chemical influence thus attracts the pollen tube. In flowers in which the style is short, the tube reaches the ovule in the course of a few hours. In plants with a long style, from one to several days may elapse before the pollen tube reaches the locule of the ovary. Once it reaches the ovary, the tube penetrates an ovule by making its way through a little hole known as the micropyle. It then grows toward a clear area of protoplasm known as the embryo sac. The embryo sac is an ovoid area, microscopic in size, filled with semifluid protoplasm containing several nuclei. (See figure.) One of the nuclei, with the protoplasm immediately surrounding it, is called the egg cell. It is this cell that the sperm cell of the pollen tube grows toward; ultimately the sperm cell reaches the egg cell and unites with it. The two cells, after coming together, unite to form a single cell. This process is known as fertilization. This single cell formed by the union of the pollen tube cell or sperm and the egg cell is now called a fertilized egg. Development of Ovule into Seed. The primary reason for the existence of a flower is that it may produce seeds from which future plants will grow. The first beginning of the growth of the seed takes place at the moment of fertilization. From that time on there is a growth, within the ovule, of a little structure called the embryo. The embryo will give rise to the future plant. After fertilization the ovule is called a seed. History of the Discoveries regarding Fertilization. - Although the ancient Greek and Roman naturalists had some vague ideas on the subject of fertilization, it was not until the latter part of the eighteenth century that it was demonstrated that pollen was necessary for the growth of the embryo within a seed. In the latter part of the eighteenth century a book appeared in which a German named Conrad Sprengel worked out the facts that the structure of certain flowers seemed to be adapted to the visits of insects. Certain facilities were offered to an insect in the way of easy foothold, sweet odor, and especially food in the shape of pollen and nectar, the latter a sweet-tasting substance manufactured by certain parts of the flower known as the nectar glands. Sprengel further discovered the fact that pollen could be and was carried by the insect visitors from the anthers of the flower to its stigma. Pollination. It was not until the middle of the nineteenth century, however, that an Englishman, Charles Darwin, discovered the true relation of insects to flowers by his investigations upon the cross-pollination of flowers. By pollination we mean the transfer of pollen from an anther to the stigma of a flower. Selfpollination is the transfer of pollen in one flower; cross-pollination is the transfer of pollen from the anthers of one flower to the stigma of another flower of the same kind. It was found by Charles Darwin and it has since stigma at the time An orchid, a flower of the type from which Charles Darwin worked out his theory of cross-pollination by insects. of pollination also shows that the pollen from another flower germinates before the pollen which has fallen from the anthers of the same flower. This latter fact alone in most cases renders it impossible for a flower to produce seeds by its own pollen. Darwin worked for many years on the pollination of many insect-visited flowers, and discovered in almost every case that showy, sweet-scented, or otherwise attractive flowers were adapted or fitted to be crosspollinated by insects. He also found, in the case of flowers that were inconspicuous in appearance, often a compensation appeared in the odor which rendered them attractive to certain insects. The so-called carrion flowers, pollinated by flies, are examples, the odor in this case being like decayed flesh. Other flowers open at night, are white, and provided with a powerful scent so as to attract night-flying moths and other insects. We shall later take up some of the many cases of the adaptation of the parts of a flower to these insect callers. Flowers adapted to be crosspollinated by insects are almost invariably irregular in shape. Let us now consider rather in detail the structure of the sweet pea, an example of such a flower. Sweet Pea. The sepals are of almost the same size and shape; that is, regular. The petals, however, are quite different from each other in form. If you pull off the parts of the corolla you will find that they separate naturally into a large expanded petal at the top of the flower; this is called the vane or standard; two petals at the sides called the wings, and a curved part below called the keel, the latter being made of two petals joined along the edge. A corolla of this kind is said to be papilionaceous from its fancied resemblance to a butterfly. What other plants do you know that have flowers of this shape? If you press the point of a pencil down upon the keel of the flower, holding it in a natural position, you will soon find that the point has taken up some pollen. Evidently a bee might carry off pollen in the same manner. Now remove the corolla from one side of the flower so as to show the stamens and