A seed is a small undeveloped plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm and angiosperm plants which occurs after fertilization and some growth within the mother plant. The emergence of the seed completes the process of reproduction in plants that begins with the development of flowers and pollination. The formation of the seed completes the process of reproduction in plants that initiated with pollination. The new seed is formed in plant development called fruits. In this section, we will study how seeds are formed.
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A seed is a small plant (in an embryo) enclosed in a coat often with some stored food. It is formed when the fertilised ovule divides by meiosis.
Fertilisation is the process of fusion of male gamete and female gamete to form a zygote. In the process of pollination, pollinating agents like wind, water, birds, or insects transfer pollen grains from the anther to the stigma of the flower. Once pollen is transferred to stigma, the male gametes from pollen grains release and fuse with the egg in the ovule to form a zygote. Fertilisation takes place, and the formed zygote divides and develops into an embryo.
Following the fertilisation, every part of the flower sheds off, except ovary. The ovary of the flower develops into the fruit while ovules develop into seeds. The creation of seed marks the process of plant reproduction. Within the seed, the growing embryo develops and matures. Fig 2
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Structure and Function of a Seed
A seed is made of a small undeveloped plant (the embryo), which is surrounded by a protective coat (the testa). Typically small in size and making zero demands upon their environment, seeds are essentially suited to perform a wide variety of functions, the associations of which are not always apparent such as -multiplication, perennation or surviving seasons such as winter), dormancy (a state of arrested development), and dispersal. Pollination and dispersal are important factors responsible for the evolutionary success of flowering plants.
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Importance of Seeds
Seeds are vital because they are the source of different foods – for example, the cereal grains, such as wheat, rice, and corn or maize, the seeds of beans, peas, peanuts, soybeans, almonds, sunflowers, hazelnuts, walnuts, pecans, and Brazil nuts. Cooking oils, production of margarine, oil paints, and lubrication are available from flaxseed, rapeseed, cottonseed, soybean, poppy seed, castor seed, coconut, sesame seed, safflower, sunflower, and various grains. Essential oils are obtained from juniper berries. Stimulants are obtained from seeds of coffee, kola, guarana, and cocoa. Spices like mustard and nutmeg seeds, oil from the aril or mace covering the nutmeg seed, the seeds, and fruits of anise, cumin, caraway, dill, vanilla, black pepper, allspice, etc. from a large group of commercial products.
We can see seeds that fall on the ground are not always be planted by humans. The seeds are dispersed by agents such as wind, water, animals, etc. Seeds need adequate sunlight, water, space, and minerals for proper growth. Scattering of seeds in different places avoids the problem of competition for basic requirements and leads to the healthy growth of the plant. The dispersal of seeds is natural as well as artificial. Gardeners follow the synthetic methods of seed dispersal. The natural means of seed dispersal are:
Dispersal by Wind
Hairy seeds of a few plants such as dandelions have feathery bristles and can be carried away by the wind. These are blown away by the wind and land in different places. Some seeds may land at locations suitable for growth.
Dispersal by Water
Sometimes, seeds float in water away from the parent plant and get dispersed. E.g. If a mangrove seed falls in the water, it is carried away through streams and rivers to grow somewhere else.
Dispersal by Animals
Animals also disperse seeds in many ways. Some plants produce seeds inside fleshy fruits that are eaten by an animal. The fruit gets digested by the animals, and the seeds are passed out through the digestive tract. Squirrels bury the seeds but might not return to take them. These seeds grow into a new plant.
1. What is Fertilisation?
Seed formation begins with the combination of a male and female gamete: a process known as fertilisation. Fertilisation, occur when both male and female gametophytes are fully mature. This usually occurs in a dual fusion process known as double fertilisation. When the pollen grain deposit on the stigma, it germinates by building a pollen tube, which grows down the style, through the micropyle and into the embryo sac, with the tube nucleus following the tube apex downwards. The tube nucleus breaks, but the two pollen sperm cells enter the embryo sac, one fusing with the diploid polar nucleus to form a triploid endosperm nucleus and the other fusing with the egg cell to form a diploid zygote or fertilised egg.
2. What are Gametes?
The sex cells of plants are known as gametes. Plants have sperms and egg cells that are needed to fuse to produce a zygote to create a new plant. The plant ovary produces a female egg cell. The male sperm cell is encased in a pollen grain. Plants have two meiotic divisions- haploid cells from male and haploid cells from female – they form a diploid for the new plant. In flowering plants, the male gamete inside the pollen grain is located on the anther. Male gametes in the anther are large nucleus-containing cells. These cells go through meiotic division, forming tetrads, bundles of gametes- they become pollen grains.