ALGAE, or HYDROPHYTA, a large order of cellular, flowerless, cryptogamic plants, found in the sea (seaweeds), in rivers, laked, marshes, hot springs, and moist places, all over the world. They consist of a brown, red, to green. Flattered, cellular, leaf-like expansion, called a thallus, sometimes 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 with the Laminaries, 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 seaweeds 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 uni-cellular, that is, are composed of a single cell, as occurs in some Desmidiease, 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 existence. 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.

Fig. 1 -- Thallu, tt, of Fucus vesiculous, the common Bladder Seaweed, with air-vesicle, v, and masses of conceptables, constituting the fructification, fr, fr, which is sometimes called gleba.
Fig. 2 -- Fructification of a Seaweed, containing spores, which are ultimately discharged at an opening, o.
Fig. 3 -- Tetraspore of one of the rose-coloured Seaweeds.

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. Rhodospermeae, rose-coloured, seaweeds, with red spores; 3. Chlorospermeae, green-coloured seaweeds, with green spores.

Algae are multiplied by the division of 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 fertilization is effected by means of union of cells, or what is called conjugation. In this process two kinds of cells unite by means of a tube, and the contents of the once passes into the other, thus giving rise to germinating spores. This is seen in Confervae, such as the green matter often seen in ponds, and called silk-weed. There are also observed in Algae two kinds of fertilizing bodies, one set called Antheridia, containing moving filaments 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 process is well seen in a green Alga called Vaucheria. The zoospores 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 circumsference of the spore.





Spores have a tendency to divide into four; such compound s pores 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 hydrocgloric acid. Diatons, a subdisivion of Algae, are so called from two Greek words signifying to cut through, in allusion to the mode of division 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 kingdoms, 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 unlatered, and in that state the streaks of the covering are easily observed.

Many of the Algae supply nutritious food. Rhodymenia palmate, one of the red seaweeds, is the dulse of the Scotch, the dillesk of the Irish. Chondrus (Sphoerococcus) 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 Caulaerpa furnish food to turtles. Laminaria 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'Urvilloea utilis is used as food in Chili Alaria esculenta, a British species, is also edible. Gigartina 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. Iradoea edulis is edible. Laurencia pinnatifida 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 seaweeds in Scotland, more especially in the Western and Northern Islands.

As regards the distribution of seaweeds, some are cosmopolitan or pelagic, as species of Ulva and Enteromorpha, which are equally abundant in high northern and southern latitudes, as they are under the equator and in temperate regions. Many Diatomaceae are distributed from pole to pole. In general, however, seaweeds are more or less limited in their distribution, so that different marine floras exist in various parts of the ocean. The marine species have been estimated at about 6000, and they are distributed in various regions. The Northern Ocean, from the pole to the 40th degree, the sea of the Antilles, the eastern coasts of South America, those of New Holland, the Indian Archipelago, the Mediterranean, the Red Sea, the Chinese and Japanese seas, all present very large marine regions, each of which possesses a peculiar vegetation. The degree of exposure to light, and the greater or less motion of the waves, are important in the distribution of Algae. The intervention of great depths of the ocean has an influence on sea plants similar to that of high mountains on land plants. Melanospermeae increase as we approach the tropics, where the maximum of the species is found. Rhodospermeae chiefly abounds in the temperate zone; while Chlorospermeae form the chief marine vegetation of the polar zone, and abound in the colder temperate zone. The green colour is characteristic of those algae which grow either in fresh water or in the shallower parts of the sea; the olive-coloured Algae are abundant between the tide-marks; while the red-coloured species occur chiefly in the deeper and the darker part of the sea.

Some seaweeds are worthy of note on account of the mode of their growth and distribution. Chorda Filum, a long cord-like seaweeds, lies in beds of 15 to 20 miles in length, the British Channel. Sargassum bacciferum constitutes the Gulf-weed, which has been noticed by all who have crossed the Atlantic. The Gulf-weed has never been seen attached, but always floating. From the abundance of this seaweed its locality is called the Sargasso Sea. The most remarkable of the seaweeds, as regards size and the extent of range, are Macrocystis pyrifera and Laminaria radiata. Masses of Macrocystis, like green meadows, are found in every latitude. Many specimens have been seen about 300 feet long; some even extend to 700 feet or upwards. A tree seaweed, Lessonia fuscescens, with a stem 10 feet long, 12 inches in circumstance, and its fronds 2-3 feet long and 3 inches broad, is found in immense masses off the Patagonian regions. D'Urvilloea utilis is another large Antarctic seaweed, which, along with Lessoniae, occurs at the Falkland Islands, formed by the surf into enormous vegetable cables, several hundred feet long, and thicker than the human body. In Britain we have a marked distribution of seaweeds as regards depth. There is a littoral zone lying between high and low water marks, divided into sub-regions characterized by the following seaweeds: - 1 Fucus canaliculatus; 2. Fucus vesiculosus; 3. Fucus nodosus; 4. Fucus serratus. Secondly, there is a laminarian zone, commencing at low-water mark, and extending for a depth of 7 to 15 fathoms. Here we meet with the great tangle seaweeds, such as Laminaria digitata and L. saccharina, along with deepwater Fuci. (J. H. B.)






The above article was written by: John Hutton Balfour, M.D., F.L.S., F.R.S.; late Professor of Botany in the University of Edinburgh; Regius Keeper of Royal Botanic Garden, 1845-79; author of Introduction to the Study of Palaeoontological Botany.