Seed-bearing plants are classified based on the number of seed leaves they have. Seeds that have only one seed leaf, such as corn, are monocots. Seeds that germinate with two seed leaves, such as beans, are dicots. Large lima beans (Phaseolus lunatus, also Phaseolus limensis) are an example of a dicot seed that has two large cotyledons that make up the bulk of the seed. The nutrients stored in the lima bean cotyledons make them a valuable source of human food.
Large Lima Beans
Lima beans are annual plants that originated in Central America, with their culture as a food crop dating back to 7,000 years ago. There are both large-seeded and small-seeded lima bean varieties. Large-seeded lima beans come from either bush types or pole beans that need support to grow as high as 16 feet. Large lima beans are green, drying to white or a speckled reddish color. Bush varieties include "Henderson 242" (Phaseolus lunatus "Henderson 242") and "Jackson Wonder" (Phaseolus lunatus "Jackson Wonder"). Large-seeded pole lima beans include "King of the Garden" (Phaseolus lunatus "King of the Garden") and "Speckled Christmas" (Phaseolus lunatus "Speckled Christmas").
When you look at a dried lima bean, the thin outside covering is the seed coat, or testa. It has a little pore, the micropyle, on the upper, indented side of the bean. It allows water to enter the seed. The indentation is the hilum, and marks where the seed was attached to the seedpod. Two large structures, the cotyledons or seed leaves, make up the bulk of the seed. These paired cotyledons make lima bean a dicot. The thick cotyledons contain food used during germination. Take apart the cotyledons along the slit that separates them to see the plant embryo between the two cotyledons.
Lima beans need water, oxygen and temperatures between 65 and 80 degrees Fahrenheit to germinate. In field conditions, the large seed is easily damaged and the seedling can have problems emerging. The large amount of carbohydrates stored in the cotyledons makes germinating seeds vulnerable to attack by fungi and bacteria. First to emerge are the hypocotyl, or stem, and the radicle, or root. As the radicle grows into the root, the cotyledons, with the plumule or shoot between them, pull out of the soil. With further growth, the true leaves enlarge and the cotyledons wither away.
One cup of lima beans contains the following percentages of the daily minimum requirements for adults: 65.8 percent for fiber, 24.9 percent for iron, 48.5 percent for manganese, 20.2 percent for magnesium, 86.5 percent for molybdenum, 39.1 percent for folate and 29.3 percent for protein. The complex carbohydrates lima beans contain are burned slowly by the body, giving the beans a low glycemic index. Each cup of limas contains 216 calories.
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- Colorado Foundation for Agriculture: Ag in the Classroom Online Lesson Plan -- The Anatomy of a Bean Seed
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- University of Wisconsin Cooperative Extension: Lima Bean Management for Wisconsin
- University of California Davis, Vegetable Research and Information Center: Beans
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