A L G M the cell may protrude an inner wall, the endosporium, and grow out into the new plant (Vaucheria), or the contents may break up into a first brood of zoospores. It is held that in Colcochcete a parenchyma results from the division of the oospore, from each cell of which a zoospore arises. Reproduction is also effected among Euchlorophycece, by means of aplanospores and akinetes. Aplanospores would seem to represent zoospores arrested in their development; without reaching the stage of motility, they germinate within the sporangium. Akinetes are ordinary thallus cells, which on account of their acquisition of a thick wall are capable of surviving unfavourable conditions. Both aplanospores and akinetes may germinate with or without the formation of zoospores at the initial stage. Among Conjugatoe reproduction is effected solely by means of conjugation of what are literally aplanospores. Among those Desmidiaccce, wdiich live a free life, two plants become surrounded by a common mucilage, in which they lie either parallel (Closterium) or crosswise (Cosmarium). Gaps then appear in the apposed surfaces, usually at the isthmus ; the entire protoplasts either pass out to melt into one another clear of the old walls, or partly pass out and fuse without complete detachment from the old walls. Among colonial Desmidiacece, the break-up of the filament is a preliminary to this conjugation ; otherwise the process is the same. The zygospore becomes surrounded with its own wall, consisting finally of three layers, the outer of which is furnished with spicular prominences of various forms. In Zygnemacece there is no dissolution of the filaments, but the wdiole contents of one cell pass over by means of a conjugation-tube into the cavity of a cell of a neighbouring filament, where the zygospore is formed by the fusion of the two protoplasts. In these cases the activity of one of the gametes, and the passivity of the other, is regarded as evidence of incipient sex. In Sirogonium there is celldivision in the parent-cell prior to conjugation; and as two segments are cut off in the case of the active gamete, and only one in the case of the passive gamete, there is a corresponding difference of size, marking another step in the sexual differentiation. In Zygogonium, although no cell-division takes place, the gametes consist of a portion only of the contents of a cell, and this is regularly the case in Mesocarpacece, which occupy the highest grade among Conjugatoe. Some Zygnemaccce and Mesocarpacece form either a short conjugating tube, or none at all, but the filaments approach each other by a knee-like bend, and the zygospore is formed at the point of contact, often being partially contained within the walls of the parent-cell. It would seem that in some cases the nuclei of the gametes remain distinct in the zygospore for a considerable time after conjugation. It is probable that in all cases nuclear fusion takes place sooner or later. In Zygnemaceoe and Mesocarpaceoe the zygospore, after a period of rest, germinates, to form a new filamentous colony ; in Desmidiacece its contents divide on germination, and thus give rise to two or more Desmids. Gametes which fail to conjugate sometimes assume the appearance of zygospores and germinate in due course. They are known as azygospores. The reproduction of Characece is characterized by a pronounced oogamy, the reproductive organs being the most highly differentiated among Chlorophycece. The antheridia and oogonia are formed at the nodes of the appendages. The oogonium, seated on a stalk cell, is surrounded by an investment consisting of five spirally-wound cells, from the projecting ends of which segments are cut off, constituting the so-called stigma. The oosphere is not differentiated within the wall of the oogonium, but certain cells known as wendungszellen, the significance of which has given rise to much speculation, are cut off from the basal portion of the parent-cell during its development. The antheridia are spherical, orange-coloured bodies of very complex structure. The antherozoid is a spirally-coiled thread of protoplasm, furnished at one end with a pair of cilia. It much more resembles the antherozoids of Bryophyta and certain Pteridophyta than any known among other Algae. The fertilized egg charged with food reserves rests for a considerable period, surrounded by its cortex, the whole having assumed a reddish-brown colour. On germination it gives rise to a row of cells in which short (nodal) and long (internodal) cells alternate. From the first node arise rhizoids ; from the second a lateral bud, which becomes the new plant. This peculiar product of germination, which intervenes between the oospore and the adult form, is the proembryo. It will be remembered that in Musci the asexual spore somewhat similarly gives rise to a protonema, from which the adult plant is produced as a lateral bud. The proembryonic branches of Characcce, one of the means of vegetative reproduction already referred to, are so called because they repeat the characters of the proembryo. Before leaving the Chlorophycece, it should be mentioned that the genus Volvox has been included by some zoologists (Biitschli, for example) among Flagellata ; on the other hand, certain green Flagellata, such as Euglena, are included by some botanists (for example, van Tieghem) among unicellular plants. A similar
