Read Ebook: A Civic Biology Presented in Problems by Hunter George W George William

Font size: 10 px 15 px 20 px 25 px 30 px 35 px

Background color: BlackWhiteGrayLight GrayDark GrayDim Gray

Text color: BlackWhiteGrayLight GrayDark GrayDim Gray

Apply/Save settings

Ebook has 642 lines and 106533 words, and 13 pages

ion the water needed to support life in the western desert lands.

Light as Condition of the Environment.--Light is another important factor of the environment. A study of the leaves on any green plant growing near a window will convince one that such plants grow toward the light. All green plants are thus influenced by the sun. Other plants which are not green seem either indifferent or are negatively influenced the source of light. Animals may or may not be attracted by light. A moth, for example, will fly toward a flame, an earthworm will move away from light. Some animals prefer a moderate or weak intensity of light and live in shady forests or jungles, prowling about at night. Others seem to need much and strong light. And man himself enjoys only moderate intensity of light and heat. Look at the shady side of a city street on any hot day to prove this statement.

Adaptations.--Plants and animals are not only fitted to live under certain conditions, but each part of the body may be fitted to do certain work. I notice that as I write these words the fingers of my right hand grasp the pen firmly and the hand and arm execute some very complicated movements. This they are able to do because of the free movement given through the arrangement of the delicate bones of the wrist and fingers, their attachment to the bones of the arm, a wonderful complex of muscles which move the bones, and a directing nervous system which plans the work. Because of the peculiar fitness in the structure of the hand for this work we say it is adapted to its function of grasping objects. Each part of a plant or animal is usually fitted for some particular work. The root of a green plant, for example, is fitted to take in water by having tiny absorbing organs growing from it, the stems have pipes or tubes to convey liquids up and down and are strong enough to support the leafy part of the plant. Each part of a plant does work, and is fitted, by means of certain structures, to do that work. It is because of these adaptations that living things are able to do their work within their particular environment.

Plants and Animals and their Natural Environment.--Those of us who have tried to keep potted plants in the schoolroom know how difficult it is to keep them healthy. Dust, foreign gases in the air, lack of moisture, and other causes make the artificial environment in which they are placed unsuitable for them.

A goldfish placed in a small glass jar with no food or no green water plants soon seeks the surface of the water, and if the water is not changed frequently so as to supply air the fish will die. Again the artificial environment lacks something that the fish needs. Each plant and animal is limited to a certain environment because of certain individual needs which make the surroundings fit for it to live in.

Changes in Environment.--Most plants and animals do not change their environment. Trees, green plants of all kinds, and some animals remain fixed in one spot practically all their lives. Certain tiny plants and most animals move from place to place, either in air, water, on the earth or in the earth, but they maintain relatively the same conditions in environment. Birds are perhaps the most striking exception, for some may fly thousands of miles from their summer homes to winter in the south. Other animals, too, migrate from place to place, but not usually where there are great changes in the surroundings. A high mountain chain with intense cold at the upper altitudes would be a barrier over which, for example, a bear, a deer, or a snail could not travel. Fish like trout will migrate up a stream until they come to a fall too high for them to jump. There they must stop because their environment limits them.

Man in his Environment.--Man, while he is like other animals in requiring heat, light, water, and food, differs from them in that he has come to live in a more or less artificial environment. Men who lived on the earth thousands of year ago did not wear clothes or have elaborate homes of wood or brick or stone. They did not use fire, nor did they eat cooked foods. In short, by slow degrees, civilized man has come to live in a changed environment from that of other animals. The living together of men in communities has caused certain needs to develop. Many things can be supplied in common, as water, milk, foods. Wastes of all kinds have to be disposed of in a town or city. Houses have come to be placed close together, or piled on top of each other, as in the modern apartment. Fields and trees, all outdoor life, has practically disappeared. Man has come to live in an artificial environment.

