Primary endosperm nucleus divides repeatedly to form a large number of free nuclei.
No cell wall formation takes place at this stage (karyokinesis). A central vacuole appears later.
It is followed by cell wall formation which is centripetal. Hence, a multicellular endosperm is formed. It is the most common type.
The process of cell plate formation may not be complete as in the case of coconut. Its peripheral portion has outer oily multicellular solid endosperm and inner free nuclear, degenerated multinucleate liquid endosperm called coconut milk.
Here wall formation occurs immediately after the division of the primary endosperm nucleus. i.e. karyokinesis is followed by cytokinesis.
Subsequent divisions also are accompanied by cell wall formation. As a result, the endosperm becomes cellular from the beginning.
It is not common. It is mostly observed in dicots.
Example- Balsam, Petunia.
The first division of the primary endosperm nucleus is cellular i.e. wall formation takes place following the first division.
However, inside each of these newly formed cells, free nuclear divisions occur. But finally, the endosperm becomes cellular following the pattern of development of nuclear endosperms.
Hence, helobial endosperm is the combination of cellular and nuclear endosperms.
It is common in monocots.
Post Fertilization changes:
Development of Embryo:
The embryo develops at the micropylar end of the embryo sac where the zygote is situated.
Most zygotes divide only after a certain amount of endosperm is formed. This is an adaptation to provide assured nutrition to the developing embryo.
Though the seeds differ greatly, the early stages of embryo development (embryogeny) are similar in both monocotyledons and dicotyledons.
The zygote gives rise to the proembryo and subsequently to the globular, heart-shaped and mature embryo.
A typical dicotyledonous embryo consists of an embryonal axis and two cotyledons. The portion of embryonal axis above the level of cotyledons is the epicotyl, which terminates with the plumule or stem tip. The cylindrical portion below the level of cotyledons is hypocotyl that terminates at its lower end in the radical or root tip. The root tip is covered with a root cap.
Formation of fruits and seeds:
As ovules mature into seeds, the ovary develops into a fruit, i.e., the transformation of ovules into seeds and ovary into fruit proceeds simultaneously.
The wall of the ovary develops into the wall of fruit called pericarp. The fruits may be fleshy as in guava, orange, mango, etc., or may be dry, as in groundnut, and mustard, etc.
Many fruits have evolved mechanisms for dispersal of seeds.
In most plants, by the time the fruit develops from the ovary, other floral parts degenerate and fall off. However, in a few species such as apple, strawberry, cashew, etc., the thalamus also contributes to fruit formation. Such fruits are called false fruits Most fruits, however, develop only from the ovary and are called true fruits.
Structure of Seed:
The ovules after fertilization, develop into seeds. A seed is made up of a seed coat and an embryo. The embryo is made up of a radicle, an embryonal axis and one (as in wheat, maize) or two cotyledons (as in gram and pea).
The outermost covering of a seed is the seed coat. The seed coat has two layers, the outer testa and the inner tegmen. The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit.
In most of the species, fruits are the results of fertilization, there are a few species in which fruits develop without fertilization. Such fruits are called parthenocarpic fruits. Banana is one such example.
Parthenocarpy can be induced through the application of growth hormones like gibberellins and such fruits are seedless. E.g. seedless grapes
Seeds, in general, are the products of fertilisation, a few flowering plants such as some species of Asteraceae and grasses, have evolved a special mechanism, to produce seeds without fertilisation, called apomixis.
Thus, apomixis is a form of asexual reproduction that mimics sexual reproduction.
There are several ways of development of apomictic seeds. In some species, the diploid egg cell is formed without reduction division and develops into the embryo without fertilisation.
E.g. Family Asteraceae, some grasses.
Practical use of Apomixis:
Hybrid varieties of several of our food and vegetable crops are being extensively cultivated. Cultivation of hybrids has tremendously increased productivity. One of the problems of hybrids is that hybrid seeds have to be produced every year.
If the seeds collected from hybrids are sown, the plants in the progeny will segregate and do not maintain hybrid characters. Production of hybrid seeds is costly and hence the cost of hybrid seeds become too expensive for the farmers.
If these hybrids are made into apomicts, there is no segregation of characters in the hybrid progeny. Then the farmers can keep on using the hybrid seeds to raise new crop year after year and he does not have to buy hybrid seeds every year.
Because of the importance of apomixis in the hybrid seed industry, active research is going on in many laboratories around the world to understand the genetics of apomixis and to transfer apomictic genes into hybrid varieties.
In many Citrus and Mango varieties, some of the nucellar cells surrounding the embryo sac start dividing, protrude into the embryo sac and develop into the embryos.
In such species, each ovule contains many embryos.
The occurrence of more than one embryo in a seed is referred as polyembryony.
Significance of fruits and seeds:
Dormancy: It is a temporary suspension of growth. The growth inhibitors prevent germination. During this period seeds are dispersed at different places.
Viability: It is a functional ability of the seed to germinate after considerable dormancy period.
Reserve Food: A fully developed embryo is nourished by the food stored in the endosperm of cotyledons.
Protective Coat: Testa, the outer hard seed coat gives protection against mechanical shocks, fluctuations in temperature and dry condition. The testa has no effect of digestive juices on it.
Dispersal: Some seeds produce wing, a hair-like structure suitable for dispersal.
Edible Fruits: Many fruits are eaten by animals and seeds are thrown.
Hence fruit and seeds are main agencies for the spread of the species.