Common objects of the microscope

by J. G. Wood

CHAPTER VI

Algæ and their Growth—Desmidiaceæ, where found—Diatoms, their Flinty Deposit—Volvox—Mould, Blight, and Mildew—Mosses and Ferns—Mare’s-Tail and the Spores—Common Sea-weeds and their Growth.

On Plate [IV]. will be seen many examples of the curious vegetables called respectively algæ and fungi, which exhibit some of the lowest forms of vegetable life, and are remarkable for their almost universal presence in all parts of this globe, and also almost all conditions of cold, heat, or climate. Many of them are well known under the popular name of sea-weeds, others are equally familiar under the titles of “mould,” “blight,” or “mildew,” while many of the minuter kinds exhibit such capability of motion, and such apparent symptoms of volition, that they have long been described as microscopic animalcules, and thought to belong to the animal rather than to the vegetable kingdoms.

Fig. [1] represents one of the very lowest forms of vegetable life, being known to the man of science as the Palmella, and to the general public as “gory dew.” It may be seen on almost any damp wall, extending in red patches of various sizes, looking just as if some blood had been dashed on the wall, and allowed to dry there. With a tolerably powerful lens this substance can be resolved into the exceedingly minute cells depicted in the figure. Generally, these cells are single, but in many instances they are double, owing to the process of subdivision by which the plant grows, if such a term may be used.

Fig. [2] affords an example of another very low form of vegetable, the Palmoglæa, that green slimy substance which is so common on damp stones. When placed under the microscope, this plant is resolvable into a multitude of green cells, each being surrounded with a kind of gelatinous substance. The mode of growth of this plant is very simple. A line appears across one of the cells, and after a while it assumes a kind of hour-glass aspect, as if a string had been tied tightly round its middle. By degrees the cell fairly divides into two parts, and then each part becomes surrounded with its own layer of gelatine, so as to form two separate cells, placed end to end.

One of the figures, that on the right hand, represents the various processes of “conjugation,” i.e. the union and fusion together of two cells. Each cell throws out a little projection; these meet together, and then uniting, form a sort of isthmus connecting the two main bodies. This rapidly widens, until the two cells become fused into one large body. The whole subject of conjugation is very interesting, and is treated at great length in the Micrographic Dictionary of Messrs. Griffith and Henfrey, a work to which the reader is referred for further information on many of the subjects that, in this small work, can receive but a very hasty treatment.

Few persons would suppose that the slug-like object on Fig. [3], the little rounded globules with a pair of hair-like appendages, and the round disc with a dark centre, are only different forms of the same organism. Such, however, is the case, and these are three of the modifications which the Protococcus undergoes. This vegetable may be seen floating like green froth on the surface of rain-water.

On collecting some of this froth and putting it under the microscope, it is seen to consist of a vast number of little green bodies, moving briskly about in all directions, and guiding their course with such apparent exercise of volition that they might very readily be taken for animals. It may be noticed that the colour of the plant is sometimes red, and in that state it has been called the Hæmatococcus.

The “still” state of this plant is shown in the round disc. After a while the interior substance splits into two portions; these again subdivide, and the process is repeated until sixteen or thirty-two cells become developed out of the single parent-cell. These little ones then escape, and, being furnished with two long “cilia” or thread-like appendages, whirl themselves merrily through the water. When they have spent some time in this state, growing all the while, they lose their cilia, become clothed with a strong envelope, and pass into the still stage from which they had previously emerged. This curious process is repeated in endless succession, and causes a very rapid growth of the plant. The moving bodies are technically called zoospores, or living spores, and are found in many other plants besides those of the lowest order.