be omitted until after the cellular structure of plants has been considered.) When the pollen grains are first formed in the anther, each consists of a single cell. Later the nucleus of this cell divides and forms two nuclei, one of which is the generative nucleus. The generative nucleus then divides. and forms two sperm nuclei. The ovule is more complex in its structure, being composed of many cells of different kinds. But here, as is the case with the pollen grain, there is one important cell that is essential in the process of reproduction, and this is known as the egg-cell (Figs. 27 and 29, A). 91. The formation of an embryo. When the pollen grain germinates and forms the tube, the sperm nucleus is carried by the tube down through the stigma and style into the cavity of the ovary, and finally through the micropyle of the ovule, until one of the sperm nuclei comes to lie beside the nucleus of the egg-cell. The two nuclei now unite in the process of fertilization to form a fertilized egg-cell. The nucleus of this cell then divides and later the cell-body, thus forming two distinct cells. Each of these divides to form two cells, and the four cells thus produced give rise to eight, then sixteen, thirty-two, and so on, until a many-celled structure is developed which is a miniature plant called the embryo. This embryo, together with other parts of the ovule, constitutes the seed. Some of the cells of the embryo will later form the roots, others the stem, and still others the leaves of the plant (Fig. 29, A-E). Hence, the new plant formed by this method of reproduction is clearly descended from two different parents, one parent flower furnishing in its pistil the egg-cell and the other in its stamen the fertilizing pollen. We may, therefore, give the following as a general definition of the process we are studying: Fertilization is the union of the nucleus of a sperm cell with the E 14 four celled stage of embryo -- micropyle produce pollen tubes which FIG. 29.- Fertilization of an ovule and may be said to begin a the early stages in the development of an embryo. (Diagrammatic.) race down the stigma and style. The tubes that first enter ovules are the ones that carry on the process of fertilization. Those that are beaten in the race are of no further use and therefore die. 92. Self-pollination and cross-pollination. - Pollination, we have said, is the transfer of pollen from the anther to the stigma. When the pollen is carried from the anther of a flower to the stigma of the pistil of the same flower, the process is known as self-pollination. In many of the flowers. that are self-pollinated, the anthers are above the stigmas, and when the pollen is ripe, the anthers burst open and allow the pollen grains to fall upon the stigma or stigmas. If pollen is carried from the anther of a flower to the stigma of the pistil of a flower of the same kind but on another plant, this transfer is called cross-pollination. Cross-pollination is often accomplished by the help of the wind, as in the flowers of the corn, of grasses, and of many trees. In these cases the pollen is dry and light, and the pistils are usually hairy or feathery to catch and hold the pollen grains. Most bright-colored and sweet-scented flowers (like the pansy and the clover) are visited by bees or other hairy insects which carry pollen on their mouth parts, bodies, and legs from one flower to another, thus insuring cross-pollination. We shall now study the pansy as a type of insect pollinated flowers. 93. Adaptations of the pansy for cross pollination. Laboratory Study No. 45. A. Floral envelopes. - When there are two circles to the floral envelopes, an outer composed of green parts and an inner made up of brightly colored parts as in the pansy, distinct names are given to the various parts. The outer circle is called the calyx, and its parts are known as sepals; the inner circle is called the corolla, and each of its parts is called a petal. 1. State the number and color of the sepals. Describe the color or 3. Locate the pairs of petals that are nearly alike in size and shape. State the position of the odd petal. 4. On which of these petals do you find the most striking spots or lines of color? 5. Make a drawing of the pansy in its natural position, front view, and natural size. Label top petals, side petals, lower petal, hairs on side petals, color spots. 6. Remove the two upper petals, and the two side petals. Now observe the tapering projection on the lower or odd petal extending upward and backward between the sepals. This is called the spur. a. Tell what you have done and seen. b. Carefully remove the lower petal with the spur 7. Slit open the spur. Is the spur hollow or solid? 9. In what two ways, therefore, may pansies attract bees? 10. On which petal would a bee be most likely to alight in visiting a pansy? What is there on this petal to guide the bee toward the supply of nectar? 11. (Optional.) What structures on the side petals might make it difficult for the bee to insert its mouth parts in this region? B. Stamens. 1. Observe the stamens arranged around the pistil. Carefully separate them with a needle or pin. State the number and situation of the stamens. 2. Carefully bend two or more stamens away from the pistil, and with the help of a magnifier look C. Pistil. on their inner surface. Tell what you have done, and state whether the openings in the yellow anthers from which the pollen is discharged are found on the inner surface (next the pistil) or on the outer surface of the anther. 1. Examine the pistil after the stamens have been removed. Carefully describe the three parts (ovary, style, and stigma) of which it is composed. 2. Observe a tiny cavity on the tip of the stigma. The inside of this cavity is the real stigma or stig matic surface. Describe the shape and state the situation of the stigmatic surface. D. Cross-pollination of the pansy by bumblebees. 1. Hold a pansy in its natural position. a. State the situation of the stamens with reference to the odd petal (i.e. are they above or below this petal?). b. On what, therefore, will pollen probably fall if it is shaken out of the anthers? 2. To determine whether or not what you have just stated is true, thrust a slender tooth-pick under the stigma and then under the stamens and into the 3. Examine a bumblebee. On what mouth-parts, head, or body) would the pollen be most FIG. 30.-Head of a likely to fall when the bee bee. |