Scientific explanation - When will what grow?
After 5-6 days, the bowls look quite different.
- Nothing has happened in the dry bowl.
- Some nice green water cress plants have grown in the bowl kept at room
temperature with water and light.
- The seeds in the shoe box have germinated, but the plants are pale and thin.
- The seeds in the bowl from the refrigerator have swollen and a mucus layer has
formed around the seeds, but they have not germinated.
Before a plant can grow, it must have the conditions necessary for photosynthesis. The most important things plants need for healthy growth are water, carbon dioxide and light. Also, biochemical processes work best at temperatures between about 15 and 35°C. From 40°C, protein denaturation occurs in cells, and at cooler temperatures, biochemical processes take place very slowly (due to the temperature dependence of reaction rates). If the plant is not adapted to these temperatures, many of its reactions are not as tightly coordinated as they would otherwise be.
The seed employs various mechanisms to ensure that the seedling does not begin to grow in less-than-ideal conditions. Watercress, for example, stores water in a mucus coating formed to protect the seedling from drying out until it develops functional roots. (Virtually) no biochemical reactions can take place without the presence of water, including germination. Some plants (cold germinators such as winter wheat, rye, sugar beet) need cold temperatures in order for their seeds to germinate. The cold decomposes the substance that suppresses their germination. This substance ensures that the plants, which are adapted to cold winter temperatures don't germinate until the spring, even though their seeds were formed in the autumn. Warm germinators, on the other hand, (maize, watercress, beans, for example) need
temperatures of at least 5°C to germinate.
The moistened seed needs only a low light stimulus to develop. However, if the plant is subsequently unable to obtain enough light, it will do everything in its power to get that light. The plant will grow thin and long and will remain pale without any leaves. The specialist term for this is "etiolation", from the French word "étoile" meaning "star". Etiolated plants may be familiar to some, as this is what happens when houseplants are placed too far from a window and are unable to get enough light.