Biology Semester II |
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| Sections: |
Introduction | Section 1 | Section 2 | Section 3 |
| Section Two: |
Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8 | Part 9 |
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Biology: Flowering Plant Reproduction: Part Eight Seeds and Fruits: We eat many parts of a plant: stems (asparagus), roots (carrots, sweet potatoes), leaves (lettuce, cabbage), buds (brussel sprouts), and most importantly fruits and seeds. Our society rests on the seeds and fruits of a small number of plants. Historically, humans have used approximately 7000 plant species as food. Today, thirty species provide 95 percent of all human food. Just four—wheat, corn, rice, and potatoes--provide most of the world’s food. In seed plants, the next generation sporophyte (the embryo) is formed within gametophyte tissues (the embryo sac or female gametophyte). These tissues are still attached to the original sporophyte plant. Thus, the embryo is protected and nourished by the original sporophyte. The seed is a structure that makes it possible to disperse the next generation some distance from the parent plan. This lowers competition between parents and offspring for space and other resources. The development of seeds was a major evolutionary advance for plants. Animals can migrate to escape worsening environmental conditions, but plants cannot. Seeds have some chemical methods of maintaining dormancy. This allows the embryo to wait out bad conditions and to germinate at a better time. Seeds are able to withstand harsh conditions for decades. Some seeds remain capable of germinating for several hundred years or more! The seed allows the next generation to be dispersed (by wind, water, or animals), hopefully to a better environment where it can survive. Following fertilization, the zygote grows by mitosis to form the mature embryo. Dicots have two “seed leaves” (cotyledons) in their embryo, while monocots have only a single cotyledon. Some embryos (many beans, for example) absorb the endosperm prior to seed germination. Others, such as corn, leave the endosperm outside the embryo. After fertilization, the flower also goes through a series of changes, losing its petals, stamens, and the stigma and style (in some cases). The ovary thickens and becomes part of the fruit enclosing a seed.
The ovary wall, after fertilization has occurred, develops into a fruit. Fruits may be fleshy, hard, multiple, or single. Seeds germinate, and the embryo grows into the next generation’s sporophyte. Fruits are classified into a number of categories. A simple fruit results from a single ovary. Simple fruits may be dry or fleshy. A fleshy fruit, such as an olive, apple, tomato, or grape, has a soft (often juicy) area surrounding the seeds. A dry fruit, such as a bean, barley, rice, or sunflower, has a dry material surrounding the seed. A compound fruit develops from a group of individual ovaries and may be either an aggregate or a multiple fruit. An aggregate fruit (such as a raspberry) develops from ovaries of single flower. If each ovary that makes up an aggregate fruit becomes a one-seeded fruit, that fruit is known as an achene. Strawberries and sunflowers are achenes. The sunflower seeds usually placed on salad bars are the seeds, whereas the sunflower seeds we buy at convenience stores are the actual, dried achene fruit. A multiple fruit, such as a pineapple, develops from ovaries that are from separate flowers that have become fused together. Germination begins when conditions are right and the dormant embryo has absorbed enough water to grow and break through the seed coat. Energy for this initial growth is stored in the endosperm or incorporated within the cotyledons of the embryo. The embryonic root usually emerges first, followed by the shoot. There are some differences between the germination of monocot and dicot seeds. The dicot embryo consists of two seed leaves (the cotyledons). The cotyledons will supply nutrients to the embryo as it grows and becomes photosynthetic. In dicots, cotyledons can be carried to the surface by the growing stem. The stem produces the primary leaves, which photosynthesize and feed the developing plant. The plumule is another name for the embryo (or that part of the dicot embryo that is not the cotyledons). The plumule consists of an epicotyl bearing young leaves, a hypocotyl that becomes the stem, and the radicle that will develop into the root. When the dicot seedling emerges, the shoot is hook-shaped to protect the delicate tissues of the plumule. As the seed germinates in darkness, the stem increases in length. Leaves remain small and protected by the cotyledons. Most dicots lift the seed above ground. On the other hand, most monocots, such as corn, leave the seed below ground. The corn plant stores its food as starch, which must be absorbed into the embryo by the cotyledon from the endosperm. The corn "seed" is actually the fruit. The corn embryo is divisible into a cotyledon, a plumule, and a radicle, which are enclosed in protective sheaths (the coleoptile and coleorhiza). The plumule and the radicle burst through these coverings when germination occurs.
The goal of this lab is to learn about the structure and function of flowers, fruits and seeds. You will also study the reproductive processes in flowering plants and discover the value of fruits to humans. Submit your completed lab to the Lab: Flowers, Fruits and Seeds assignment link for grading. For information on how this assignment will be graded, please visit the Course Information section.
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Lab: Flowers, Fruits and Seeds 100 points