NEET Biology Notes Reproduction in Plants Development
Development of Endosperm
As the consequence of triple fusion, a male gamete fuses with the secondary nucleus, a triploid structure called Primary Endospesm Mother Cell (PEMC) is formed that divides by mitotic division and forms a mass of nutritive cells called as endosperm. Endosperm develops as a result of triple fusion from primary endosperm nucleus. It provides nutrition to the developing embryo.
- Endosperm is of two kinds in a single seed, in case of coconut (Cocos nucifera), i.e. liquid endosperm and cellular endosperm.
- Endosperm is hard and stony in Areca nut and datepalm.
- Endosperm sometimes has irregular or uneven surface due to surface layCtflS®! due to endosperm itself. Such endosperm is called ruminate endosperm.
- Effect of genotype of pollen on endosperm is called xenia. This effect was named by Fock in 1881.
Development of Embryo
The entrie series, pf, events that occur between fertilisation and matqijity of a plant is called embryogenesis. During development, the cells become progressively more specialised or differentiated. The first stage in the development of a plant zygote is pre-determined mode of development. It gives rise to an organised mass of cells called the embryo.
- Generally, a zygote divides transversly resulting in a small apical cell and large basal cell, from the 2-celled stage until the initiation of organs. The embryo is commonly called proembryo. It subsequently develops in globular, heart-shaped and mature embryo,
In dicot embryo, an embryonal axis and two cotyledons present. 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 terminates at the lower end in the radicle or root tip. The root tip is covered with root cap.
- In monocots embryo, possess only one cotyledon. In the grass family, the cotyledon is called scutellum that is situated towards one side of the embryonal axis.
- At lower end, the embryonal axis has the radicle and root cap enclosed in an undifferentiated sheath called coleorrhiza.
- The portion of the embryonal axis above the level of attachment of scutellum is the epicotyl.
- Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure the coleoptile.
Development of Seed
A seed is a fertilised mature ovule that possesses an embryonic plant, stored food (sometimes absent) and a protective coat or coats. Therefore, an ovule is changed into seed after some changes. The two integuments develop into seed coats. This outer one is called testa and inner one is tegmen. Some post-fertilisation changes in a seed are as follows:
- Ovule (megasporangium) forms seed.
- Ovary (carpel) forms fruit.
- Egg cell forms embryo.
- Nucellus forms perisperm.
- Secondary nucleus forms endosperm.
- Outer integument forms testa (i.e. outer seed coat).
- Inner integument forms tegmen (i.e. inner seed coat).
- Micropyle forms an opening in the seed (i.e. micropyle).
It required water, oxygen and suitable temperature. Initially, water is taken through micropyle.
Food stored in cotyledon is used by embryo during seed germination. Oxygen is needed for fast respiration to yield energy and temperature for proper functioning of enzymes.
- Hypogeal Germination
In hypogeal germination, the cotyledons remain inside the soil. Hypogeal germination occurs in pea.
- Epigeal Germination
In epigeal germination, the cotyledons are pushed above the soil. Epigeal germination occurs in bean.
- Viviparous Germination
The condition, when seeds germinate inside the fruit, while it is still attached to the parent, is called vivipary. Viviparous is found in mangroove plants such as Rhizophora.
Development of Fruit
A fruit is a seed containing part that develop from a fertilised ovary.
Fruits are classified into three kinds
- Simple Fruits
Fruits, which develop from single ovary of a single flower.
- Aggregate Fruits
Fruits formed from polycarpellary, apocarpous flowers. Each carpel forms a fruitlet and all fruitlets together form an aggregate fruit.
- Composite Fruits
Composite or multiple fruits are those, which develop from inflorescence, where flowers are crowded together.
Besides, there are schizocarpic fruits, that are many seeded and break into many pieces on maturity. These are lomentum as in Acacia and Mimosa; cremocarp as in coriander and carrort; carcerulus as in Ocimum; samara as in Hiptage, Acer and regma as in Geranium.
Most fruits develop only from the ovary and are called true fruits. In some species such as apple, strawberry, cashew, etc., the thalamus also contributes to fruit formation. Such fruits are called false fruits.
Dispersal of Seeds and Fruits
It takes place by following ways
Dispersal by Wind
For wind dispersal seeds are
- light and powdery, e.g. orchid
- Parachute mechanism, e.g. Pinus
- Censer mechanism, e.g. poppy, etc.
Dispersal by Water
Adapted for floating by developing fibrous, porous mesocarp and waterproof spongy coat or epicarp, e.g. lotus, coconut, etc.
Dispersal by Animals
For this, fruits and seeds develop following characteristics
- Attractive and sweet, e.g. mango and tomato.
- The presence of hooks or spines, e.g. Xanthium
- Sticky seeds, e.g. Cleome.
Mechanical Dispersal or Dispersal by Explosion
At maturity some fruits dry up and burst open throwing the the seeds out.
eg-: oxalis and impatiens
Special Modes of Seed Development and Fruit Formation Seeds, normally develop by the process of fertilisation. However, in some plants there are some other modes also for their development.
- Apomixis It 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.
- Polyembryony 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. Occurrence of more than one embryo in a seed is referred as polyembryony.
- Parthenogenesis It is the formation of embryo from an unfertilised egg or female gamete. Parthenogenesis have great practical uses in plant breeding and genetics. The useful characteristics of plants can be preserved for many generation.
- Amphimixis It is actually the process of sexual reproduction, which involves meiosis and syngamy. By meiosis, the diploid cells of the sporophyte give rise to haploid gametophytes, which produce male and female gametes. Syngamy, i.e. fusion of haploid gametes, results in the restoration of the diploid sporophytic generation.
seeds out, e.g. Oxalis and Itppatiens.
- Parthenocarpy It was earlier defined as the production of fruits without pollination or other stimulus. But now, it is defined as, the formation of fruits without fertilisation, e.g. banana.
Significance of Seed and Fruit Formation
- Seeds and fruits offer several advantages to angiosperms.
- Seeds have better adaptive strategies for dispersal to new habitats and help the species to colonise in other areas.
- They have sufficient food reserves, young seedlings are nourished until they are capable of photosynthesis on their own.
- The hard seed coat provides protection to the young embryo.
- Being the products of sexual reproduction they generate new genetic varieties leading to variations.
- Dehydration and dormancy of mature seeds are crucial for the storage of seeds, which can be used as food throughout the year and to raise crops for the next season.