microscope each of these threads is seen to be a series of cells joined end to end. All the cells are practically the same in shape and structure, however, so that a study of one will make clear the structure of all. Inclosing each cell there is a thin cell wall. The first structures one is likely to notice within the cell are the chlorophyll bodies. In the pond scum known as Spirogyra the chlorophyll is arranged in spiral bands, and it is this which has given the plant its name (Fig. 85, B). In other forms the chlorophyll is differently arranged, A, two conjugating threads of Spirogyra; B, single cell of Spirogyra. sometimes in star-shaped masses, one in each half of the cell, and sometimes diffused throughout the cell. If a little iodine is added to the specimen when it is being examined under the microscope, a nucleus may be distinguished near the center of each cell (Fig. 85, B). In the cell-body and nucleus the protoplasm appears as a clear and almost transparent mass. The thread or filament continues to increase in length by the growth and division of certain individual cells that compose it. At the close of the season most of the filaments perish, but some of them undergo peculiar changes. The bands of chlorophyll lose their definiteness, the protoplasm becomes massed, tiny outgrowths from the sides of the cells occur, and these continue to extend till they meet similar outgrowths from a neighboring filament (Fig. 85, A). These outgrowths unite, and thus a tube from one cell to the other is formed. The contents of one cell pass through to another, and the two masses fuse. A thick wall forms about the united mass and the old cell walls decay and fall away, leaving these thick-walled zygospores on the bottom of the pond. In the spring the protoplasm within each of these zygospores begins to grow, breaks through the thick wall, and proceeds to form a new filament by cell division. The formation of the zygospores is known as conjugation; it is a kind of sexual reproduction, though the two cells taking part in the process are the same in appearance. If one observes pond scum on a sunny day, bubbles will be seen escaping from the mass. A test of this gas proves it to be oxygen, and as we should expect, it occurs in connection with the process of carbohydrate manufacture the same as in other green plants. In fact it has been proved that these simple plants manufacture foods, digest, assimilate, respire, and reproduce as do the higher plants we have studied. The differences, then, between a simple plant like Spirogyra and a bean plant or an oak tree are mainly those of structure and adaptations for the performance of functions which are largely common to both. Indeed, it is evident that every cell of the Spirogyra is in contact with the water, from which all the substances needed are obtained by absorption. Hence, any special adaptations for securing food materials or of giving off wastes, such as are found in higher plants, are unnecessary. 187. Suggestions for the study of Spirogyra. No. 67. It is desirable that pupils should see the "pond scum" in its habitat, even if they do not collect material for work. The escape of bubbles may be noticed at this time or in the laboratory. The mass should be described as to color and "feel," and the fine threads noted by floating the mass in a saucer of water. A filament should then be studied under the microscope, and the parts of a single cell described, and several cells should be drawn. If fresh zygospore material can be obtained, this should also be studied, and the parts described above noted and drawn; otherwise charts or pictures may be used. 188. Pleurococcus and other algæ. - Another and still simpler form of plant life is known as Pleurococcus. It may be readily obtained from the trunks on the north side of FIG. 86. Pleurococcus. - (Sedgwick and Wil large trees. It appears as a very thin green layer closely adhering to the bark. If a little of this material is scraped off and placed under the compound microscope, it will be found that it is made up of a large number of tiny circular green cells which adhere to each other more or less, since in the process of reproduction one cell divides to form two, each of which is considered to be an individual plant. Thus the whole mass is made up of a large number of one-celled plants. son.) The Spirogyra and Pleurococcus are only two of a large number of simple plants known as algæ. They differ widely in form, but none of them develop roots, stems, or leaves. Among the most common algæ are the marine forms known as sea weeds, of which there are many kinds. 189. Suggestions for the study of Pleurococcus. No. 68. As indicated above, material for the study of Pleurococcus may be easily obtained by removing pieces of bark from trees having a considerable quantity of this plant on their surface. If collected in a dry season, the bark should be placed under a bell-jar with sufficient water to make the air moist, and allowed to stand for several days. The place in which the Pleurococcus is found should be described, and also the appearance of a mass of the plants. Single cells should then be studied under the high power of the compound microscope, and the cell and its contents described and drawn. D. Fungi. (See Chapter IX, 147–166.) III. SUMMARY OF A CLASSIFICATION OF THE PLANT KINGDOM Division I-Spore-producing plants. Sub-division 1 - Fungi (including bacteria, yeast, molds, mushrooms, rusts, smuts). Sub-division 2 Algae (including Spirogyra, Pleurococcus, and sea weeds). Sub-division 3 - Mosses and their relatives. Division II- Seed-producing plants. Sub-division 1-Gymnosperms (including pines, spruces, hem locks). Sub-division 2-Angiosperms, composed of: Class I-Monocotyledons (e.g. corn, lilies, gladiolus). Class II Dicotyledons one of which is composed of 160 or more families, Rose family-composed of 14 genera, one of which is the |