The following points highlight the top two exercises to study germination of dormant and non-dormant seeds.
To show the normal process of germination of seeds of castor and pea.
Seeds of castor and pea, pots, garden soil, water.
Normal Germination Process:
Put some castor and pea seeds in the properly moistened garden soil filled in the pots and observe.
Oxygen, water and suitable temperature are the three main conditions necessary for germination. When these three conditions are appropriately available, the seeds absorb water from the soil through micropyle and start swelling. The seed coat becomes softened. Soon a seedling starts appearing.
During this period, the food material, stored in cotyledons or endosperm, is converted into soluble form and used by the growing embryo. Cell divisions start in the growing parts of embryo, i.e., radicle and plumule. The first part of the embryo which comes out of the seed coat is the radicle. Being positively geotropic, the radicle grows towards the soil.
The seed coat soon ruptures and the plumule, which lies between cotyledons, comes out. The radicle develops into roots while the plumule gives rise to shoot or stem.
On the basis of the behaviour of cotyledons the germination may be of following two types, i.e., epigeal and hypogeal.
(i) Epigeal germination:
It is a kind of germination (Fig. 215) in which the cotyledons are borne above the ground level, becoming the first photosynthetic organs of the seedling, as in castor (Fig. 215).
(ii) Hypogeai germination:
It is a kind of germination, in which the cotyledons remain below ground (Fig. 216). Their stored food is used up in the early growth of the epicotyl and the hypocotyl, as in pea (Fig. 216).
To study germination of non-dormant and dormant seeds.
If the three normal conditions necessary for germination (i.e., necessary oxygen, water and temperature) are available, the seeds of most plants germinate. But in some plants (e.g., Xanthium, Ginkgo and Cucurbita) the seeds do not germinate inspite of the availability of these three necessary conditions. Such seeds are called dormant seeds and this phenomenon is called seed dormancy.
Seed dormancy is, therefore, the inability on the part of the viable seeds to germinate under suitable conditions.
Causes of seed dormancy:
Some of the major causes of the seed dormancy are following:
(i) Mechanically hard seed coat.
(ii) Seed coat impervious to water and air.
(iii) Presence of immature or rudimentary embryo in the seed.
(iv) Presence of some specific chemical inhibitors which inhibit the entry of oxygen and water in the seed coat.
Germination of non-dormant seeds:
Non-dormant seeds (e.g., pea, castor, bean, etc.) can be germinated by providing suitable conditions of germination, i.e., proper amount of oxygen and water and required temperature as mentioned in the earlier exercise of normal process of germination (Exercise No. 1).
Germination of dormant seeds:
Dormant seeds (e.g., Xanthium, Cucurbita, Ginkgo, etc.) germinate only when the causes of dormancy are removed. It is, therefore, necessary to first ascertain or know the cause of dormancy (mentioned above in this exercise) and then to remove this cause. No generalized method for dormant seeds of all plants can, therefore, be applied for their germination.
For example, if the dormancy is due to mechanically hard seed coat, the seed coat should first be made less harder by dipping the seeds in water or some chemicals and then they should be sown under necessary conditions for germination of seeds.