The flowering plants show two generations i.e. sporophyte and gametophyte, whichalternate with one another, during life cycle.A sporophyte is a dominant multicellular, diploid generation which is formed from zygote or fertilized egg. This generation produces spores (microspores and megaspores) by meiosis. T
he gametophyte is a small multicellular, haploid generation, which is formed from spores. The male gametophyte develops from microspore or pollen grain; and the female gametophyte develops from megaspore. The male and female gametophytes produce male and female gametes (sperms and egg) by mitosis. The gametophytes in flowering plants are short lived, much reduced and dependent for protection on sporophyte.
Double fertilization
The pollen tube grows via style through micropyle into the ovule.
Seed and Fruit Formation
Seed and Fruit Formation, Ovule after fertilization forms seeds. Zygote forms the embryo by mitosis. This consists of tiny radicle, a plumule with one or two short special leaves called cotyledons. Cotyledons store the food of endosperm in some seeds and become thick e.g. bean seed. The outer integument of ovule forms the testa and the inner integument forms tegment. Testa and tegmen constitute seed coat. Seeds provide protection and nourishment to the embryo.
After the formation of seeds the ovary wall becomes fleshy or scaly and forms fruit wall. The floral parts i.e.sepals, petals, stamen, stigma shrivel and fall off. This ripened ovary with seeds is the fruit.
Parthenocarpic fruits may be formed by the ovary in which no fertilization occurs as it happens in fruits of banana, seedless varieties of grapes etc. in some other plants, unfertilized egg develops into new plant and this process is called parthenogenesis.
Dispersal of seeds and fruits
The seeds are the embryos of flowering plants in dormant stage. Fruits protect the seeds and help in the dispersal of the plant i.e. seeds and fruits may be carried a long distance from the parent plant. This reduces competition for light, water and other nutrients among the members of same species. The seeds and fruits may be dispersed by wind, by animals or by water.
Dispersal by wind
The smaller seeds and fruits are carried by wind to distant places. These seeds and fruits are light in weight and may also have wings or hairs. This helps them to be carried away by wind, away from the parent plant.
Examples are seeds of cotton, Bombax (sumbal), calotropis, grasses and Dandelion; and fruits of maple etc.
Dispersal by animals
The fruits and seeds of some plants have hooks or spines, which may be entangled in the fur of many mammals and are taken away from the site of parent plant. For example, famedicago (maina), cocklebur, and Achyranthus (Puthkanda)
Some fruits and seeds are eaten by animals. The seeds are not digested and are passed out of the body of the animal alongwith faeces. Thus these are carried away from the site of the parent plant to new growing places e.g. guava, mulberry and ficus.
Dispersal by water
The fruits and seeds of some plants which grow in or near water are dispersed by water. The fruits of these plants may contain spongy tissues or fibrous fruit walls which allow them to float on the surface of water to be carried to distant places e.g fruit of lotus, coconut, and daffodils.
A (i) Seeds of dandelion have filamentous tufts that are dispersed by wind.
A (ii) Winged fruits of maple dispersed by wind.
Fig.16.10 Examples of fruits and seeds. (a) dispersed by wind (b) dispersed by animals (c) dispersed by water
Structure of Seed
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?
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.
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.
- In which tubes the seeds germinate?
- Did all the seeds in these tubes germinate, if not, why? Name the conditions necessary for the germination of seeds?