REFERENCE BOOKS

ELEMENTARY

ADVANCED

LABORATORY SUGGESTIONS

On any bright warm day in the fall we will find insects swarming everywhere in any vacant lot or the less cultivated parts of a city park. Grasshoppers, butterflies alighting now and then on the flowers, brightly marked hornets, bees busily working over the purple asters or golden rod, and many other forms hidden away on the leaves or stems of plants may be seen. If we were to select for observation some partially decayed tree, we would find it also inhabited. Beetles would be found boring through its bark and wood, while caterpillars are feeding on its leaves or building homes in its branches. Everywhere above, on, and under ground may be noticed small forms of life, many of them insects. Let us first see how we would go to work to identify some of the common forms we would be likely to find on plants. Then a little later we will find out what they are doing on these plants.

Try to discover members of the six different orders named above. Collect specimens and bring them to the laboratory for identification.

The Life History of the Milkweed Butterfly.--If it is possible to find on our trip some growing milkweed, we are quite likely to find hovering near, a golden brown and black butterfly, the monarch or milkweed butterfly . Its body, as in all insects, is composed of three regions. The monarch frequents the milkweed in order to lay eggs there. This she may be found doing at almost any time from June until September.

The Adult.--After a week or more of inactivity in the pupa state, the outer skin is split along the back, and the adult butterfly emerges. At first the wings are soft and much smaller than in the adult. Within fifteen minutes to half an hour after the butterfly emerges, however, the wings are full-sized, having been pumped full of blood and air, and the little insect is ready after her wedding flight to follow her instinct to deposit her eggs on a milkweed plant.

Plants furnish Insects with Food.--Food is the most important factor of any animal's environment. The insects which we have seen on our field trip feed on the green plants among which they live. Each insect has its own particular favorite food plant or plants, and in many cases the eggs of the insect are laid on the food plant so that the young may have food close at hand. Some insects prefer the rotted wood of trees. An American zo?logist, Packard, has estimated that over 450 kinds of insects live upon oak trees alone. Everywhere animals are engaged in taking their nourishment from plants, and millions of dollars of damage is done every year to gardens, fruits, and cereal crops by insects.

Homes and Shelter.--After a field trip no one can escape the knowledge that plants often give animals a home. The grass shelters millions of grasshoppers and countless hordes of other small insects which can be obtained by sweeping through the grass with an insect net. Some insects build their homes in the trees or bushes on which they feed, while others tunnel through the wood, making homes there. Spiders build webs on plants, often using the leaves for shelter. Birds nest in trees, and many other wild animals use the forest as their home. Man has come to use all kinds of plant products to aid him in making his home, wood and various fibers being the most important of these.

Insects and Flowers.--Certain other problems can be worked out in the fall of the year. One of these is the biological interrelations between insects and flowers. It is easy on a field trip to find insects lighting upon flowers. They evidently have a reason for doing this. To find out why they go there and what they do when there, it will be first necessary for us to study flowers with the idea of finding out what the insects get from them, and what the flowers get from the insects.

The Use and Structure of a Flower.--It is a matter of common knowledge that flowers form fruits and that fruits contain seeds. They are, then, very important parts of certain plants. Our field trip shows us that flowers are of various shapes, colors, and sizes. It will now be our problem first to learn to know the parts of a flower, and then find out how they are fitted to attract and receive insect visitors.

Field Work.--Is Color or Odor in a Flower an Attraction to an Insect?--Sir John Lubbock tried an experiment which it would pay a number of careful pupils to repeat. He placed a few drops of honey on glass slips and placed them over papers of various colors. In this way he found that the honeybee, for example, could evidently distinguish different colors. Bees seemed to prefer blue to any other color. Flowers of a yellow or flesh color were preferred by flies. It would be of considerable interest for some student to work out this problem with our native bees and with other insects by using paper flowers and honey or sirup. Test the keenness of sight in insects by placing a white object in the grass and see how many insects will alight on it. Try to work out some method by which you can decide whether a given insect is attracted to a flower by odor alone.