pistil. Notice that the stamens are separated into two groups, those in one group united by their filaments to each other. How many are there in each group? This condition of stamens is said to be diadelphous. Draw the flower from the side, showing the above parts before you pull off the stamens. Look for the three parts of the pistil. Draw and label them. Notice the little hairs covering parts of the pistil; can you imagine their use? In old flowers you can see that the ovary is forming the characteristic pod of the pea family. Insects as Pollinating Agents. We have seen in a general way that insects assist in the cross-pollination of flowers. Let us now turn our attention to the insects to see how in their structure and habits they are adapted to play the important part that they fill in the cross-pollination of flowers. No one who sees a hive of bees with their wonderful communal life can fail to see that these insects play a great part in the life of the flowers near the hive. A famous observer named Sir John Lubbock tested bees and wasps to see how many trips they made daily from the hive to the flowers, and found that the wasp went out on 116 visits during a working day of 16 hours, while the bee made but a few At this point, at least one field trip should be introduced for the purpose of studying under natural conditions the cross-pollination of flowers by insects. For suggestions for such a trip, see Hunter and Valentine, Manual, page 207. Many of the following exercises on fall flowers may profitably be taken in the field and reported on by the pupil as class exercises. Excellent suggestions for a field trip may be found in Andrews, Botany All the Year Round. less visits, and worked only a little less time than the wasp worked. It is evident that in the course of so many trips to the fields a bee must light on and cross-pollinate many hundreds of flowers. a Bumblebees; a, queen; b, worker; c, drone. Laboratory Study of a Bumblebee. The careful study of the bumblebee will give us some insight into the manner in which the bee transfers pollen. Living specimens may be chloroformed and then used dry, or material which has been preserved in formol or alcohol will answer the purpose. The body of an insect is divided into three regions; these may easily be found in the bee. The anterior or front part is the head, the middle is the thorax, and the posterior or hind part is the abdomen. The abdomen in the working bees is terminated in a sharp sting. Look carefully at the abdomen; you will find it is cut up into a series of little rings which taper off at the hind end of the body. These rings are called segments. Notice that the legs and wings are attached to the thorax. The wings are found on the dorsal or upper side of the body, the legs on the lower or ventral surface. Look at a leg with your hand lens. Is it all of one piece or jointed? When a plant or animal structure is fitted to do certain kind or kinds of work, we say that the structure is adapted to its functions. Can you determine how the leg is adapted for movement and for clinging to an object? Can you find any parts adapted to hold pollen? Notice the collection of stiff hairs on the joint next the body. In the honeybee these hairs are so arranged that they act as receptacles for pollen, which the bee stores there to carry home to the hive. Bees, wasps, and many other insects use pollen as food. The body of the bee, as well as the head and legs, are more or less completely covered with tiny hairs. Are the hairs any better developed in certain parts of the body? If so, do you think they would be of use in carrying pollen, and why? Pollination not intended by the Bee. The cross-pollination of flowers is not planned by the bee; it is simply an incident in the course of the food gathering. The bee visits a large number of flowers of the same species during the course of a single visit from the hive, and it is then that cross-pollination takes place. 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? Is Color or Odor in a Flower an Attraction to an Insect? Try to decide whether color or odor has the most effect in attracting bees to flowers. 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 are 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. Test the keenness of sight in insects by placing a white object (a white golf ball will do) 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. The Eyes of the Bumblebee.-Look carefully at the large eyes located on the sides of the head. They are made up of a large number of little units, each of which is considered to be a very simple eye. The large eyes are therefore called the compound eyes. All insects are provided with compound eyes, and in addition to these (in some cases) with simple eyes. The simple eyes of the bee may be found by a careful observer in front and above the compound eyes. One would suppose that with so many eyes the sight of insects would be extremely keen, but such does not seem to be the case. A lily; P, petal; S., stamen (anther); Insects can, as we have already The bee is attracted to a flower for food. This food may consist of pollen and nectar. |