full of Terrible Arrows, how easily can he shoot the deleterious Miasms into those Juices or Bowels of Men's Bodies, which shall soon Enflame them with a Mortal Fire!"

From this point of view, efforts at sanitation would be in vain, for the "divel" would probably pay scant respect to such things as antiseptics, quarantines, and the purity of water supplies. Nowadays, through knowing the cause of epidemics which, like the cholera and the plague, formerly swept away thousands in their course, we are able to check these diseases before they have fairly started. The average duration of human life is about fifteen years greater than it was half a century ago. Some diseases, such as yellow fever, have been almost completely exterminated. Surgery has been robbed of the worst of its dangers. Infant mortality has been more than cut in half. Cures or preventives have been discovered for some of the most deadly diseases, and the spread of most infectious diseases from person to person has been greatly checked. A large part of this great achievement of science has been the direct outgrowth of our knowledge of the world of microscopic life.

REFERENCES

DUCLAUX, E., Pasteur: The History of a Mind. Philadelphia, Saunders, 1920.

GARRISON, F. H., An Introduction to the History of Medicine (3rd ed.).

Philadelphia, Saunders, 1921.

HOLMES, S. J., Louis Pasteur. N. Y., Harcourt, Brace, 1924. MACCALLUM, W. G., Pathology. Philadelphia, Saunders, 1922. VALLERY-RADOT, R., The Life of Pasteur. Garden City, N. Y., Doubleday, Page, 1923.

WHITE, A. D., A History of the Warfare of Science with Theology. 2 vols. N. Y. Appleton, 1895.

ZINSSER, H., A Textbook of Bacteriology (5th ed.). N. Y., Appleton, 1922.

CHAPTER VII

THE GREEN PLANT

Out of the group of one-celled organisms that we have been considering have arisen two great diverging branches of the tree of life which have culminated in the higher green plants on the one hand and the higher animals on the other. Why life as it developed should have followed just two main paths, instead of many, or only one, is not apparent. Possibly life may have branched out quite differently in other worlds. We do not know. It is evident that without the green plants animal life could not have made a great deal of headway on our planet; but while plants receive benefits in several ways from the presence of animals, they are by no means as dependent on animals as the latter are dependent upon plants. The green plant is the laboratory in which is manufactured food that supports practically all the higher terrestrial forms of animal life. The student of general biology, therefore, cannot well afford to omit the green plant from his program of study.

Since an understanding of the life of a plant depends upon a knowledge of the way in which it is organized, we shall consider first some of the general features of plant structure. A typical green plant consists of three main subdivisions: root, stem, and leaves. The root fixes the plant in the soil and absorbs the water and salts needed for its life; the stem supports the leaves and flowers, and serves as a channel for the transportation of the substances required for growth. The leaves function as organs of absorption and transpiration, and of the synthesis of food materials under the influence of light. In adaptation to their peculiar functions, leaves commonly consist of a thin expanded part, or blade, which presents a relatively large surface exposed to light and air, and a petiole, or stalk of attachment, although

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the stalk is not infrequently absent, as in sessile leaves, whose blades are joined directly to the stem. If we make a cross section through the blade of a typical leaf, we shall find that it includes only a few layers of cells. There is an outside epidermis, consisting generally of a single layer of thin cells usually devoid of

FIG. 38-Cross section of the leaf of Fagus showing different kinds of cells: ep, epidermis; k, crystals included in some of the cells; pl, palisade cells; s, sp, cells of the loose parenchyma with spaces between them; st, stoma, or opening through the lower epidermis. (After Strasburger.)

A

B

chlorophyll. In the epidermis of the lower side of the leaf there are, at intervals, peculiar pairs of cells called the guard cells, more or less semicircular in outline, surrounding an opening, or stoma. Through alterations in form, brought about by changes in turgor, the guard cells may open or close the stomata between them and thus regulate the freedom of communication between the interior of the leaf and the outside air. The epidermis is often furnished with hair cells, which are sometimes long and fleecy, as in the leaves of the mullein, and sometimes short and stellate, or occasionally sharp and stiff, as in the nettle. Hair cells very commonly protect the surface of the leaf from too rapid evaporation.

FIG. 39-A, stoma cells from the lower epidermis of a leaf with an opening (stoma) between them. B, starch grains showing concentric layers.

Just below the upper epidermis the cells (palisade cells) are usually elongated and placed closely side by side at right angles to the surface; below this layer the cells are more loosely arranged with air spaces between them. The thin-walled cells making up the body of the leaf belong to the tissue called parenchyma, as contrasted with the woody fibers and vascular ducts. This rather loose parenchyma is supported by a framework of stronger fibers constituting the so-called veins. In examining a leaf of an oak, apple, blackberry, or Begonia—a very large number of leaves show essentially the same arrangementseveral larger veins may be seen diverging from the central axis, or midrib, and giving off branches which divide and redivide, forming a network of supporting tissue. These veins are composed of so-called fibrovascular bundles, and they act not merely as a supporting skeleton for the looser tissues of the leaf, but as conducting channels for the transfer of sap.

The fibrovascular bundle is a sort of unit of composition, out of which a large part of the permanent tissues of the plant is constituted. Each such bundle contains one or more ducts which consist of elongated cells placed end to end. When the duct is fully formed, the cells are dead, and the cross partitions, where the cells meet, have disappeared, thus affording a continuous passage for the transfer of fluid. The walls of these ducts are often marked with pits, rings, or a spiral thickening of cellulose. Alongside the ducts are several elongated tapering cells, with relatively thick walls. These cells (which give the toughness and hardness of woody tissue) together with the ducts, constitute the xylem, or woody part of the bundle. Another part, the phloëm, or bast, lying on the outer side of the bundle, consists mostly of elongated cells (bast tissue) among which are the so-called sievetube cells. The latter are arranged end to end, the contiguous ends being perforated by several apertures, which permit the passage of fluids from one cell to the next. Between the xylem and phloëm, is a layer of younger, relatively undifferentiated cells, the cambium, which forms a growing layer, from which cells are added to both of the other groups.

The stems of the higher, or flowering, plants fall into two classes as regards the arrangements of the fibrovascular bundles. In the endogenous stems, exemplified by such plants as the palm and Indian corn, the woody bundles are scattered through the parenchyma, or pithy part of the stem. In the exogenous stems, which occur in most trees, and in the majority of flowering plants, the pith forms a

FIG. 41-Section of an exogenous stem of a yearold maple under different degrees of magnification: a, central pith; b, spiral ducts; d, pitted ducts; e, annular, or ringed ducts; f, inner bark; g, green bark; h, layer of cork; i, epidermis. The f on the upper side of the figure represents a medullary ray. (After Gray.)

FIG. 40-A section

of a stalk of Indian corn showing fibrovascular bundles in the pith. A typical endogenous stem. (After Gray.)

central core surrounded by the wood. The latter is formed from the inner, or xylem, portion of a large number of fibrovascular bundles, closely packed together. The bark is formed from the outer part of such bundles, the cambium of adjacent bundles constituting a continuous layer around the stem. As the cambium is the layer of actively dividing and

growing cells, it adds new layers to the outside of the wood and to the inside of the bark. In a cross section of a tree there may be seen the concentric rings of woody deposit marking the stages