Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/546

Rh 508 flattened, cellular, leaf-like expansion, called a thallus, sometime? stalked, which bears the organs of reproduction. Some have root-like processes by which they are attached to rocks. These do not act like the nourishing roots of flowering plants ; they simply fix the plants and enable them to sway about in the water. This is markedly the case Avith the Laminarias, or large tangles of our coasts. The leafy appendages of seaweeds are called fronds. They vary in size, colour, and consistence. Some of the red and green delicate fronds form beautiful objects when carefully dried and laid out on drawing-paper. In order to dry sea weeds they must be first washed carefully in fresh water to separate saline matters, and then placed within drying- paper and subjected to pressure. Very delicate seaweeds should be floated out in water, drawing-paper being placed under them, and their fronds being carefully arranged on the paper before they are raised out of the water. They must then be dried partially in the air, and afterwards under pressure between sheets of drying-paper. Seaweeds are composed entirely of cells, which in some instances become elongated so as to have the appearance of tubes. Some Algae are nni-cellular, that is, are composed of a single cell, as occurs in some Desmidieae, as Closterium. At other times they are composed of numerous cells, which are kept together by a gelatinous matter, but separating easily from each other so as to have an independent exist ence. This is observed in the red snow plant (Protococcus or Palmella nivalis). The cells of seaweeds are sometimes joined together so as to form a linear series, and to give them a thread-like appearance ; and in such a case, when the divisions between the cells are marked, the whole appears like a beaded necklace of cells. When the cells are united both lengthwise and laterally they then form an expanded flat frond. In some instances the frond is gelatinous. The germinating bodies or spores of seaweeds are cells often contained in cavities (Fig. 2). They vary in colour, and the fronds have frequently the same colour as the spores. In reference to their colour, Algae have been divided into three sub-orders : 1. Melanospermeae, brown coloured seaweeds (Fig. 1), with olive-brown spores; 2. Rhodospermese, rose-coloured seaweeds, with red spores ; 3. Chlorospermeae, green-coloured seaweeds, with green ipores. ~ Fig. 2. Fig. 1. Fig. 3. Fig. 1. Thallus, It, of Fucus ttsicuJoitis, the common Bladder Seaweed, vith air-vesicle, r, and masses of conccptacles constituting the fructification, fr,fr, which is sometimes called gleba. Fig. 2. Fructification of a Seaweed, con taining spores, which are ultimately discharged at an opening, o. Fig. 3. Tetraspore of one of the rose-coloured Seaweeds. Algae are multiplied by the division cf cells and by spores. By cell-division there is a multiplication of cells, and by separation from the parent plant these cells may bear buds. True fertilisation is effected by means of union of cells, or what is called conjugation. In this pro cess two kinds of cells unite by means of a tube, and the contents of the one passes into the other, thus giving rise to germinating spores. This is seen in Conferva}, such as the green matter often seen in ponds, and called silk-tceed. There are also observed in Algae two kinds of fertilising bodies, one set called Antheridia, containing moving fila ments or spermatozoids ; and the other called Archegonia, containing a rudimentary cell, which, after contact with the spermatozoids, becomes a spore forming a new plant. The spores produced by some Algae move about in water, and have been called Zoospores. Their spontaneous movements are effected by means of vibratile slender threads called cilia. These zoospores are contained in a cell, which ultimately bursts and scatters them. The pro cess is well seen in a green Alga called Vaucheria. The zoo- spores move about for a certain time, and ultimately the spores get fixed to a rock or the wood of a pier, and then the cilia disappear. Cilia sometimes occur in pairs at one end of a spore, numbering two or three ; at other times they are placed round the whole circumference of the spore. Spores have a tendency to divide into four ; such com pound spores are called tetraspores (Fig. 3). They are common in the sub-order Rhodospermeae. They seem to differ from ordinary spores, and to be more of the nature of buds. In some Algae, such as Corallines, there is a coating of calcareous matter which conceals their tissue. This can be removed by means of hydrochloric acid. Diatoms, a subdivision of Algae, are so called from two Greek words signifying to cut through, in allusion to the mode of divi sion into two valves. They are microscopic one-celled bodies, covered externally by a siliceous or flinty coat. They are on the confines of the animal and vegetable king doms, and have been referred sometimes to the one and sometimes to the other. Their mode of reproduction by conjugation and spores seems to indicate their alliance with Algae, although some still place them among infusorial animalcules. The siliceous markings of Diatoms are very beautiful microscopic objects. After exposure to the action of fire or nitric acid, the silex remains unaltered, and in that state the streaks of the covering are easily observed. Many of the Algae supply nutritious food. RJwdymenia palmata, one of the red seaweeds, is the dulse of the Scotch, the dillesk of the Irish. Chondrus (Sphcerococcus] crispus and C. mammillosus, two Rhodosperms, receive the name of carrageen or Irish moss. Their fronds consist in part of a substance allied to starch, which is extracted by putting them in water, and on cooling it forms a jelly. Species of Ulva, one of the Chlorosperms, supply the green laver. Species of Caulerpa furnish food to turtles. Lamm- aria digitata, and Laminaria saccharina, under the name of tangle, are eaten in the north of Europe. Dulse and tangle was formerly a common cry in the streets of Edinburgh. D Urvillcea utilis is used as food in Chili Alaria esculenta, a British species, is also edible, (jliyartina speciosa is used for jelly in the Swan River settlement. Gracilaria lichenoides, under the name of Ceylon moss, is used for soups and jellies. Gracilaria spinosa supplies the Agar-Agar in China. Nostoc edule is a Chinese article of diet. The edible nests of China are supposed to be formed from seaweeds. Plocaria tenax is used in China to furnish glue. Iridcca edulis is edible. Laureneia pin- natifida is called pepper- dulse on account of having pungent qualities. Seaweeds form an excellent manure. They are used on many farms situated near the sea-shore. Seaweeds after burning yield barilla, an impure carbonate of soda. Kelp was for many years prepared from sea weeds in Scotland, more especially in the Western and Northern Islands.