Mouth Parts of the Bee.--The mouth of the bee is adapted to take in the foods we have mentioned, and is used for the purposes for which man would use the hands and fingers. The honeybee laps or sucks nectar from flowers, it chews the pollen, and it uses part of the mouth as a trowel in making the honeycomb. The uses of the mouth parts may be made out by watching a bee on a well-opened flower.

Suggestions for Field Work.--In any locality where flowers are abundant, try to answer the following questions: How many bees visit the locality in ten minutes? How many other insects alight on the flowers? Do bees visit flowers of the same kinds in succession, or fly from one flower on a given plant to another on a plant of a different kind? If the bee lights on a flower cluster, does it visit more than one flower in the same cluster? How does a bee alight? Exactly what does the bee do when it alights?

Butter and Eggs .--From July to October this very abundant weed may be found especially along roadsides and in sunny fields. The flower cluster forms a tall and conspicuous cluster of orange and yellow flowers.

The corolla projects into a spur on the lower side; an upper two-parted lip shuts down upon a lower three-parted lip. The four stamens are in pairs, two long and two short.

Other Examples of Mutual Aid between Flowers and Insects.--Many other examples of adaptations to secure cross-pollination by means of the visits of insects might be given. The mountain laurel, which makes our hillsides so beautiful in late spring, shows a remarkable adaptation in having the anthers of the stamens caught in little pockets of the corolla. The weight of the visiting insect on the corolla releases the anther from the pocket in which it rests so that it springs up, dusting the body of the visitor with pollen.

In some flowers, as shown by the primroses or primula of our hothouses, the stamens and pistils are of different lengths in different flowers. Short styles and long or high-placed filaments are found in one flower, and long styles with short or low-placed filaments in the other. Pollination will be effected only when some of the pollen from a low-placed anther reaches the stigma of a short-styled flower, or when the pollen from a high anther is placed upon a long-styled pistil. There are, as in the case of the loosestrife, flowers having pistils and stamens of three lengths. Pollen only grows on pistils of the same length as the stamens from which it came.

The milkweed or butterfly weed already mentioned is another example of a flower adapted to insect pollination.

Other Flower Visitors.--Other insects besides those already mentioned are pollen carriers for flowers. Among the most useful are moths and butterflies. Projecting from each side of the head of a butterfly is a fluffy structure, the palp. This collects and carries a large amount of pollen, which is deposited upon the stigmas of other flowers when the butterfly pushes its head down into the flower tube after nectar. The scales and hairs on the wings, legs, and body also carry pollen.

Flies and some other insects are agents in cross-pollination. Humming birds are also active agents in some flowers. Snails are said in rare instances to carry pollen. Man and the domesticated animals undoubtedly frequently pollinate flowers by brushing past them through the fields.

Pollination by the Wind.--Not all flowers are dependent upon insects or other animals for cross-pollination. Many of the earliest of spring flowers appear almost before the insects do. Such flowers are dependent upon the wind for carrying pollen from the stamens of one flower to the pistil of another. Most of our common trees, oak, poplar, maple, and others, are cross-pollinated almost entirely by the wind.

Flowers pollinated by the wind are generally inconspicuous and often lack a corolla. The anthers are exposed to the wind and provided with much pollen, while the surface of the stigma may be long and feathery. Such flowers may also lack odor, nectar, and bright color. Can you tell why?

Imperfect Flowers.--Some flowers, the wind-pollinated ones in particular, are imperfect; that is, they lack either stamens or pistils. Again, in some cases, imperfect flowers having stamens only are alone found on one plant, while those flowers having pistils only are found on another plant of the same kind. In such flowers, cross-pollination must of necessity follow. Many of our common trees are examples.

Other Cases.--The stamens and pistil ripen at different times in some flowers. The "Lady Washington" geranium, a common house plant, shows this condition. Here also cross-pollination must take place if seeds are to be formed.

