Inside the bracts are the whorls of brightly colored, irregular flowers called the ray flowers. They appear to act, in some instances at least, as an attraction to insects by showing a definite color (see the common dogwood, Cornus florida). d A composite head. Section through composite head, show ing a disk flower (a), a ray flower (c), In most cases the ray flowers are imperfect. latter is easily obtained in the fall of the year.) Determine what parts of the ray flower are missing. The flowers occupying the center of the cluster are the disk flowers. Examine such a flower under the hand lens. Determine if the flower is perfect. A careful observer will find in cosmos that the anthers are united in a ring around the pistil. This is a typical condition in the Compositæ. REFERENCE BOOKS FOR THE PUPIL Andrews, Botany all the Year Round, pages 222-236. American Book Company. FOR THE TEACHER Darwin, Forms of Flowers. D. Appleton and Company. Darwin, Orchids Fertilized by Insects. D. Appleton and Company. Darwin, Fertilization in the Vegetable Kingdom. Chapters I. and II. D. Appleton and Company. Campbell, Lectures on the Evolution of Plants. The Macmillan Company. Gray, Structural Botany. American Book Company. Lubbock, British Wild Flowers. The Macmillan Company. Year Book, U.S. Department of Agriculture, 1896, 1897, 1898, 1899, 1900. HUNTER'S BIOL. 4 V. FRUITS A Typical Fruit, the Pea or Bean Pod. In the study of the flower of the sweet pea it was seen 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 locule of the ovary has become almost filled with a number of almost spherical bodies, attached along one edge of the ovary. These we recognize as the young seeds. Study of Bean or Pea Fruit. - The pod of the pea or string bean will show us these facts more clearly. Examine, and then draw, natural size, an un Fruit of the honey locust; a legume, showing the attachment of the seeds. The walls of the pod are called valves. Show the ing your drawing. opened pod. Label the ovary, peduncle, remains of style, stigma and calyx if you can find them. Then split open the pod and make another drawing that will show the seeds. Label the stalk that attaches each seed to the wall of the pod. This is called the funiculus. That part of the ovary wall which bears the seeds is the placenta. The entire ovary wall is called the pericarp. above parts by label 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, to protect, and ultimately to distribute the seeds 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. FORMS OF FRUITS; THE ACHENE, THE SIMPLEST FRUIT.-The forms taken by fruits are very numerous. Naturally the simplest of all fruits would be one produced from a simple pistil or carpel containing only one seed. A collection of such fruits may be seen in the ripened flower of the buttercup, smartweed, or buckwheat. A single one of these fruits, that is, a single ripe ovary, is called an achene. In the fruits of the strawberry and blackberry the receptacle of the flower has grown up to form the fleshy mass that we call fruit, while the achenelike fruits are found growing on the outside of this. Such a collection of fruits is called an accessory fruit. Why? Dry and Fleshy Fruits. - Fruits, are easily grouped into two great classes, depending upon the amount of water they contain when ripe. They are hence called dry or fleshy. The blackberry; a fruit made up of many separate ripe carpels. In the following list decide which are dry and which fleshy fruits. Bean, apple, acorn, orange, grain (wheat or corn), pumpkin. How would the seeds in fleshy fruit be able to get out of the fruit? What is the use of the fleshy part to the fruit? Pome. The botanist finds several kinds of fleshy fruits. One which we meet with most frequently is called the pome. An apple is an excellent example. Study of an Apple. - In order to understand the formation of the apple fruit it will be necessary to go back to the apple blossom. This gives us an explanation for the dried structures found at the opposite end from the apple stem. These are the dried ends of the sepals. Notice the skin. Of what use, is it to the fruit? Remove the skin from the apple. Leave the pared apple exposed to the air a few hours. What happens? The formation of the fruit can be understood better if we cut several sections through it at right angles to the stem. In a cross section find the locules of the ovary; how many are there? The tough walls directly inclosing the seeds are the pericarp or ovary wall proper. The fleshy part of the Young apples, and apple blossoms. apple, then, appears to be formed from some other parts of the flower. Botanists are undecided as to whether it is calyx tube, receptacle, or part of both structures.1 S PI Draw a cross section of an apple to show the above points. In a longitudinal section the relation of the fruit to the flower is better shown. The stem was the peduncle of the flower; the ovary wall, placenta, and seeds may easily be seen. In young specimens the funiculus, a thread attaching the seed to the placenta, may be found. A short distance outside the ovary wall is seen a faint line. This is composed of somewhat stringlike structures, the tubes such as we found in the strings of the string bean. These bundles of ducts are called the fibrovascular bundles. It is through these ducts that the apple has received most of the food material which was used to form the fruit. We shall learn more about the structure of the fibrovascular bundles in the study of the stems of different plants. Draw a longitudinal section of the apple, natural size. Öther fruits, built on the same plan of structure as the apple, are the pear, quince, and hip. These fruits are classed as pomes. Longitudinal section of a pome; p, peduncle; f, fibrovascular bundles; 8, seeds; pl, placenta; e, carpel. PEPO.- Another fruit of somewhat similar structure as the pome is the pepo. The pumpkin, gourd, and squash are examples. Cut a squash open in cross sections The ovary wall can be made out close to the numerous squash? The outer fleshy part and the rind grow from the receptacle of the 'See Hunter and Valentine, Manual, page 73. In this and other suggestions for laboratory work it is expected that the teacher will select from the following pages only a portion of the given material. flowers. From what you have observed, make up a definition of a pepo fruit. Draw a cross section of the squash, natural size. BERRY. A gardener or vegetable vender rarely calls a tomato a berry. Tomatoes, however, are considered excellent examples of this type of fruit.' In botanical language, a berry is any pulpy, juicy mass containing seeds, this mass inclosed in a rather tough but thin covering, as a rind or skin. In popular language, a berry is any small round edible fruit containing small seeds. DRUPE. Another fleshy fruit is the drupe or stone fruit. Thi is illustrated by the peach or cherry. In the drupe a juicy interior is surrounded by a skin; the center of the fruit is occupied by a stone which contains the seed. This stony covering is made up of the inner wall of the pericarp (ovary wall) which has separated from the part which forms the flesh of the fruit. The connection between these two layers is well seen in a clingstone peach or a very young cherry. Classify as many of the following named fruits as you can: plum, apricot, egg plant, watermelon, lemon, pomegranate, cranberry, black haw, pear, date, olive. Make up your classification in tabular form. The cherry; a stone fruit or drupe. Dry Fruits. A dry fruit may split open to allow the escape of seeds. The pea is an example. Such a fruit is said to be dehiscent. Study an open pea pod. When it splits, it separates along both edges of the two sides or valves. Such a fruit is called a legume. Follicle. If the ovary splits along one edge of a valve only, the fruit is called a follicle. The milkweed pod is an example. Capsule. When the ovary forming the fruit is compound, the ovary having several locules, it is said to be a capsule. 1 For laboratory work on the tomato, see Hunter and Valentine, Manual, page 71. 2 For laboratory exercises on dry dehiscent fruits, see Hunter and Valentine, Manual, pages 66, 67, 68. |