Inside a seed is the embryo of a plant, plus a food source for that embryo, all contained within a protective seed coat. Here, you can observe seed germination, in which the embryo begins to digest the food and grow into a seedling. While this process usually happens in soil, the key component for germination is water.
At appropriate temperatures, most seeds begin their germination by absorbing water through a tiny hole in the seed coat. The moisture starts the metabolic processes of the embryo that’s contained within the seed. When hydrated by absorption of water, enzymes in the seed are activated. They begin digesting the food stored inside to generate energy for the embryo’s growth.
The developing root emerges from the seed first. As the root grows longer and thicker, it develops tiny root hairs, which help the developing plant take up water and nutrients. Shoots with pale-green leaflike structures emerge after the roots. Eventually, these leaves will turn a deeper green color and begin to photosynthesize, capturing and storing light energy and carbon dioxide from the air.
Photosynthesis in leaves supplies the plant with the energy and matter it needs to grow. Newly germinated seedlings, however, are not yet photosynthesizing. Instead, in early stages of growth, the embryo digests and assimilates the energy and matter from the food present in the seed. Depending on the type of seed, this food store contains a mixture of proteins, fats, sugars, and starches. This stored food isn’t just important to the developing plant embryo; it’s also important to human diets. About 45 percent of the calories humans consume globally comes from seed grains like rice, wheat, and corn.
A common misconception is that plants get their mass from soil. In this soil-free experiment, you can prove to yourself that plants don’t strictly require soil to grow. In fact, many plants grow very well hydroponically in water cultures, as long as the appropriate nutrients such as nitrogen, phosphorous, and potassium are provided. Your seedlings will eventually need more space than the seed germinator can provide, but given the right lighting conditions, they’ll begin to photosynthesize, accumulating mass from the carbon dioxide in the air and the water you provide.