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uncertainty exists with reference to certain groups of Phceophycece, and the matter will1thus arise again. 3. PiiuiOPHYCKu:. —The Phceophycece are distinguished by the possession of a brown colouring matter, phycophsein, in addition to chlorophyll. They consist of the following groups:— Fucacece, Phceosporece, Dictyotaccce, Cryptomonadaceoe, Peridinaceoe, and Diatomacece. Of these the first three include multicellular plants, some of them of great size; the last three are unicellular organisms, with little in common with the rest excepting the possession of a brown colouring matter. Fucacece and Phceosporece are doubtless closely allied, and to these Dictyotacece may be joined, though the relationship is less close. They constitute the Euphceophycece, and will be dealt with in the first place. Euphceophycece are almost exclusively marine, growing on rocks and stones on the coast, or epiphytic upon other Algae. In tidal seas they range from the limits of high water to some distance beyond the low-water line. On the British coasts zones are observable in passing from high- to low-water mark, characterized by the prevalence of different species, thus :—Pelvetia canaliculata, Fucus platycarpus, Fucus vesiculosus, Ascophyllum nodosum, Fucus scrratus, Laminaria digitata. Some species are minute filamentous plants, requiring the microscope for their detection ; others, like Lessonia, are of considerable bulk ; or, like Macrocystis, of enormous length. In Fucacece, Dictyotacece, and Laminariacece and Sphacelariacece, among Phceosporece, the thallus consists of a true parenchyma ; elsewhere it consists of free filaments, or filaments so compacted together, as in Cutleriacece and Desmarestiacece, as to form a false parenchyma. In Fucacece and Laminariacece the inner tissue is differentiated into a conducting system. In Laminariacece the inflation of the ends of conducting cells gives rise to the so-called trumpet-hyphse. In Nereocystis and Macrocystis a zone of tubes occurs, which present the appearance of sieve-tubes even to the eventual obliteration of the perforations by a callus. While there is a general tendency in the group to mucilaginous degeneration of the cell-wall, in Laminaria digitata there are also glands secreting a plentiful mucilage. Secondary growth in thickness is effected by the tangential division of superficial cells. The most fundamental external differentiation is into holdfast and shoot. In Laminariacece secondary cylindrical props arise obliquely from the base of the thallus. In epiphytic forms the rhizoids of the epiphyte often penetrate into the tissue of the host, and certain epiphytes are not known to occur excepting in connexion with a certain host; but to what extent, if any, there is a partial parasitism in these cases has not been ascertained. In filamentous forms there is a differentiation into branches of limited and branches of unlimited growth (Sphacelaria). In Laminariacece there is a distinction of stipe and blade. The blade is centrally - ribbed in Alaria and laterally - ribbed in Macrocystis. It is among the Sargassacece that the greatest amount of external differentiation, rivalling that of the higher leafy plants, is reached. A characteristic feature of the more massive species is the occurrence of air-vesicles in their tissues. In Fucus vesiculosus they arise in lateral pairs ; in Ascophyllum they are single and median ; in Macrocystis one vesicle arises at the base of each thallus segment; in Sargassum and Halidrys the vesicles arise on special branches. They serve to buoy up the plant when attached to the sea-bottom, and thus light is admitted into the forest-like growths of the gregarious species. When such plants are detached they are enabled to float for great distances, and the great Sargasso Sea of the North Atlantic Ocean is probably only renewed by the constant addition of plants detached from the shores of the Caribbean Sea and Gulf of Mexico. Growth in length is effected in a variety of ways. . In Dictyota, Sphacelariacece, and Fucacece there is a definite apical cell. In the first it is a biconvex lens, from which segments are continually cut off parallel to the posterior surface ; and in the second an elongated dome, from which segments are cut off by a transverse wall. While, however, in Dictyota the product of the subsequent division in the segment enlarges with each subdivision, the divisions in the cylindrical segment of Sphacelariacece are such that the whole product after subdivision, however many cells it may consist of, does not exceed in bulk the segment as cut off from the apical cell. In Dictyotacece the apical cell occasionally divides 1 Phaeophyceee include :— 1. Cyclosporince {Fucacece)—4 families, 32 genera, 347 species. 2. Tetrasporince {Dictyotacece)—1 family, 17 genera, 130 species.^ 3. Phceozoosporinece {Phceosporece)—24 families, 143 genera, 5/1 species. , (De Toni’s Sylloge Algarum.) 4. Peridiniales—3 families, 32 genera, 167 species. 5. Cryptomonadacece (including Chrysomonadacece)—2 families, 28 genera, 50-60 species. 6. Bacillariales {Diatomacece)—about 150 genera, and 5000 species, fossil and recent. . ... . (Engler and Prantl’s Pftanzeufamihen.)
