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gators; and as the result of his own observations declares that Algae, so far from being as polymorphic as they have been described, vary only within relatively narrow limits, and present on the whole as great fixity as the higher plants. It certainly supports his view to discover, on subjecting to a careful investigation Botrydium granulation, a Siphonaceous Alga whose varied forms had been described by Kostafinski and Woronin, that these authors had included in the life-cycle, stages of a second Alga described previously by Kiitzing, and now described afresh by Klebs as Protosiphon botryoides. In Botrydium the chromatophores are small, without pyrenoids, and oil-drops are present; in Protosiphon the chromatophores form a network with pyrenoids, and the contents include starch. Klebs insists that the only solution of such problems is the subjection of the Algae in question to a rigorous method of pure culture. It is interesting to learn that Senn, pursuing the methods described by Klebs, has confirmed Chodat’s observation of the passage of Raphidium into a Dactylococcus-st&gQ, although he was unable to observe further metamorphosis. He has also seen Pleurococcus viridis dividing so as to form a filament, but has not succeeded in seeing the formation of zoospores as described by Chodat. While, therefore, there is much evidence of a negative character against the existence of an extensive polymorphism among Algae, some amount of metamorphosis is known to occur. But until the conditions under which a particular transformation takes place have been ascertained and described, so that the observation may be repeated by other investigators, scant credence is likely to be given to the more extreme polymorphistic views. In comparison with the higher plants, Algae exhibit so much simplicity of structure, while the conditions under w c Ph sio ^i ^ they grow are so much more readily logy. °* controlled, that they have frequently been the subject of physiological investigation with a view chiefly to the application of the results to the study of the higher plants. (See Physiology op Plants). In the literature of vegetable physiology there has thus accumulated a great body of facts relating not only to the phenomena of reproduction, but also to the nutrition of Algae. With reference to their chemical physiology, the gelatinization of the cell-wall, which is so marked a feature, is doubtless attributable to the occurrence along with cellulose of pectic compounds. There is, however, considerable variation in the nature of the membrane in different species ; thus the cell-wall of (Edogonium, treated with sulphuric acid and iodine, turns a bright blue, while the colour is very faint in the case of Spirogyra, the wall of which is said to consist for the most part of pectose. While starch occurs commonly as a cell-content in the majority of the Green Algae no trace of it occurs in Vaucheria and some of its allies, nor is it known in the whole of the Phceophycece and Rhodophycece. In certain Euphceophyceoe bodies built up of concentric layers, and attached to the chromatophores, were described by Schmitz as phaeophyceanstarch; they do not, however, give the ordinary starch reaction. Other granules, easily mistaken for the “ starch ” granules, are also found in the cells of Phoeophyceoe ; these possess a power of movement apart from the protoplasm, and are considered to be vesicles and to contain phloroglucin. The colourless granules of Florideoe, which are supposed to constitute the carbohydrate reserve material, have been called floridean-starch. A white efflorescence which appears on certain Brown Algae (Saccorhiza bulbosa, Laminaria saccharina), when they are dried in the air, is found to consist of mannite. Mucin is known in the cellsap of Ace tabular ia. Some Siphonales (C odium) give rise to proteid crystalloids, and they are of constant occurrence

among Florideoe. The presence of tannin has been established in the case of a great number of freshwater Algae. By virtue of the possession of chlorophyll all Algae are capable of utilizing carbonic acid gas as a source of carbon in the presence of sunlight'. The presence of phycocyanin, phycophaein, and phycoerythrin considerably modifies the absorption spectra for the plants in which they occur. Thus in the case of phycoerythrin the maximum absorption, apart from the great absorption at the blue end of the spectrum, is not, as in the case where chlorophyll occurs alone, near the Fraunhofer line B, but farther to the right beyond the line D. By an ingenious method devised by Engelmann, it may be shown that the greatest liberation of oxygen, and consequently the greatest assimilation of carbon, occurs in that region of the spectrum represented by the absorption bands. In this connexion Pfeffer points out that the penetrating power of light into a clear sea varies for light of different colours. Thus red light is reduced to such an extent as to be insufficient for growth at a depth of 34 metres, yellow light at a depth of 177 metres, and green light at 322 metres. It is thus an obvious advantage to Red Algse, which flourish at considerable depths, to be able to utilize yellow light rather than the red, which is extinguished so much sooner. The experiment of Engelmann referred to deserves to be mentioned here, if only in illustration of the use to which Algae have been put in the study of physiological problems. Engelmann observed that certain Bacteria were motile only in the presence of oxygen, and that they retained their motility in a microscopic preparation in the neighbourhood of an Algal filament when they had come to rest elsewhere on account of the exhaustion of oxygen. After the Bacteria had all been brought to rest by being placed in the dark, he threw a spectrum upon the filament, and observed in what region the Bacteria first regained their motility, owing to the liberation of oxygen in the process of carbon-assimilation. He found that these places corresponded closely with the region of the absorption band for the Algse under experiment. Although Algse generally are able to use carbonic acid gas as a source of carbon, some Algse, like certain of the higher plants, are capable of utilizing organic compounds for tins purpose. Thus Spirogyra filaments, which have been denuded of starch by being placed in the dark, form starch in one day if they are placed in a 10 to 20 per cent, solution of dextrose. According to Bokorny, moreover, it appears that such filaments will yield starch from formaldehyde when they are supplied with sodium oxymethyl sulphonate, a salt which readily decomposes into formaldehyde and hydrogen sodium sulphite, an observation which has been taken to mean that formaldehyde is always a stage in the synthesis of starch. With reference to the assimilation of nitrogen, it would seem that Algae, like other green plants, can best use it when it is presented to them in the form of a nitrate. Some Algse, however, seem to flourish better in the presence of organic compounds. In the case of Scenedesmus acutus, it is said that the Alga is unable to take up nitrogen in the form of a nitrate or ammoniacal salt, and requires some such substance as an amide or a peptone. On the other hand it has been held by Frank and other observers that atmospheric nitrogen is fixed by the agency of Green 'Algae in the soil. (For the remarkable symbiotism between Algse and Fungi, see Fungi.) Excepting where the thallus is impregnated with silica, as in Diatomacece, or carbonate of lime, as in Corallinaceoe, Characece, and some Siphonales, it is perhaps not surprising that Algse should not have been extensively preserved in the fossil form. Considering, however, that it is gener-