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 shortstyled flower, or when the pollen from a high anther is placed upon a long-styled pistil. = Flowers which have this peculiar condition are said to be dimorphic (Greek of two forms). There are, as in the case of the loosestrife, trimorphic flowers having pistils and stamens of three lengths. Condition of stamens and pistils in the spiked loosestrife (Lythrum salicaria). Charles Darwin, who worked out the fertilization of this flower, describes it as follows: "When bees suck the flowers, the anthers of the longest stamens. are rubbed against the abdomen and inner sides of the hind legs as is likewise the stigma of the long-styled form (see diagram). The anthers of the midlength stamens and the stigma of the midstyled form are rubbed against the upper side of the thorax and between the front pair of legs. And, lastly, the anther of the shortest stamens and the stigma of the short-styled form are rubbed against the proboscis and the chin; for the bees in sucking the flowers insert only the front part of the head into the flower.... It follows that insects will generally carry the pollen of each form from the stamens to the pistil of corresponding length." 1 Protection of Pollen. - Pollen, in order to be carried effectively by the wind, insects, or other agencies, must be dry. In some flowers the irregular form of the corolla protects the pollen from dampness. Other flowers close up at night, as the morning-glory and four-o'clock. Still others, as the bellflower, droop during a shower or at night. Pollen is also protected from insect visitors which would carry 1 Forms of Flowers, page 159. off pollen but give the flower no return by cross-pollinating it. In some flowers access of ants, plant lice, or other small crawling insects to the stamens is rendered difficult by hairs which are developed upon the filaments or on the corolla. Sometimes a ring of sticky material is found making a barrier around the stalk underneath the flower. Many other adaptations of this sort might be mentioned. Artificial Cross-Pollination and its Practical Benefits to Man. Artificial cross-pollination is practiced by plant breeders and can easily be tried in the laboratory or at home. First the anthers must be carefully removed from the bud of the flower so as to eliminate all possibility of self-pollination. The flower must then be covered so as to prevent access of pollen from without; when the ovary is sufficiently developed, pollen from another flower, having the characters desired, is placed on the stigma and the flower again covered to prevent any other pollen reaching the flower. The seeds from this flower when planted may give rise to plants with some characters like each of the plants from which the pollen and egg cell came. Artificial fertilization has been made of great practical value to man. REFERENCE BOOKS ELEMENTARY Sharpe, A Laboratory Manual for the Solution of Problems in Biology. American Book Andrews, Botany all the Year Round, pages 222-236. American Book Company. ADVANCED Bailey, Plant Breeding. The Macmillan Company. Campbell, Lectures on the Evolution of Plants. The Macmillan Company. Coulter, Barnes, and Cowles, A Textbook of Botany, Part II. American Book Company. Darwin, Different Forms of Flowers on Plants of the Same Species. D. Appleton & Co. Darwin, Fertilization in the Vegetable Kingdom, Chaps. I and II. D. Appleton & Co. Darwin, Orchids Fertilized by Insects. D. Appleton and Company. Gray, Structural Botany. American Book Company. Lubbock, British Wild Flowers. The Macmillan Company. Müller, The Fertilization of Flowers. The Macmillan Company. V. FRUITS AND THEIR USES Problem VIII. A study of fruits to discover (a) Their uses to a plant. (b) The means of scattering. (c) Their protection from animals and other enemies. (Laboratory Manual, Prob. VIII.) 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 almost filled with a number of almost ovoid bodies, attached along one edge of the inner wall. These we recognize as the young seeds. The pod of a bean, pea, or locust illustrates well the growth from the flower. The flower stalk, the ovary, and the remains of the style, the stigma, and the calyx, can be found on most unopened pods. If the pod is opened, the seeds will be found fastened to the ovary wall each by a little stalk. That part of the ovary wall which bears the seeds is the placenta. The walls of the pod are called valves. Fruit of the black locust; a legume, showing the, attachment of the seeds. The pod, which is in reality a ripened ovary with other parts of the pistil attached to it, is considered as a fruit. By definition, a fruit is a ripened ovary together with any parts of the flower that may be attached to it. The chief use of the fruit to the flower is to hold and to protect the seeds; it may ultimately distribute them where they can reproduce young plants. Formation of Seeds. Each seed has been formed as a direct result of the fertilization of the egg cell (contained in the embryo sac of the ovule) by a sperm cell of the pollen tube. Seed Dispersal. If you will go out any fall afternoon into the fields, a city park, or even a vacant lot, you can hardly escape seeing how seeds are scattered by the parent plants and trees. Several hundred little seedling trees may often be counted under the shade of a single maple or oak tree. But nearly all these young trees are doomed to die, because of the overshading and crowding. Plants, like animals, are dependent upon their Young cedars around parent tree. Photographed by Overton. surroundings for food and air. They need light even more than animals need it, because the soil directly under the shade of the old tree gives only raw food material to the plants, and they must have sunlight in order to make food. This overcrowding is seen in the garden where young beets or lettuce are growing. The gardener assists nature by thinning out the young plants so that they may not be handicapped in their battle for life in the garden by an insufficient supply of air, light, and food. 1 At this point a field trip may well be taken with a view to finding out how the common fall weeds scatter their seeds. Fruits and seeds obtained upon this trip will make a basis for laboratory work on the adaptations of seed and fruit for dispersal. It is evidently of considerable advantage to a plant to be able to place its progeny, which are to grow up from seeds, at a considerable distance from itself, in order that the young plant may be provided with a sufficient space to get nourishment and foothold. This is the result which plants have to accomplish. Some accomplish the result more completely than others, and thus are the more successful ones in the battle of life. Adaptations for Seed Dispersal; Fleshy Fruits with Hard Seeds. Plants are fitted to scatter their seeds by having the special means either in the fruit or in the seed. Various agents, as the wind, water, or squirrels, birds, and other animals, make it possible for the seeds to be taken away from the plant. The blackberry, a fruit having small seeds scattered by birds. Fleshy fruits, that is, such fruits as contain considerable water when ripe, are eaten by animals and the seeds passed off undigested. Most wild fleshy fruits have small, hard, indigestible seeds. Birds are responsible for much seed planting of berries or other small fruit. Bears and other berry-feeding animals aid in this as well. Some seeds have especial adaptations in the way of spines or projections. Insects make use of these projections in order to carry them away. Ants plant seeds which they have carried to their nests for a food supply. Nuts are planted by squirrels and blue jays. Suggestions for Field Work. Examine the fruit of huckleberry, blackberry, wild strawberry, wild cherry, black haw, wild grape, tomato, currant. Report how many of the above have seeds with hard coatings. Notice that in most, if not in all, edible fruits, the fruit remains green, sour, and inedible until the seeds are ripe. In the state of nature, how might this be of use to a plant? Hooks and Spines. Some fruits which are dry and have a hard external covering when ripe possess hooks or spines which enable the whole fruit to be carried away from the parent plant by animals or other moving objects. Cattle are responsible for the spread of |