Summary.--If we now collect our observations upon flowers with a view to making a summary of the different devices flowers have assumed to prevent self-pollination and to secure cross-pollination, we find that they are as follows:--

REFERENCE BOOKS

ELEMENTARY

ADVANCED

LABORATORY SUGGESTIONS

Cells of Various Sizes and Shapes.--Plant cells and animal cells are of very diverse shapes and sizes. There are cells so large that they can easily be seen with the unaided eye; for example, the root hairs of plants and eggs of some animals. On the other hand, cells may be so minute, as in the case of the plant cells named bacteria, that several million might be present in a few drops of milk. The forms of cells may be extremely varied in different tissues; they may assume the form of cubes, columns, spheres, flat plates, or may be extremely irregular in shape. One kind of tissue cell, found in man, has a body so small as to be quite invisible to the naked eye, although it has a prolongation several feet in length. Such are some of the cells of the nervous system of man and other large animals, as the ox, elephant, and whale.

Pollen.--Pollen grains of various flowers, when seen under the microscope, differ greatly in form and appearance. Some are relatively large, some small, some rough, others smooth, some spherical, and others angular. They all agree, however, in having a thick wall, with a thin membrane under it, the whole inclosing a mass of protoplasm. At an early stage the pollen grain contains but a single cell. A little later, however, two nuclei may be found in the protoplasm. Hence we know that at least two cells exist there, one of which is called the sperm cell; its nucleus is the sperm nucleus.

A Typical Fruit,--the Pea or Bean Pod.--If a withered flower of any one of the pea or bean family is examined carefully, it will be found that the pistil of the flower continues to grow after the rest of the flower withers. If we remove the pistil from such a flower and examine it carefully, we find that it is the ovary that has enlarged. The space within the ovary has become nearly filled with a number of nearly ovoid bodies, attached along one edge of the inner wall. These we recognize as the young seeds.

The Necessity of Fruit and Seed Dispersal to a Plant.--We have seen that the chief reason for flowers, from the plant's standpoint, is to produce fruits which contain seeds. Reproduction and the ultimate scattering of fruits and seeds are absolutely necessary in order that colonies of plants may reach new localities. It is evident that plants best fitted to scatter their seeds, or place fruits containing the seeds some little distance from the parent plants, are the ones which will spread most rapidly. A plant, if it is to advance into new territory, must get its seeds there first. Plants which are best fitted to do this are the most widely distributed on the earth.

REFERENCE BOOKS

ELEMENTARY

ADVANCED

LABORATORY SUGGESTIONS

Food in the Cotyledons.--The problem now before us is to find out how the embryo of the bean is adapted to grow into an adult plant. Up to this stage of its existence it has had the advantage of food and protection from the parent plant. Now it must begin the battle of life alone. We shall find in all our work with plants and animals that the problem of food supply is always the most important problem to be solved by the growing organism. Let us see if the embryo is able to get a start in life from food provided for it within its own body.

Footnote 3: Iodine solution is made by simply adding a few crystals of the element iodine to 95 per cent alcohol; or, better, take by weight 1 gram of iodine crystals, 2/3 gram of iodide of potassium, and dilute to a dark brown color in weak alcohol or distilled water.

Test for Oils.--If the substance believed to contain oil is rubbed on brown paper or is placed on paper and then heated in an oven, the presence of oil will be known by a translucent spot on the paper.

Place in a test tube the substance to be tested; for example, a bit of hard-boiled egg. Pour over it a little strong nitric acid and heat gently. Note the color that appears--a lemon yellow. If the egg is washed in water and a little ammonium hydrate added, the color changes to a deep orange, showing that a protein is present.

Another characteristic protein test easily made at home is burning the substance. If it burns with the odor of burning feathers or leather, then protein forms part of its composition.

Share on: WhatsApp @Hash Facebook Tweet