A botanical seed consists of an embryonic plant that is in resting form. Seed germination is the basic phase in the growth of any plant.
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Life cycle of any plant is divided into different phases and seed germination is basic stage of the growth of any plant. A seed contains the essence of a plant in a resting, embryonic condition. Whenever a seed gets a favorable environment, the stages of germination start taking place. A dormant seed lying in the ground needs warmth, oxygen, and water to develop into a plant.
The seed coat is the outer covering of a seed which protects the embryo from any kind of damage, caused by the natural elements or due to the invasion of parasites, and prevents it from drying. A seed coat may be thick and hard, or thin and soft. The endosperm inside the seed coat contains a temporary nutritional reserve, which is packed around embryo in the form of cotyledons or seed leaves. Plants are classified as monocots and dicots depending upon number of cotyledons their seeds contain. Monocots are angiosperm seeds that contain a single seed-leaf or cotyledon, whereas, dicots are angiosperm seeds that contain two cotyledons.
All seeds need adequate oxygen, water, and the right temperature for germination. Some seeds also need proper lighting. Some can germinate well in presence of light, while others may require darkness to start germination. Water is necessary for initiation and maintenance of an appropriate rate of metabolism. Soil temperature is equally important for appropriate germination. Optimum soil temperature for each seed varies from species to species.
- The seed absorbs water and seed coat bursts. It is the first sign of germination. There is an activation of enzymes, increase in respiration, and plant cells get duplicated. A chain of chemical changes starts which leads to the development of the plant embryo.
- Chemical energy stored in the form of starch is converted to sugar, which serves as food for the embryo during the germination process. Soon, the embryo gets nourished and enlarged, and the seed coat bursts open.
- The growing plant emerges out. Tip of the root first emerges, growing downwards, and helps to anchor the seed in place. It also allows the embryo to absorb minerals and water from soil.
- Some seeds require special treatment of temperature, light or moisture to start germination.
Steps in seed germination can be different in dicots and monocots.
Germination in Dicots
During germination process of dicots, primary root emerges through the seed coat when seed is buried in soil. The hypocotyledonous stem (hypocotyl) emerges from seed coat and grows upwards through soil. As it grows up, it takes the shape of a hairpin, which is known as hypocotyl arch. Epicotyl structures, known as the plumule, are protected by two cotyledons from any kind of damage. When hypocotyl arch emerges out of the soil, it continues growing straight, the direction of growth being influenced by the direction of light. Cotyledons spread apart, exposing the epicotyl, which contains two primary leaves and the apical meristem. In many dicots, cotyledons supply nutritional matter to the developing plant and they also turn green (owing to the to the production of chlorophyll), which enable the plant to acquire nutrition via photosynthesis.
Germination in Monocots
During germination of seeds such as oats or corn, the primary root emerges from the seed and grows downwards. Then, the plant’s primary leaf emerges and grows upwards. It is protected by a cylindrical, hollow structure known as coleoptile. Once the seedling grows above soil surface, the growth of coleoptile stops and it is pierced by the primary leaf.
Of course, all the seeds that happen to land in soil are not equally lucky to get the proper environment to germinate. Many seeds tend to dry up and cannot develop into a plant. Only those seeds that get sufficient amounts of water, oxygen, and the right temperature along with soil germinate into plants.