why do seeds need light to germinate

When seeds are developing, the seeds go through several stages, the last being dehydration to become an inactive seed. The inactive seed is what can be bought at the store in packets. For seeds to be active again, the proper conditions such as warmth, water, and oxygen are needed. These conditions lead to the seed germinating. Germination comes from the Latin root germinare, which means to sprout, and germen, meaning seed. Lets determine if sunlight on the soil is necessary for germination. To make germination trays take one egg carton and cut off the top half and place it under the bottom half. Take the second egg carton and cut off the top half, placing it under the third carton. The trays under the cartons will catch excess water. Use a pencil to punch a hole in each top layer compartment in both cartons to allow drainage. Fill each compartment approximately half way with potting soil. Sprinkle a few seeds in each compartment of the egg cartons, then add enough soil to cover the seeds.

Water each compartment until the soil is moist. Close the lid of the second egg carton. Place the egg cartons in the window to receive sunlight. Check each day for growth and if the soil is moist. Water as needed. 1. In which egg carton did the seeds germinate? 2. Did more seeds germinate in one of the cartons? If so, which one? 3. Is light necessary for germination? Germination is the process by which seeds sprout. Seeds must go through several stages of development before being ready to sprout. Light on the soil was not necessary for seed germination because warmth, water, and oxygen are the only conditions necessary for a seed to germinate. This is because the seed contains the plant embryo and stores enough food to give the baby plant all the energy it needs to sprout. After the seeds sprout and use up all the stored food, they do need light to grow. The light is needed for photosynthesis, which thereafter provides the plant s food source.

Janice Van Cleave s Plants. Janice Van Cleave, John Wiley and Sons, Inc. : New York, 1997. ISBN 0-471-14687-0
S. Olesik, WOW Project, Ohio State University, 2003. Why do some seeds germinate only in the dark? It may be easier to answer the question "Why are some seeds inhibited from germination when in the light". The key to this phenomenon is Phytochrome and you need to research that pigment and its different forms to write a full explanation. You will also need to find out about RED and FAR-RED light. Phytochrome is present in one form in white/sun light and present in a different form after a period in the dark. Normally, the light form decays to the dark form after a few HOURS, but it is converted back again after a few MINUTES in the light. So, a seed on the soil surface gets enough light to keep the dark form of phytochrome low for sufficient time to prevent germination initiation.

It is only when the seed is in permanent darkness that the dark form of phytochrome is active for long enough to trigger germination. Obviously, this prevents such seeds germinating until they are buried. This mechanism is even more interesting. some seeds will ONLY germinate in the light (I know these are NOT the seeds you asked about), but if the light has first passed through leaves, then germination is inhibited. This enables such seeds to germinate as soon as they receive direct sunlight, but to remain dormant whilst they are under other plants which would give a developing seeding too much competition. It turns out that the spectral quality of light that has passed through leaves is such that it converts the phytochrome to the 'dark' form (as I have called it) - even though the seed is on the surface of the soil. I hope you will find this a fascinating area to research.

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