long slender threads, sway slowly about as if in search of some solid object. In this stage they are soft and flexible, and contain little woody tissue. If a tendril succeeds in encountering a support, it coils around it. In response to the stimulus of contact, woody tissue develops in the tendril which rapidly increases in tensile strength. In many plants the tendrils which do not succeed in meeting with a solid object wither up and die. The Virginia creeper puts out tendrils whose tips, when coming in contact with an object, expand into flattened discs, or suckers. These tendrils, which are more apt to encounter a support on the FIG. 49-Leaves of the sensitive plant Mimosa. The leaf on the right has responded to a touch by drooping downward and folding together its leaflets. (From Verworn.) shaded side of the plant, are negatively phototropic. In a few plants the leaf stalks function as tendrils by actively coiling about twigs of the plant which serves as a support. It is not uncommon for plants to fold up or spread out their leaves in response to changes of light or temperature. The so-called sleep movements may be observed in the leaflets of clover and oxalis, which become folded together in the dark but expanded in the light. An extreme irritability is developed in the sensitive plant, Mimosa pudica. Even a light touch will cause the compound leaves of this plant to droop downward and the leaflets to be folded together. After this reaction has been evoked several times in close succession, it becomes less readily elicited, as if the plant had become fatigued by its efforts. Claude Bernard discovered that this power of response is temporarily lost if the plant is exposed to the action of ether. "What a singular thing,” remarks this writer, "plants can be anesthetized like animals, and absolutely the same phenomena can be observed in the two!" We are all familiar with the opening of flowers in the morning sun and the closure of their petals in the evening. You may readily observe this reaction in the heads of a common dandelion. Many flowers the tulip and crocus, for instance-open when it is warm, and close when it becomes cool. There are specialized responses in several of the insectivorous plants, e. g., the sundew, which enable them to entrap the insects that afford a part of their nutriment. The behavior of these curiously modified forms of plant life is described in an interesting work on Insectivorous Plants by Mr. Darwin, who has treated the subject with his customary thoroughness and sagacity. FIG. 50-Flowers of Leontodon hastilis, closed in the dark and expanded in the light. (After Detmer.) All of these activities of plants we have described are the consequences of the irritability of protoplasm. Although they have no nervous system, plants receive and transmit stimuli, and they execute appropriate movements in adjusting themselves to the forces of the outer world. Plants respond to gravity, heat, cold, sunshine and darkness, moisture and dryness, and contact with solid objects in ways which are as appropriate for the maintenance of life as our own responses to a dish of pudding or an automobile coming toward us on the street. If we were so constituted as not to be affected by the pudding (or other aliments) and failed to dodge the automobile, we should not be long for this world. Plants are equally dependent for their FIG. 51—Leaf of the sundew Drosera seen from above, showing numerous tentacles with expanded glandular tips. (From Darwin's Insectivorous Plants.) continuous existence upon their appropriate behavior. Their life, like that of every living being, is one of continual adjustment. REFERENCES CAMPBELL, D. H., A University Textbook of Botany. N. Y., Mac millan, 1907. COULTER, J. M., BARNES, C. R., AND COWLES, H. C., Textbook of DARWIN, C. R., Insectivorous Plants. N. Y., Appleton, 1899. GAGER, C. S., Fundamentals of Botany. Philadelphia, Blakiston, 1916. STRASBURGER, E., A Text-book of Botany. London, Macmillan, 1921. CHAPTER VIII THE STRUCTURE AND LIFE OF HIGHER ANIMALS Having endeavored in the preceding chapter to impart some elementary knowledge of higher green plants, let us now make a somewhat similar study of the life of animals. The fundamental life processes in plants and animals, from the simplest onecelled forms to an oak tree or a human being, have very much in common. One organism differs from another largely in the way in which it solves essentially the same problem of getting on in the world and perpetuating its kind. The ways in which the problem is solved are multitudinous; each kind of organism has its own peculiar solution. In some cases it is a relatively simple one; in others it is highly elaborate; but beneath the very conspicuous differences in the ways in which the problem is met by the diatom, the oak tree, the earthworm, and the man of affairs, there are fundamental likenesses in comparison with which most of the differences are, from the viewpoint of the general biologist, relatively superficial. In considering the elaboration of life processes along the various lines in which it has been carried out, it is well to bear in mind that every living thing is a vortex through which material continually flows. There are, therefore, certain functions, such as absorption, assimilation, respiration, destructive metabolism, and excretion, which are common to all life. Whether we are writing on the life of bacteria or composing a treatise on human physiology, we should be treating of these same functions. A higher animal differs from a very simple protozoön chiefly because it has a number of specialized organs, such as its stomach, heart, lungs, kidneys, muscles, and nervous system, in which to carry out functions which both organisms have in common. The higher animal, of course, does many more things than its hum bler relative can, but they reduce themselves to the same general classes. It will perhaps be advantageous to begin our study of the life of the multicellular animals, or Metazoa, with one of their simpler representatives, the fresh-water Hydra. This common inhabitant of ponds and streams has a cylindrical body about one-fourth of an inch in length, which is usually attached to some object at its lower end, or foot. The mouth, which is at the opposite end of the body, is surrounded by several tentacles, which are remarkably extensile and contractile. These tentacles are adhesive, and are armed with numerous nettling, or poison, cells which poison the small animals upon which the Hydra feeds. Once a hapless water flea or mosquito larva is caught by the tentacles, it is drawn to the mouth and swallowed. The body of Hydra is essentially an elongated sack consisting of two layers of cells, separated by a thin non-cellular lamella, the mesogloa. The outer layer is known as the ectoderm, and the inner as the entoderm. The former is composed of epithelial cells, whose shape varies considerably in different parts of the body. Some of them are drawn out at the base into a fiber, which is closely applied to the mesogloea. The fibrous part of the cell is specialized for the function of contraction, the remaining part being much like an ordinary cell of epithelium. These cells are, therefore, called epithelio-muscular cells, and they represent a sort of intermediate stage between an epithelium cell and a muscular fiber. Many of the ectoderm cells, especially on the tentacles, possess peculiar stinging organs, or nematocysts. The latter are elliptical bodies, which contain, coiled up within them, a long thread, which is capable of being discharged with great force. This thread is formed by an inpushing of the outer end of the capsule. When it is discharged it is actually everted, or turned inside out, and in penetrating the tissues of an organism, it introduces some of its contained poison. Since Hydra is, in most respects, a very simple metazoan, it is curious to find that it is armed with cell organs of so highly organized and specialized a kind. Nematocysts are found |