Algæ
The algæ comprise most of the green floating “scum” which covers the surfaces of ponds and other quiet waters. The masses of plants are often called “frog spittle.” Others are attached to stones, pieces of wood, and other objects submerged in streams and lakes, and many are found on moist ground and on dripping rocks. Aside from these, all the plants commonly known as seaweeds belong to this category; these latter are inhabitants of salt water.
The simplest forms of algæ consist of a single spherical cell, which multiplies by repeated division or fission. Many of the forms found in fresh water are filamentous, i.e. the plant body consists of long threads, either simple or branched. Such a plant body is termed a thallus. This term applies to the vegetative body of all plants that are not differentiated into stem and leaves. Such plants are known as thallophytes (p. [181]). All algæ contain chlorophyll, and are able to assimilate carbon dioxide from the air. This distinguishes them from the fungi.
Nostoc.—On wet rocks and damp soil dark, semitransparent irregular or spherical gelatinous masses about the size of a pea are often found. These consist of a colony of contorted filamentous algæ embedded in the jelly-like mass. The chain of cells in the filament is necklace-like. Each cell is homogeneous, without apparent nucleus, and blue-green in colour, except one cell which is larger and clearer than the rest. The plant therefore belongs to the group of blue-green algæ. The jelly probably serves to maintain a more even moisture and to provide mechanical protection. Multiplication is wholly by the breaking up of the threads. Occasionally certain cells of the filament thicken to become resting-spores, but no other spore formation occurs.
Fig. 264.—Filament of Oscillatoria, showing one dead cell where the strand will break.
Oscillatoria.—The blue-green coatings found on damp soil and in water frequently show under the microscope the presence of filamentous algæ composed of many short homogeneous cells (Fig. [264]). If watched closely, some filaments will be seen to wave back and forth slowly, showing a peculiar power of movement characteristic of this plant. Multiplication is by the breaking up of the threads. There is no true spore formation.
Fig. 265.—Strand of Spirogyra, showing the chlorophyll bands. There is a nucleus at a. How many cells, or parts of cells, are shown in this figure?
Spirogyra.—One of the most common forms of the green algæ is spirogyra (Fig. [265]). This plant often forms the greater part of the floating green mass (or “frog spittle”) on ponds. The thread-like character of the thallus can be seen with the naked eye or with a hand lens, but to study it carefully a microscope magnifying two hundred diameters or more must be used. The thread is divided into long cells by cross walls which, according to the species, are either straight or curiously folded (Fig. [266]). The chlorophyll is arranged in beautiful spiral bands near the wall of each cell. From the character of these bands the plant takes its name. Each cell is provided with a nucleus and other protoplasm. The nucleus is suspended near the centre of the cell (a, Fig. [265]) by delicate strands of protoplasm radiating toward the wall and terminating at certain points in the chlorophyll band. The remainder of the protoplasm forms a thin layer lining the wall. The interior of the cell is filled with cell-sap. The protoplasm and nucleus cannot be easily seen, but if the plant is stained with a dilute alcoholic solution of eosine they become clear.
Fig. 266.—Conjugation of Spirogyra. Ripe zygospores on the left; a, connecting tubes.
Spirogyra is propagated vegetatively by the breaking off of parts of the threads, which continue to grow as new plants. Resting-spores, which may remain dormant for a time, are formed by a process known as conjugation. Two threads lying side by side send out short projections, usually from all the cells of a long series (Fig. [266]). The projections or processes from opposite cells grow toward each other, meet, and fuse, forming a connecting tube between the cells. The protoplasm, nucleus, and chlorophyll band of one cell now pass through this tube, and unite with the contents of the other cell. The entire mass then becomes surrounded by a thick cellulose wall, thus completing the resting-spore, or zygospore (z, Fig. [266]).
Zygnema is an alga closely related to spirogyra and found in similar places. Its life history is practically the same, but it differs from spirogyra in having two star-shaped chlorophyll bodies (Fig. [267]) in each cell, instead of a chlorophyll-bearing spiral band.
Fig. 267.—Strand, or Filament of Zygnema, freed from its gelatinous covering.
Vaucheria is another alga common in shallow water and on damp soil. The thallus is much branched, but the threads are not divided by cross walls as in spirogyra. The plants are attached by means of colourless root-like organs which are much like the root-hairs of the higher plants: these are rhizoids. The chlorophyll is in the form of grains scattered through the thread.
Vaucheria has a special mode of asexual reproduction by means of swimming spores or swarm-spores. These are formed singly in a short enlarged lateral branch known as the sporangium. When the sporangium bursts, the entire contents escape, forming a single large swarm-spore, which swims about by means of numerous lashes or cilia on its surface. The swarm-spores are so large that they can be seen with the naked eye. After swimming about for some time they come to rest and germinate, producing a new plant.
Fig. 268.—Thread of Vaucheria with Oögonia and Antheridia.
The formation of resting-spores of vaucheria is accomplished by means of special organs, oögonia (o, Fig.[ 268]) and antheridia (a, Fig. [268]). These are both specially developed branches from the thallus. The antheridia are nearly cylindrical, and curved toward the oögonia. The upper part of an antheridium is cut off by a cross wall, and within it numerous ciliated sperm-cells are formed. These escape by the ruptured apex of the antheridium. The oögonia are more enlarged than the antheridia, and have a beak-like projection turned a little to one side of the apex. They are separated from the thallus thread by a cross wall, and contain a single large green cell, the egg-cell. The apex of the oögonium is dissolved, and through the opening the sperm-cells enter. Fertilization is thus accomplished. After fertilization the egg-cell becomes invested with a thick wall and is thus converted into a resting-spore, the oöspore.
Fig. 269.—Fucus. Fruiting branches at s, s. On the stem are two air-bladders.
Fucus.—These are rather large specialized algæ belonging to the group known as brown seaweeds and found attached by a disk to the rocks of the seashore just below high tide (Fig. [269]). They are firm and strong to resist wave action and are so attached as to avoid being washed ashore. They are very abundant algæ. In shape the plants are long, branched, and multicellular, with either flat or terete branches. They are olive-brown. Propagation is by the breaking off of the branches. No zoöspores are produced, as in many other seaweeds; and reproduction is wholly sexual. The antheridia, bearing sperm-cells, and the oögonia, each bearing eight egg-cells, are sunken in pits or conceptacles. These pits are aggregated in the swollen lighter coloured tips of some of the branches (s, s, Fig. [269]). The egg-cells and sperm-cells escape from the pits and fertilization takes place in the water. The matured eggs, or spores, reproduce the fucus plant directly.
Fig. 270.—Nitella.
Nitella.—This is a large branched and specialized fresh-water alga found in tufts attached to the bottom in shallow ponds (Fig. [270]). Between the whorls of branches are long internodes consisting of a single cylindrical cell, which is one of the largest cells known in vegetable tissue. Under the microscope the walls of this cell are found to be lined with a layer of small stationary chloroplastids, within which layer the protoplasm, in favourable circumstances, will be found in motion, moving up one side and down the other (in rotation). Note the clear streak up the side of the cell and its relation to the moving current.