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Various substances produced by plants prevent the germination of seeds until conditions are
favorable for this development. These substances help to insure the survival of different plant
species by limiting the density of plant populations and by giving the plants that do grow from
seeds a better start in life.Most fruits contain such growth inhibitors which delay seed germination until the fleshy part of the fruit has decayed and the inhibitor substance has leached away into the soil. Some growth inhibitors act on the seeds of other species, thus limiting their competition for growing space.
It has been found that the seed coats of certain grasses contain germination inhibitors which can be extracted by soaking them in water over night. By pouring off the liquid and keeping it in a refrigerator to prevent spoilage, it can be used to water pots in which seeds have been planted. When these post are compared with controls, the effect of the inhibitor can be measured. Since very little work has been done with such inhibitors, this field offers excellent opportunities for original research.
In order to eliminate the possibility that fungi may affect the results of the experiments, it is suggested that all of the seeds be germinated in sand which has been boiled in water. After the same number of seeds have been planted in each pot, water the experimental pots with inhibitor water and the control plants with tap water.
Several kinds of seeds and several suspected inhibitors should be tested. Some common substances which seem to reduce seed germination are tomato juice, coumarin (an ingredient of synthetic vanilla), lemon peel, onion juice, apple cider, and the water in which carrots or spinach have been cooked. Graph paper is convenient for keeping daily records and for presenting the results in an effective way.
How Deep Should Seeds Be Planted?
How does the depth at which seeds are planted affect their germination? What is the maximum depth from which seedlings are able to emerge from the ground? Data that throw some light on these questions can be obtained by planting the seeds along the sides of an aquarium. Start near the bottom and firm the soil with a straight edge or ruler so that it has a slant as shown in the diagram. Add additional layers of soil and continue upward to the surface. A different kind of seed could be planted on each side of the aquarium.
Caution should be exercised in drawing conclusions from this experiment. If the aquarium is kept indoors the temperature will be the same at all depths. Outdoors the temperature usually decreases with depth, especially in the spring.
Effect of Oxygen Deficiency on Seed Germination
Place some moist soil in each of 2 1-gallon glass jugs and drop fast-growing seeds, such as radish or squash seed, upon the moist soil. Add a little more soil to just cover the seeds and shake the jug gently so that the surface becomes level.
Unroll a strip of steel wool, moisten it, and suspend it inside one of the jugs by means of a string. Close this experimental jug and the chondral jug with corks or stoppers. The moist steel wool should take up most of the oxygen from the experimental jug.
Keep both jugs under the same conditions of light and temperature. Observe both jugs at regular intervals and make notes of any differences in the germination of the seeds.
Effect of Excess Oxygen on Seed Germination
Plant seeds in two glass jugs, as described above. In the experimental jug, insert a 2-hole stopper with glass tubing, rubber tubing, and clamps as shown. Open both clamps to let oxygen into the jug. Then close both clamps and record the results over a period of 2 or 3 weeks, comparing these seeds with those in the jug which contains only air.
A convenient way to supply oxygen is by adding hydrogen peroxide to the material from the inside of a flashlight cell. The gas should be bubbled through water to remove other gases which may be produced. Additional oxygen may be added from time to time in this way.
The experiments described on this page have not been performed many times in this particular way, and further checking of the results is needed before any general conclusions can be drawn. The experiments should be tried with different kinds of seeds under carefully controlled conditions.