Structure of Seed – A seed consists essentially of a seed coat, stored food and a young plant, the embryo. The seed coat may be made of inner tegmen and outer testa. On the seed coat a scar (hilum) is present which shows the place of attachment of a seed in the fruit is present. At one end of the hilum is the micropyle. Through micropyle the seed absorbs water during the process of germination.
The embryo consists of epicotyl,one or two seed leaves called cotyledons, and a hypocotyle. The portion of the embryo above the point of attachment of cotyledons to embryo axis, and below the plumule is called epicotyl. The plumule forms shoot.the portion of the embryo below the point of attachment of cotyledons to embryo axis but above the radicle, is called hypocotyls. The radicle forms root.
The seed also contain the food, derived from the endosperm tissue. The food may remain as such or may be digested and stored in the cotyledons. Thus the cotyledons become thick due to the presence of stored food. This food is supplied to the developing plant during the process of germination of seeds. In the grain of maize, the cotyledon is modified into an organ called scutellum a term that comes from the Latin word meaning “Shield”. The food stored in scutellum is used first during germination of seed. Later on scutellum absorbs the food of endosperm and supply it to the developing embryo.
“The maize grain is infact a single seeded fruit in which seed coat fruit wall is fused.”
Practical Work: investigating structure of seed of gram and maize
With the help of hand lens locate micropyle and hilum of the two seeds. With the help of scalpel cut the seeds longitudinally and observe different parts of the embryo along with endosperm with the help of (fig 16.11)
1. What is the function of micropyle?
2. Why can’t you find endosperm in gram seed?
16.5.1 Seed germination
During germination (growth and development) of a seed the water is absorbed through micropyle, and the formerly dormant embryo resumes growth and emerges from the seed. The root is normally the first structure emerging the seed, growing rapidly and absorbing water and minerals from the soil.
Much of the water is transported to the cells in embryo. As its cells elongate the stem pushes out of the soil. There are two types of germination, epigeal and hypogeal. When the cotyledons appear above Fig 16.12 (a) Hypogeal germination in gram seed the ground by the elongation of hypocotyl, the type of seed germination is called epigeal, e.g. seeds of melons, cucumber and beans[Fig 16.12(b)].
When the cotyledons remain below the soil, and epicotyl elongates bringing only plumule above the soil surface, it is described as hypogeal germination, e.g. seed of gram and maize grain[Fig 16.12(a)]
Germination is the onset of growth of a seed, often lollowing a period of dormancy, in response to suitable environmental conditions.
16.5.2 Conditions
For germination to take place the seed must be viable (living) and should have sufficient food for its germination. The following environmental conditions must exist.
In some seeds such as of tobacco plant light is also necessary to trigger germination of seeds
(i) Moisture or water (ii) Air(O2) (iii) Suitable temperature
(i) Moisture or water
Through micropyle, the seed absorbs water, which swells it causing the seed coat to burst. This helps the root to emerge out of seed. Water absorbed by the germination seed helps to activate the enzymes to digest the food of seed, which is made available to the growing embryo. It also helps to elongate hypocotyls and epicotyl, to grow out.
(ii) Air
Because oxidative metabolism usually takes over soon after a plant embryo starts to grow, most seeds require oxygen for germination. So air is very important for germination as it contains about 21% of oxygen, which is used during respiration.
(iii)Suitable temperature
Germination of seeds occur over a wide temperatue range between 5-30 C. the optimum temperature for the germination of seeds of most plants ranges between 25-30 C. optimum or suitable temperature is necessary for the enzymes to function.
During early germination of seeds, the vital mobilization of food reserves stored in the embryo cotyledons or the endosperm is mediated by hormones, which in some cases are gibberellins.
Practical work: investigating the conditions necessary for germination
Set up four test tybes each with ten seeds labeled A, B, C and D as shown in Fig.16.13.
Put tubes A, C and Din a warm place, in laboratory,
Put tube B in a refrigerator.
The test tube will have the following environmental conditions:
- Water, suitable temperature and air
- Water and the air (low temperature)
- Suitable temperature and air
- Water and suitable temperature (no air)
Observe the test tubes regularly for about one week.
1. In which tubes the seeds germinate?
2. Did all the seeds in these tubes germinate, if not, why? Name the conditions necessary for the germination of seeds?