LITMIR.BIZ

      Fig. 24.—Forms of diatoms. A, Pinnularia. i, seen from above; ii, from the side. B, Fragillaria (?). C, Navicula. D, F, Eunotia. E, Gomphonema. G, Cocconeis. H, Diatoma. All × 300.

      These plants are all characterized by the presence of a brown pigment, in addition to the chlorophyll, which almost entirely conceals the latter, giving the plants a brownish color, ranging from a light yellowish brown to nearly black. One order of plants that possibly belongs here (Diatomaceæ) are single celled, but the others are for the most part large seaweeds. The diatoms, which are placed in this class simply on account of the color, are probably not closely related to the other brown algæ, but just where they should be placed is difficult to say. In some respects they approach quite closely the desmids, and are not infrequently regarded as related to them. They are among the commonest of organisms occurring everywhere in stagnant and running water, both fresh and salt, forming usually, slimy, yellowish coatings on stones, mud, aquatic plants, etc. Like the desmids they may be single or united into filaments, and not infrequently are attached by means of a delicate gelatinous stalk (Fig. 25).

      Fig. 25.—Diatoms attached by a gelatinous stalk. × 150

      They are at once distinguished from the desmids by their color, which is always some shade of yellowish or reddish brown. The commonest forms, e.g. Navicula (Fig. 24, C), are boat-shaped when seen from above, but there is great variety in this respect. The cell wall is always impregnated with large amounts of flint, so that after the cell dies its shape is perfectly preserved, the flint making a perfect cast of it, looking like glass. These flinty shells exhibit wonderfully beautiful and delicate markings which are sometimes so fine as to test the best lenses to make them out.

      This shell is composed of two parts, one shutting over the other like a pill box and its cover. This arrangement is best seen in such large forms as Pinnularia (Fig. 24, A ii).

      Most of the diatoms show movements, swimming slowly or gliding over solid substances; but like the movements of Oscillaria and the desmids, the movements are not satisfactorily understood, although several explanations have been offered.

      They resemble somewhat the desmids in their reproduction.

      The True Brown Algæ.

      These are all marine forms, many of great size, reaching a length in some cases of a hundred metres or more, and showing a good deal of differentiation in their tissues and organs.

      Fig. 26.—A, a branch of common rock weed (Fucus), one-half natural size. x, end of a branch bearing conceptacles. B, section through a conceptacle containing oögonia (og.), × 25. C, E, successive stages in the development of the oögonium, × 150. F, G, antheridia. In G, one of the antheridia has discharged the mass of spermatozoids (an.), × 150.

      One of the commonest forms is the ordinary rock weed (Fucus), which covers the rocks of our northeastern coast with a heavy drapery for several feet above low-water mark, so that the plants are completely exposed as the tide recedes. The commonest species, F. vesiculosus (Fig. 26, A), is distinguished by the air sacs with which the stems are provided. The plant is attached to the rock by means of a sort of disc or root from which springs a stem of tough, leathery texture, and forking regularly at intervals, so that the ultimate branches are very numerous, and the plant may reach a length of a metre or more. The branches are flattened and leaf-like, the centre traversed by a thickened midrib. The end of the growing branches is occupied by a transversely elongated pit or depression. The growing point is at the bottom of this pit, and by a regular forking of the growing point the symmetrical branching of the plant is brought about. Scattered over the surface are little circular pits through whose openings protrude bunches of fine hairs. When wet the plant is flexible and leathery, but it may become quite dry and hard without suffering, as may be seen when the plants are exposed to the sun at low tide.

      The air bladders are placed in pairs, for the most part, and buoy up the plant, bringing it up to the surface when covered with water.

      The interior of the plant is very soft and gelatinous, while the outer part forms a sort of tough rind of much firmer consistence. The ends of some of the branches (Fig. 26, A, x) are usually much swollen, and the surface covered with little elevations from which may often be seen protruding clusters of hairs like those arising from the other parts of the plant. A section through one of these enlarged ends shows that each elevation corresponds to a cavity situated below it. On some of the plants these cavities are filled with an orange-yellow mass; in others there are a number of roundish olive-brown bodies large enough to be easily seen. The yellow masses are masses of antheridia; the round bodies, the oögonia.

      If the plants are gathered while wet, and packed so as to prevent evaporation of the water, they will keep perfectly for several days, and may readily be shipped for long distances. If they are to be studied away from the seashore, sections for microscopic examination should be mounted in salt water (about 3 parts in weight of common salt to 100 of water). If fresh material is not to be had, dried specimens or alcoholic material will answer pretty well.

      To study the minute structure of the plant, make a thin cross-section, and mount in salt water. The inner part or pith is composed of loosely arranged, elongated cells, placed end to end, and forming an irregular network, the large spaces between filled with the mucilaginous substance derived from the altered outer walls of these cells. This mucilage is hard when dry, but swells up enormously in water, especially fresh water. The cells grow smaller and more compact toward the outside of the section, until there are no spaces of any size between those of the outside or rind. The cells contain small chloroplasts like those of the higher plants, but owing to the presence of the brown pigment found in all of the class, in addition to the chlorophyll, they appear golden brown instead of green.

      No non-sexual reproductive bodies are known in the rock weeds, beyond small branches that occur in clusters on the margins of the main branches, and probably become detached, forming new plants. In some of the lower forms, however, e.g. Ectocarpus and Laminaria (Fig. 28, A, C), zoöspores are formed.

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