Nature of the Endosperm in Angiosperms
Nature of the Endosperm in Angiosperms
Nemec (1910), brink and Copper (1947), suggested that fusion of second male gamete with the polar nuclei serve two functions:
- It stimulates the development of endosperm.
- It helps in the formation of the tissue which is physiologically more suitable for the nourishment of the embryo. Thus, it can be concluded that the endosperm in angiosperms is neither diploid nor haploid but it is an undifferentiated tissue which shows different degrees of polyploid from 2n to 15n. For example, in Oenothera 2n, in Polygonum 9n and most of the angiosperms 3n; in Ditepalanthus 4n; in Fritillaria 5n; in Peperomia 9n and in Pereromia hispidula 15n.
Significance of Double Fertilization in Angiosperms
- In angiosperms the nuclear division stops abruptly at 8-nucleate stage. During fertilization one of the male gametes untie with the egg and the other male gamete unites with the fusion nucleus to form endosperm nucleus. The endosperm nucleus rests for sometimes and then starts free nuclear division resulting in the formation of endosperm tissue. Therefore, it can be safely concluded that fusion of second male gamete with the fusion nucleus during double fertilization provides a stimulus without which the fusion nucleus would have remained inactive and the process of free nucleus division which had been terminated at 8-nucleate stage before fertilization, remained suspended for good, resulting in the non-formation of endosperm for the developing embryo.
- The process of double fertilization serves as a safeguard against unnecessary waste of energy by the angiospermous plants. Unlike gymnosperms the endosperm formation in angiosperms follows fertilization and in case the fertilization fails for certain reasons, there is no endosperm formation is not linked with fertilization resulting in the wastage of energy because the endosperm is there and the embryo is not.
- Xenia and metaxenia are also brought about through the courtesy of double fertilization which leads to endosperm formation.
When sexual reproduction in a flower is replaced by some form of asexual reproduction, the phenomenon is called Apomixis. During this phenomenon structures concerned in sexual reproduction are involved.
It may occur as various forms Such as:
- Parthenogenesis: In, this case the embryo develops from unfertilized egg, i.e., there is no fusion of male gamete and the egg cell.
It is a rare phenomenon in angiosperms. In these plants the meiosis does not take place during megasporogenesis and the egg nucleus is diploid. In some plants the embryo develop from haploid (n) egg.
- Apogamy: Sometimes the embryo develops from a cell of embryo-sac other than the egg. This is known as apogamy. This phenomenon is noted in, Allium, Iris etc. The embryo developing from a synergid or antipodal cell may be haploid or diploid.
- Sporophytic Budding: Sometimes the embryos arise by budding from the cells of nucellus or integuments (diploid cells of sporophyte) which project into the embryo-sac. This is known as sporophytic budding, e.g., in Citrus, etc.
In many angiosperms, more than one visible embryos develop in the seeds. Formation of more than one embryos in an ovule is called polyembryony. It is rare in angiosperms and more common in gymnosperms.
The polyembryony arises in various ways:
- The oospore produces more than one embryos during development as a result of splitting. This termed as Cleavage Polyembryony and is found in Allium, Citrus, etc.
- Additional embryos called Adventitious Embryos may be produced as a result of sporophytic budding, e.g., in Lemon (from cells of nucellus) and Onion (from cells of integuments).
- An ovule may contain more than one functional megaspores which develop into embryo-sacs and oospheres, which on-fertilization give rise to more than one embryos.
- Embryos maybe formed from synergids, in addition to the embryo formed from oosphere or from antipodal cells.
Seed Formation in Angiosperms
The seed develops from the ovule after a few changes. The two integuments develop into two Seed Coats, the outer Testa and the inner Tegmen. In some seeds the testa and legumen fuse to form a single coat. The micropylar of the ovule persists in the seed in the form of a pore while a small scar, the Hilum, represent the point of attachment of the ovule to the funicle and it can be seen on one side of the seed.
In some seeds an additional investment grows up around the tests and surrounds the seed. It may develop from placenta, funicle and micropylar and is known as Aril, as in Litchi. In certain seeds a small fleshy outgrowth of the seed is formed at the micropyle called Caruncle e.g., Caster-oil seed. Nucellus is generally exhausted. In some cases, it persists as a small thin layer called Perisperm.
The synergids and the antipodal cells are completely disorganized.
Development of Fruit in Angiosperms
The stimulus of fertilization result in the formation of fruit as a result of changes in the ovary walls. The style and stigma dry up and disappear and the ovary enlarge. The wall of a ripened ovary form the fruit wall or Pericarp which may remain soft and fleshy or become dry” and hard. When the ovary of the flower grows into the fruit, it is known as True Fruit. But sometimes other parts of the flowers such as receptacal or perianth leaves take part in the formation of fruit, such fruit is called False Fruit or Pseudopodia, e.g., Apple, Pear, Mulberry, Pineapple, etc.
Parthenocarpy in Angiosperms
As a rule, the development of ovule into seed and ovary into a fruit takes place after pollination and fertilization. In most species of flowering plants, the pollination result in failure of seed and fruit formation. But in some plants the fruits are formed even in the absence of pollination and fertilization. Such seeds are seedless and are said to be Parthenocarpic while the phenomenon is called Parthenocarpy. The examples are Oranges, grapes, Bananas, Pineapple.