with its densely-packed whorled branches, and in Microdictyon, Anadyomene, Struvea, and Boodlea the branches, spreading in one plane, become bound together in a more or less close network. 1 Chlorophyceae include :— 1. Confervoideae—12 families, 77 genera, 1021 species. 2. Siphoneae—9 families, 26 genera, 271 species. 3. 1’rotococcoideae—2 families, 90 genera, 342 species. 4. Conjugatas—2 families, 33 genera, 1296 species. (De Toni’s Sylloge Algarum.) 5. Characeae—2 families, 6 genera, 181 species. (Engler and Prantl’s Planzenfamilien). S. I. —34
