Fig. 18. Zoospores.
The Confervaceæ are generally reproduced by zoospores. In most cases the endochrome within a cell divides itself into numerous segments, each of which becomes a minute zoospore, and escapes into the water through a rupture in the cell wall. This is the case in a very graceful genus of Confervæ, of which the Chætophora elegans is a species. It consists of filamental strings of cells, ending in a capillary bristle, with lateral branches like narrow fronds. It is reproduced by zoospores. One half of each zoospore is round, opaque, and full of matter; the other half hyaline, and tapering to a beak furnished with four cilia. It frequently happens in this genus of Confervaceæ, where the filaments are divided at equal distances into little joints or compartments, that the zoospores issue from the terminal cell first, then from the next, and so on in succession till the upper part of the branch is left empty, while the lower part is still forming zoospores. After moving in water for a time, the zoospores retreat to a shady place, fix themselves to some substance, and begin to grow. These plants rapidly cover a large surface of water; for each individual cell may produce 100 zoospores, and as the development and dissemination of them continues during the whole summer, one plant may yield an enormous number.
The Sphæroplea annulina is a rare and very remarkable Conferva, whose cinnabar-coloured spores make the surface of the water in which it floats, like a pool of blood. It has no root, being merely a filament with capillary extremities, formed of elongated cells joined end to end. The spores only grow on the filaments that are exposed to the sun and air; the filaments that are below the water are green and barren. The spores are filled with red matter, grains of starch, and red oil, the outer or cellulose coat being so plaited, that the spore looks like a red star with white rays.
When a spore germinates, it produces a minute cell ending in capillary fibres, which increases in length by the continual bisection of its central cells, while the other Confervæ grow by the bisection of those that are terminal. During this growth, the red contents of the spore are so changed by a remarkable succession of chemical processes, that the primordial cells in the filament of the young plant are filled with a colourless viscous matter, an aqueous liquid, granules of starch, and chlorophyll. In some of the cells the starch disappears, while the green matter and the other materials arrange themselves into a series of rings, alternating with empty spaces or vacuoles. After a time, the green changes to a yellowish red, and then each ring in succession resolves itself into a multitude of minute active particles, which move with incredible velocity in the void spaces of the cell, till at last the whole cell swarms with them. They are analogous to the pollen of flowering plants, and thence are called spermatozoids. Their form is cylindrical, thick, broad, and yellow at one end, sharp at the other, with a colourless beak, and long cilia. The parent cell is at last pierced by their united efforts, and out they rush in great confusion into the water; some whirl round their centres, others swim in a circle, many describe cycloidal curves by a series of leaps, and a few swim in straight lines.
During the preceding changes another process is in progress, within what may be called female cells. In these the starch, mixed with green matter and a plastic substance, arrange themselves also into green and vacant rings, and after various and complicated changes, each green ring forms itself into a kind of plastic primordial free cell, which, after being fertilized by the moving bodies, gets a stronger coat. The green matter becomes first of a red-brown colour, then red; and after leaving the parent cell it is invested with a plaited cellulose coat, and becomes a star-like resting spore which may produce a new plant. No cryptogamic plant exhibits a greater variety in the modes of action of the vital forces, none more activity in the motile powers. In some of these Confervæ, the moving male filaments, or spermatozoids, instead of escaping singly from their prison cell in confusion into the water, are discharged in a mass enclosed in a capsule furnished with cilia, which moves with its lively burden like a zoospore, till a lid falls off which sets them free.
Some pretty plants allied to the Confervæ are called Batrachospermeæ, from the resemblance which their beaded filaments bear to the spawn of a frog. They are all inhabitants of fresh water, chiefly of gently flowing streams, and are so flexible that they yield to every movement of the water, and when taken out of it are like a mass of jelly. Their colour is usually a brownish-green, but sometimes it is of a reddish or bluish purple. The central stem of the plant, though originally formed of a single row of large cylindrical cells placed end to end, gets an investment of cells, or rather branches, which ultimately becomes a thick cylindrical stem, bearing, at nearly regular intervals, whorls of short radiating branches, each composed of rounded cells, arranged in a bead-like row, and sometimes branching again. Some of the radiating branches grow out into transparent points, which may possibly be antheridia, and contain motile bodies; for within certain cells in other branches resting spores are found, which are agglomerated and form the large dark globular masses that are seen in the midst of the whorls.
The Hydrodictyon utriculatum is another allied plant of singular structure, which grows in fresh-water pools in the midland and southern counties of England. It resembles a regularly reticulated green purse, from four to six inches long, and is composed of a vast number of tubular cylindrical cells, which adhere to one another by their rounded extremities, the points of junction corresponding to the knots or intersections of the network. Each of these cells may form within itself from 7,000 to 20,000 gonidia, which at a certain stage of their development are observed to be in active motion in its interior; subsequently, by mutual adhesion, they form into groups which lay the foundation of new net-plants, when set free by the dissolution of their envelope. Besides these groups, there are certain cells which produce from 30,000 to 100,000 more minute bodies of a longer shape, each of which is furnished with four long cilia, and a red spot. These escape from their cell in a swarm, move freely in the water for a time, then come to rest, and sink to the bottom, where they remain, heaped together in green masses. Their future fate is unknown, but they are believed to be male filaments similar to those described, and are generally called spermatozoids.
The Nostochineæ are either an assemblage of cells loosely united into numerous green chaplets, or distinctly beaded filaments, generally twisted, and occasionally branched; they are imbedded in a firm gelatinous frond of different form, sometimes globular, sometimes spreading in branched masses, often of considerable size. They are frequently seen on damp shady walks in gardens: they shrink to a film in dry weather, and reappear so suddenly in rain that they have been called fallen stars. They are reproduced by spontaneous division of their filaments; the segments escape from the gelatinous mass, move slowly in the direction of their length, after a time come to rest, secrete a gelatinous envelope, and not only grow in length by transverse bisection, but split longitudinally into new filaments which are separated by their gelatinous secretions. These movements, discovered by M. Thuret, are evidently intended to disperse the plant.
Vesicular cells, destitute of endochrome, sometimes furnished with cilia, and of a larger size than the others, are occasionally seen at the end or middle of a filament of the Nostocs, sometimes situated at intervals along their length; and near to these are sporangial cells, a little larger than the ordinary cells. From analogy, it is believed that the vesicular cells are antheridia, and that the sporangial cells contain germs which, after being fertilized by the spermatozoids, are set free and become resting spores. In some species, the sporangial cells are oblong, and contain vividly green matter; in others, the cells are elliptical and brown.
The species are widely distributed. Hormosiphon arcticus, a species consisting of a modification of cellulose, abounds to such a degree in the herbless polar regions, that it affords a welcome variety of food. Each plant lies on a small depression of the snow, which covers the soft and almost boggy slopes bordering the arctic seas, but it is carried by the winds in every direction, rolling over the snow and ice to a distance of several miles. Two northern species of Nostoc were found by Dr. Hooker in Kerguelen’s Land, growing on wet rocks near the sea; one of them was the common Nostoc commune. Other species occur in the warm springs in India, as well as in the arctic and antarctic regions, and an aquatic species is much used in China as a wholesome food. The genus Monormia forms floating masses of jelly on the surface of brackish water. The necklaces are of vast length, and, together with the jelly in which they are imbedded, wave with the slightest motion of the water. Floating masses grow on large ponds or lakes, which give the water a green tint.