El Gran Escape de la Planta

Caso 3 - Background Information

Seeds come in different sizes, shapes, and colors. Some are edible and some are not. Some seeds germinate readily while others need specific conditions to be met before they will germinate.

When looking at a seed, realize and convey to the students that within every seed lives a tiny plant or embryo. An idea to convey is that a small child can hold in their hand 500 radishes, several hundred thousand petunias, or an entire meadow if you consider each seed to be a plant and relate it to the size of each seed.

Plants have the ability to move in the form of seeds. They can't just get up and walk to a new location, but structures on the seed may allow it to move to a new location. Some of the moving forces might be wind, water, animals, and gravity. Have students collect seeds of garden and wild plants. Look at them and speculate how they might be moved from place to place.

Seed structure. The outer covering of a seed is called the seed coat. Seed coasts help protect the embryo from injury and also from drying out. Seed coats can be quite thin and soft as in beans or very think and hard as in locust or coconut seeds. Endosperm, which is a temporary food supply, is packed around the embryo in the form of special leaves called cotyledons or seed leaves. These generally are the first parts visible when the seed germinates. Plants are classified based upon the number of seed leaves (cotyledons) in the seed. Plants such as grasses and grass relatives can be monocots, containing one cotyledon. Dicots are plants that have two cotyledons. Seeds remain dormant or inactive until conditions are right for germination. All seeds need water, oxygen, and proper temperature in order to germinate. Some seeds require proper light also. Some germinate better in full light while other require darkness to germinate.

When a seed is exposed to the proper conditions, water and oxygen are taken in through the seed coat. The embryo¹s cells start to enlarge and the seed coat breaks open and root or radicle emerges first followed by the shoot or plumule which contains the leaves and stem.

Many factors contribute to poor germination. Overwatering results in a lack of proper oxygen levels. Planting seeds to too deep results in the seed using up all of its stored energy before reaching the soil surface, and dry conditions result in the lack of sufficient moisture to start and sustain the germination process.

Some seeds may be inhibited from germinating because of either physical or physiological dormancy. Physical dormancy usually takes the form of a seed coat that is so hard that water and oxygen cannot get through until the coat breaks down. These seeds benefit from being soaked or scratched lightly before planting. Morning glories and locust seeds are examples. Physiological dormancy results when the internal chemistry of the seed needs to be corrected by exposing the seed to proper temperatures. Seeds such as apple will not germinate unless exposed to cold temperatures for a period of time. Both of these adaptations prevent the seed from germinating until conditions are right for their survival.

Nonflowering plants like ferns reproduce by seed-like structures called spores. These spores are generally found on the undersides of leaves and look like tiny tufts of velvet. Spores are ripe when they readily fall off the leaf. A collection of ferns in the classroom will generally yield a nice display of spores, arrangement of spores, and colors. You may want to try to germinate them but it takes a very long time for germination to occur and for a plant to develop.

The seed viewer activity allows the student to observe the germination process without the interference of a soil medium. The seeds used in the activity will germinate and grow for a short time while being nourished by food stored in the cotyledons. Eventually they will need nutrients from a soil medium to continue to grow. You may want to transplant a few germinated seeds in soil and note differences between those in the seed viewer and those in the soil. This activity can also sharpen observation skills. Students can not how the seed changes physically during germination. You might also incorporate different or no light levels, different temperatures, or different moisture levels and see how or if it affects germination.

The seed within activity is best done with seed that has been soaked in water overnight. Lima beans are good examples of dicot seeds and corn can be used for monocot seeds. The beans will easily open into two halves. The corn will have to be cut in half lengthwise to view the inside. Students should draw what they see and offer explanations of the differences.

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