A seed is the dormant phase of a plant species. It is the stage in the life cycle when the plant is not actively growing, but is, alive. A seed consists essentially of three types of tissue:
- Protective tissue;
- A food reserve tissue; and
- An embryo.
- STRUCTURE OF SEEDS
The protective tissue is called the seed coat or testa, and is actually dead. The embryo is derived from the fertilised egg cell and goes on to develop into a new plant. The food reserve tissue is a tissue that stores fuel which is used up when the seed eventually germinates. The reserve tissue can take one of two forms:
- Endospermic; and
|Embryo: An embryo is that tissue which will grow to develop into a new plant. An embryo consists of many cells and is derived from a fertilised egg cell.
The seed develops in the ovary from the ovule which contains an egg cell. The egg cell fuses with a sperm that eventually gives rise to the embryo. The rest of the ovule tissue is called the endosperm and is a storage tissue.
Some seeds begin to dry out early in their development and the resultant dry seed contains an embryo and an endosperm.
Other seeds however, continue to develop further before they dry out. In these seeds the embryo grows larger and in doing so uses up the food reserves in the endosperm. The result is a seed with little or no endosperm, but a large embryo. Most of this large embryo is actually a reserve tissue called the Cotyledon.
In Monocotyledons, there is only one Cotyledon; in Dicotyledons there are two. At this stage, it is important that
the student should realise that the storage tissue of monocotyledons and dicotyledons can be endospermic or cotyledonous depending on the species concerned. See Figure 1 on the following page.
Figure 1: An ovule showing the an endosperm reserve and a cotyledon reserve.
Figure 1. illustrates how a seed develops and shows diagrammatically the difference between seeds with the different types of reserve tissue. From the diagram the student will observe two structures in the embryo:
- The radical: this develops into the root system of the new plant; and
- The plumule: which develops into the stem and leaves and the surface structures of the new plant.
SPECIFIC SEED STRUCTURE
THE MAIZE SEED:
Maize (Zea Mays) forms the bulk of the diet of most people in Southern Africa. Maize is a monocotyledon belonging to the family graminea (the grasses). The reserve tissue is an endosperm. Figure 2 shows a maize seed.
It was mentioned earlier that the ovule is contained in an ovary which develops into a fruit. However, in Maize the ovary wall and the ovule wall or coat, fuse together to form the testa. This type of seed is called a Caryopsis.
THE BROAD BEAN:
The Broad Bean is a dicotyledon and a member of the legumiPCIe. The storage reserve is cotyledonous. The point where the bean is joined to the pod is called the Hilum. Figure 3 shows a broad bean seed.
Figure 2: Maize: This is a monocotyledon with the food reserve in the endosperm.
Figure 3: Beans This is a dicotyledon with the food reserves in the cotyledons.
THE PROCESS OF GERMINATION
When a seed forms, it begins to dehydrate until eventually the seed tissues have lost about 85% of their water content. At this stage the seed is mature and inactive. Before the seed can grow again it has to reabsorb lost moisture. Germination can be regarded as the reactivation of the seed tissues.
The process follows the course of reabsorbing water which then induces the tissues to live and induces embryo growth. As it grows it depletes the food reserves of the seed. Gradually, normal plant growth is restored and new plants are formed.
During germination the radical of the embryo grows first and breaks through the testa and penetrates the surrounding soil. The radical grows towards gravity and this is said to be positively geotropic. (Atropism is a directional growth in response to a stimulus, e.g. gravity or light).
The portion of radical just next to the seed is called the hypocotyl. The plumule then begins to grow and form the epicotyl which then develops into the stem and leaves.
Before a seed will germinate conditions have to be right. There must be adequate moisture in the soil which must be adequately aerated because a germinating seed needs oxygen. The temperature must also be right for germination.
TYPES OF GERMINATION
There are two types of germination:
- Epigeal; and
Monocotyledons do not undergo epigeal germination. However, not all dicotyledons undergo epigeal germination either. In this type of germination, the hypocotyls elongate and push the cotyledons up out of the soil. In some, but not all cases the cotyledons turn green and begin to photosynthesise. The plumule then grows and gives rise to the proper leaves. As can be seen from Figure 4 the hypcotyl is bent. Because of this fact this type of germination can be seriously hindered by the crusting of the soil surface which some types of soil are prone to. This is why soya beans and cotton can have problems germinating as they undergo epigeal germination.
All monocotyledons undergo this type of germination. An example is the maize seed and the broad bean, the former a monocotyledon and the latter a dictyledon. This type of germination is much easier on harder soil because the cotyledons are not pushed above the surface as they remain underground. In maize germination, the plumule spikes its way up through the soil and is protected by the Coleoptile. The broad bean has no Coleoptile but the plumule is hooked and this protects the growing tip. Figure 6 shows a germinating maize seed.
- SEED DORMANCY
Some seeds are capable of germination as soon as they reach maturity. Other seeds for various reasons enter a dormant phase and will not germinate even when conditions are perfect. The advantages of dormancy are complex, but it usually means that a seed cannot germinate and expose itself to unsuitable conditions, e.g. at the wrong time of the year. Seed dormancy can be broken by a number of factors depending upon the type of plant. Some seeds need a period of time to elapse before germination will occur. Other seeds, such as tobacco require illumination by sunlight before they will germinate. Chilling some seeds will break their dormancy. In this last case a period of cold weather followed by warmer weather indicates to the seed that Winter is over.
Figure 4: Epigeal germination of a common soya bean.
Figure 5: Hypogeal germination of a pea plant (this is the same for a maize plant).
Figure 6: Germination of a maize plant.
|Viable: capable of living or being alive.
The quality of seed can be assessed in two ways:
- Firstly, how much will actually germinate (viability); and
- Secondly, how long will it remain viable.
Checking viability can be done by selecting a sample of seed and providing it with suitable conditions for germination. The percentage of seed germinating after about a week gives one a measure of viability. The appearance of a radical is regarded as positive germination.
All seeds lose their viability with time, however, some more quickly than others. Tea seeds virtually lose all viability within 6 months. Wheat on the other hand can remain viable for nearly 30 years. Keeping quality can easily be assessed by conducting similar experiments as those for viability checking over specified periods of time.