Seed and Seed Germination Lecture PDF

Seed and Seed Germination The seed is a ripened fertilized ovule. The seed serves the functions of: 1) Perennation 2) Dispersal 3) Reproduction of the parent plant. In angiosperms, the seeds are enclosed within fruits. In gymnosperms seeds are exposed. ١  Fruit including see...

Seed and Seed Germination The seed is a ripened fertilized ovule. The seed serves the functions of: 1) Perennation 2) Dispersal 3) Reproduction of the parent plant. In angiosperms, the seeds are enclosed within fruits. In gymnosperms seeds are exposed. ١  Fruit including seeds Flower ٢ ٣  General structure of the seed 1- Seed Coat (Testa) Seed Coat (Testa) is the protective covering of the seed. Function of seed coat: Seed coat protects the seed from fungi, bacteria and insects. The seed is attached to fruit through the hilum (a scar on the seed coat). Micropyle is a tiny pore in the testa near the hilum. Micropyle admits water to the embryo before germination. ٤ 2- The embryo The embryo is a young plant enclosed within the seed coat. Embryo consists of an embryo axis and cotyledons. The embryo axis consists of plumule, epicotyl, cotyledonary node, hypocotyle, and radicle. Plumule grows into the shoot system of the plant. Radicle grows into the root system of the plant. The embryo axis has a node called cotyledonary node that bears one or two cotyledons. The part of embryo axis between plumule and cotyledonary node is called epicotyle. The part of embryo axis between cotyledonary node and radicle is called hypocotyle. ٥  Mature plant ٦ Two cotyledons of a dicot seed ٧  Functions of cotyledons: 1. Storage of food. 2. Protect the embryo. 3. Photosynthetic organs after germination. The seed contains reserve food either in cotyledons or in a special tissue called endosperms. ٨  Seed germination Germination is the process by which an embryo grows from a seed. Germination results in the formation of the seedling. Germination consists of three processes: (1) Absorption of water, causing a swelling and splitting of the seed coat. (2) Enzymatic activity and increased respiration. (3) Cell enlargement and divisions leading to the growth of radicle and plumule. ٩ Factors affecting seed germination 1- External factors (i) Water The seeds must absorb water to become active and show germination. Water softens the seed coats, causes their rupturing, and converts the insoluble food into soluble. (ii) Oxygen Oxygen is necessary for respiration which releases the energy needed for growth of embryo. ١٠ iii) Suitable Temperature The minimum temperature is the lowest temperature at which seeds can germinate. The maximum temperature is the highest temperature at which seeds can germinate. Temperatures above maximum temperature or below minimum temperature can damage seeds or make them go into dormancy. At optimal temperatures, germination is rapid. The optimum temperature for most of crop plants is around 25-30°C. ١١ (iv) Light Some seeds need light to germinate. By contrast, other seeds need darkness to germinate. Some seeds are light-indifferent (not sensitive either for darkness or lightness). Most seeds germinate best under dark conditions. ١٢ 2- Internal factors Seed viability is the capacity of the seed to germinate. Some viable seeds might not germinate because they developed a dormancy (or sleep) period. Dormancy is a period of time where the viable seeds do not germination even when supplied with the factors adequate for germination. Reasons of seed dormancy: 1. The embryo is not fully mature. 2. The seed may not have sufficient amounts of growth hormones required for the growth of embryo. 3. Presence of hard and impermeable seed coats. ١٣  Types of seed germination 1- Epigeal germination In epigeal germination the cotyledons come out of the soil. The cotyledons become green and function as the first leaves of the seedling. ١٤  Epigeal germination ١٥ 2- Hypogeal germination In hypogeal germination, the cotyledons remain inside the soil. The cotyledons do not take part in photosynthesis. ١٦  Hypogeal germination ١٧  Stages of seed germination ١٨