- - AGRICULTURE CORE CURRICULUM - - (CLF6000) Advanced Core Cluster: ORNAMENTAL HORTICULTURE (CLF6250) Unit Title: SEXUAL & ASEXUAL PROPAGATION ____________________________________________________________________________ (CLF6253) Topic: POLLINATION & SEED Time Year(s) PRODUCTION 2 hours 3 / 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson, the student will be able to: Learning Outcome #: (D-5) - Describe two situations of pollen transfer that can be called self-pollination (e.g., within one flower or between flowers on the same plant). (D-6) - Describe three mechanisms plants use to ensure cross- pollination, e.g., effects of flower structure, dioecious plants (unisexual plants), and pistils and stamens maturing at different times. (D-6) - Demonstrate how to cross-pollinate plants in class. (D-7) - List and describe, in order of occurrence, the major events that take place in plant reproduction from pollination through fertilization, ending with seed production. Special Materials and Equipment: Handouts of flower parts, flowers for pollination and dissection, film or video on pollination, and dissection microscope References: Ingels, Jack E. (1985). ORNAMENTAL HORTICULTURE: PRINCIPLES AND PRACTICES. Albany, NY: Delmar Publishers. Hartmann, H. T., Flocker, W. J., & Kofranek, A. M. (1981). PLANT SCIENCE: GROWTH, DEVELOPMENT, AND UTILIZATION OF CULTIVATED PLANTS. Englewood Cliffs, NJ: Prentice-Hall. Evaluation: Quiz by instructor TOPIC PRESENTATION: POLLINATION AND SEED PRODUCTION A. The seed is the result of complex growth and developmental events. 1. Pollen formation is the first step in this development. a. Pollen is made in anthers (the male part), which are part of the flower's stamens. b. A pollen grain consists of two or more cells inside a protective enclosure. c. Pollen grains can travel through dry air or be carried by animals or wind without being killed by dehydration. d. The pollen grains play a part similar to an animal's sperm cells, travelling to the female structures where eggs are found. e. When a pollen grain lands on the stigma of a pistil (the female part), it grows a pollen tube that carries a male nucleus into the pistil. f. A single anther can produce thousands or millions of pollen grains. g. Pollen grains of various plant taxa differ in size, shape, color, and surface ornamentation. They can thus be used in classifying plants. 2. Another step is the production of pistils with ovules. a. A pistil has a sticky tip (the stigma) that captures pollen, a long tube (the style) that guides the pollen tube, and an enlarged basal ovary that contains ovules. b. An ovule consists of an egg and several other cells that aid in reproduction, enclosed in layers of protective cells called integuments. c. A single pistil may contain many ovules. 3. Pollination is the next step in the development of seeds. a. Pollination is the transfer of pollen from the anther to the stigma (top part of the pistil) of a flower. b. Once the pollen grain has reached the stigma it stays in place because of a sticky exudate produced by the stigma. c. This fluid also provides nourishment for the pollen grain. d. The pollen grain sends out a long tube (pollen tube) that grows down the style of the flower and finally reaches the ovule within the ovary. 4. The next step is fertilization. a. A nucleus from the pollen tube unites with the egg nucleus in an ovule, making a zygote (the first cell of the new plant). 1) The zygote gets half its genes from the plant parent that made the pollen, and half from the plant parent that made the egg. b. A second pollen nucleus combines with nuclei of the ovule (not the egg or zygote) to make a cell that will form nutritive tissue called endosperm. c. Such double fertilization occurs only in flowering plants. 5. Each fertilized ovule usually develops into a seed. a. The zygote grows into an embryo by mitotic division, cell differentiation, and cell expansion. b. The other fertilized cell develops into nutritive endosperm tissue by expanding, accumulating foods, and dividing repeatedly. c. The integuments develop into a seed coat that hardens and may accumulate waterproofing compounds. d. In some seeds, the embryo consumes the endosperm before the seed is mature. 1) This is true of roses, and many other plants. 2) The endosperm cells break down, and the embryo's cotyledons absorb the released food. 3) The cotyledons thicken and retain the food until the seed germinates. e. In other seeds, the endosperm persists until the seed germinates. 1) This is true of cereals such as corn. 2) At germination, the endosperm breaks down, and the released food is absorbed by cotyledons. B. Seeds may be fertilized by either self-pollination or cross-pollination. 1. In self-pollination, pollen and ovules are made by the same plant, or by plants of the same clone. a. Pollen may tranfer from the stamen to the pistil on the same flower. b. Also, pollen may transfer from one flower to the another flower on the same plant. c. Repeated self-fertilization results in inbreeding. Inbred plants often show reduced vigor and make fewer fruits and seeds. 2. In cross-pollination, pollen and ovules come from genetically different plants. a. Some plants cannot self-pollinate. 1) Some plants have distinct male and female individuals. Examples would be different kinds of holly (Ilex) and trees. 2) Some plants (Ginkgo) have barriers that prevent pollen tube growth if the pollen comes from a plant with the same genes as the ovule-making parent. 3) Some plants have pistil and stamen maturing at different times to prevent self-pollination. b. Cross-fertilization often leads to vigorous, highly productive plants, an effect called hybrid vigor. c. Cross-pollination is useful in plant breeding, and can be controlled by removing anthers from a plant before pollination, so that the grower can supply pollen from another selected plant. __________________________________________________________ ACTIVITY: 1. Collect pollen grains from several different plants. View them under a microscope noting different sizes, shapes, and colors. 2. Germinate pollen grains in a sugar solution. Observe growth of pollen tubes. Determine germination percents. 3. Dissect flowers of local crop or ornamental plants. Separate flower parts (petals, stamen, pistil, and sepals). Observe their characteristics of shapes, sizes, and colors. 4. Demonstrate how to cross-pollinate plants. Equipment: two flowers, small paper bags, twisties, scissors, tweezers, magnifying glass, small paint brush. a. Remove the stamen from the first flower. b. Transfer the pollen by taking the anther from the second flower and touching the pistil on the first flower (a paint brush can be used instead). c. Cover the second flower with a paper bag and wrap with a twistie. d. Label the bag-covered flower with name, date, and type of flower. __________________________________________________________ 1/6/91 MT/clh #%&C