In the second sub-kingdom of plants is embraced an enormous assemblage of plants, differing widely in size and complexity, and yet showing a sufficiently complete gradation from the lowest to the highest as to make it impracticable to make more than one sub-kingdom to include them. They are nearly all aquatic forms, although many of them will survive long periods of drying, such forms occurring on moist earth, rocks, or the trunks of trees, but only growing when there is a plentiful supply of water.

All of them possess chlorophyll, which, however, in many forms, is hidden by the presence of a brown or red pigment. They are ordinarily divided into three classes—I.The Green AlgÆ (ChlorophyceÆ); II.Brown AlgÆ (PhÆophyceÆ); III.Red AlgÆ (RhodophyceÆ).

Class I.—Green AlgÆ.

The green algÆ are to be found almost everywhere where there is moisture, but are especially abundant in sluggish or stagnant fresh water, being much less common in salt water. They are for the most part plants of simple structure, many being unicellular, and very few of them plants of large size.

We may recognize five well-marked orders of the green algÆ—I.Green slimes (ProtococcaceÆ); II.ConfervaceÆ; III.Pond scums (ConjugatÆ); IV.SiphoneÆ; V.Stone-worts (CharaceÆ).

Order I.—ProtococcaceÆ.

The members of this order are minute unicellular plants, growing either in water or on the damp surfaces of stones, tree trunks, etc. The plants sometimes grow isolated, but usually the cells are united more or less regularly into colonies.

A common representative of the order is the common green slime, Protococcus (Fig.11, A, C), which forms a dark green slimy coating over stones, tree trunks, flower pots, etc. Owing to their minute size the structure can only be made out with the microscope.

Scraping off a little of the material mentioned into a drop of water upon a slide, and carefully separating it with needles, a cover glass may be placed over the preparation, and it is ready for examination. When magnified, the green film is found to be composed of minute globular cells of varying size, which may in places be found to be united into groups. With a higher power, each cell (Fig.11, A) is seen to have a distinct cell wall, within which is colorless protoplasm. Careful examination shows that the chlorophyll is confined to several roundish bodies that are not usually in immediate contact with the wall of the cell. These green masses are called chlorophyll bodies (chloroplasts). Toward the centre of the cell, especially if it has first been treated with iodine, the nucleus may be found. The size of the cells, as well as the number of chloroplasts, varies a good deal.

With a little hunting, specimens in various stages of division may be found. The division takes place in two ways. In the first (Fig.11, B), known as fission, a wall is formed across the cell, dividing it into two cells, which may separate immediately or may remain united until they have undergone further division. In this case the original cell wall remains as part of the wall of the daughter cells. Fission is the commonest form of cell multiplication throughout the vegetable kingdom.

The second form of cell division or internal cell division is shown at C. Here the protoplasm and nucleus repeatedly divide until a number of small cells are formed within the old one. These develop cell walls, and escape by the breaking of the old cell wall, which is left behind, and takes no part in the process. The cells thus formed are sometimes provided with two cilia, and are capable of active movement.

Internal cell division, as we shall see, is found in most plants, but only at special times.

Closely resembling Protococcus, and answering quite as well for study, are numerous aquatic forms, such as Chlorococcum (Fig.12). These are for the most part destitute of a firm cell wall, but are imbedded in masses of gelatinous substance like many CyanophyceÆ. The chloroplasts are smaller and less distinct than in Protococcus. The cells are here oval rather than round, and often show a clear space at one end.

Owing to the absence of a definite membrane, a distinction between fission and internal cell division can scarcely be made here. Often the cells escape from the gelatinous envelope, and swim actively by means of two cilia at the colorless end (Fig.12, B). In this stage they closely resemble the individuals of a Volvox colony, or other green Flagellata, to which there is little doubt that they are related.

There are a number of curious forms common in fresh water that are probably related to Protococcus, but differ in having the cells united in colonies of definite form. Among the most striking are the different species of Pediastrum (Fig.11, D, E), often met with in company with other algÆ, and growing readily in aquaria when once established. They are of very elegant shapes, and the number of cells some multiple of four, usually sixteen.

The cells form a flat disc, the outer ones being generally provided with a pair of spines.

New individuals arise by internal division of the cells, the contents of each forming as many parts as there are cells in the whole colony. The young cells now escape through a cleft in the wall of the mother cell, but are still surrounded by a delicate membrane (Fig.11, E). Within this membrane the young cells arrange themselves in the form of the original colony, and grow together, forming a new colony.

A much larger but rarer form is the water net (Fig.11, G), in which the colony has the form of a hollow net, the spaces being surrounded by long cylindrical cells placed end to end. Other common forms belong to the genus Scenedesmus (Fig.11, F), of which there are many species.

Order II.—ConfervaceÆ.

Under this head are included a number of forms of which the simplest ones approach closely, especially in their younger stages, the ProtococcaceÆ. Indeed, some of the so-called ProtococcaceÆ are known to be only the early stages of these plants.

A common member of this order is Cladophora, a coarse-branching alga, growing commonly in running water, where it forms tufts, sometimes a metre or more in length. By floating out a little of it in a saucer, it is easy to see that it is made up of branching filaments.

The microscope shows (Fig.13, A) that these filaments are rows of cylindrical cells with thick walls showing evident stratification. At intervals branches are given off, which may in turn branch, giving rise to a complicated branching system. These branches begin as little protuberances of the cell wall at the top of the cell. They increase rapidly in length, and becoming slightly contracted at the base, a wall is formed across at this point, shutting it off from the mother cell.

The protoplasm lines the wall of the cell, and extends in the form of thin plates across the cavity of the cell, dividing it up into a number of irregular chambers. Imbedded in the protoplasm are numerous flattened chloroplasts, which are so close together as to make the protoplasm appear almost uniformly green. Within the chloroplasts are globular, glistening bodies, called “pyrenoids.” The cell has several nuclei, but they are scarcely evident in the living cell. By placing the cells for a few hours in a one percent watery solution of chromic acid, then washing thoroughly and staining with borax carmine, the nuclei will be made very evident (Fig.13, B). Such preparations may be kept permanently in dilute glycerine.

If a mass of actively growing filaments is examined, some of the cells will probably be found in process of fission. The process is very simple, and may be easily followed (Fig.13, C). A ridge of cellulose is formed around the cell wall, projecting inward, and pushing in the protoplasm as it grows. The process is continued until the ring closes in the middle, cutting the protoplasmic body completely in two, and forms a firm membrane across the middle of the cell. The protoplasm at this stage (C iii.) is somewhat contracted, but soon becomes closely applied to the new wall. The whole process lasts, at ordinary temperatures (20°-25°C.), from three to four hours.

At certain times, but unfortunately not often to be met with, the contents of some of the cells form, by internal division, a large number of small, naked cells (zoÖspores) (Fig.13, D), which escape and swim about actively for a time, and afterwards become invested with a cell wall, and grow into a new filament. These cells are called zoÖspores, from their animal-like movements. They are provided with two cilia, closely resembling the motile cells of the ProtococcaceÆ and VolvocineÆ.

There are very many examples of these simple ConfervaceÆ, some like Conferva being simple rows of cells, others like Stigeoclonium (Fig.14, A), ChÆtophora and Draparnaldia (Fig.14, B, C), very much branched. The two latter forms are surrounded by masses of transparent jelly, which sometimes reach a length of several centimetres.

Among the marine forms related to these may be mentioned the sea lettuce (Ulva), shown in Figure15. The thin, bright-green, leaf-like fronds of this plant are familiar to every seaside student.

Somewhat higher than Cladophora and its allies, especially in the differentiation of the reproductive parts, are the various species of Œdogonium and its relatives. There are numerous species of Œdogonium not uncommon in stagnant water growing in company with other algÆ, but seldom forming masses by themselves of sufficient size to be recognizable to the naked eye.

The plant is in structure much like Cladophora, except that it is unbranched, and the cells have but a single nucleus (Fig.16, E). Even when not fruiting the filaments may usually be recognized by peculiar cap-shaped structures at the top of some of the cells. These arise as the result of certain peculiarities in the process of cell division, which are too complicated to be explained here.

There are two forms of reproduction, non-sexual and sexual. In the first the contents of certain cells escape in the form of large zoÖspores (Fig.16, C), of oval form, having the smaller end colorless and surrounded by a crown of cilia. After a short period of active motion, the zoÖspore comes to rest, secretes a cell wall about itself, and the transparent end becomes flattened out into a disc (E, d), by which it fastens itself to some object in the water. The upper part now rapidly elongates, and dividing repeatedly by cross walls, develops into a filament like the original one. In many species special zoÖspores are formed, smaller than the ordinary ones, that attach themselves to the filaments bearing the female reproductive organ (oÖgonium), and grow into small plants bearing the male organ (antheridium), (Fig.16, B).

The sexual reproduction takes place as follows: Certain cells of a filament become distinguished by their denser contents and by an increase in size, becoming oval or nearly globular in form (Fig.16, A, B). When fully grown, the contents contract and form a naked cell, which sometimes shows a clear area at one point on the surface. This globular mass of protoplasm is the egg cell, or female cell, and the cell containing it is called the “oÖgonium.” When the egg cell is ripe, the oÖgonium opens by means of a little pore at one side (Fig.16, A).

In other cells, either of the same filament or else of the small male plants already mentioned, small motile cells, called spermatozoids, are formed. These are much smaller than the egg cell, and resemble the zoÖspores in form, but are much smaller, and without chlorophyll. When ripe they are discharged from the cells in which they were formed, and enter the oÖgonium. By careful observation the student may possibly be able to follow the spermatozoid into the oÖgonium, where it enters the egg cell at the clear spot on its surface. As a result of the entrance of the spermatozoid (fertilization), the egg cell becomes surrounded by a thick brown wall, and becomes a resting spore. The spore loses its green color, and the wall becomes dark colored and differentiated into several layers, the outer one often provided with spines (Fig.16, F). As these spores do not germinate for a long time, the process is only known in a comparatively small number of species, and can hardly be followed by the ordinary student.

Much like Œdogonium, but differing in being branched, is the genus BulbochÆte, characterized also by hairs swollen at the base, and prolonged into a delicate filament (Fig.16, G).

The highest members of the ConfervaceÆ are those of the genus ColeochÆte (Fig.17), of which there are several species found in the United States. These show some striking resemblances to the red seaweeds, and possibly form a transition from the green algÆ to the red. The commonest species form bright-green discs, adhering firmly to the stems and floating leaves of water lilies and other aquatics. In aquaria they sometimes attach themselves in large numbers to the glass sides of the vessel.

Growing from the upper surface are numerous hairs, consisting of a short, sheath-like base, including a very long and delicate filament (Fig.17, B). In their methods of reproduction they resemble Œdogonium, but the reproductive organs are more specialized.