Class 11 Biology
The word fruit conjures up images of colourful, juicy and tasty platter full of fruits. Most of us love to have fruits. In this lesson, you will learn about fruits from the perspective of biology. You will learn about structure of fruits and about different types of fruits.
Fruit is a mature, ripened ovary which develops after fertilization. If a fruit develops without fertilization, it is called parthenocarpic fruit. Banana is a good example of parthenocarpic fruit. Now-a-days, parthenocarpy is artificially induced to produce seedless watermelon, oranges and guavas.
A fruit usually consists of a wall and one or many seeds. The wall of the fruit is called pericarp. The pericarp can be dry or fleshy. A fleshy pericarp is differentiated into three layers, viz. epicarp, mesocarp and endocarp.
Depending on the nature of pericarp, there are three types of fruits, viz. berry, drupe and pome.
Berry: A fruit with juicy pericarp and without stone is called berry. The whole pericarp is edible in a berry. Examples of berry are: grapes, raspberry, etc.
Drupe: A fruit with a stone is called drupe. A stone is composed of a hardened endocarp and seed. The seed is enclosed in the endocarp. The stone is not edible. Mango, avocado and coconut are examples of drupe. In case of a mango, the skin of the fruit is exocarp, and the juicy edible portion is mesocarp. The seed is enclosed in a hard cover and the hard cover is endocarp. In case of a coconut, the hard shell is mesocarp. The brown layer on the edible part of a coconut is endocarp, and the white innermost layer is seed.
Pome: A pome is an accessory fruit. When the fruit is composed of ovary and some accessory tissues, it is called an accessory fruit. Apple is a good example.
Function of Fruit: Fruit is a means for dissemination of seeds. When an animal eats a fruit, it helps in dispersal of seeds by various means. For example, many birds swallow the fruit along with seeds. So, the seeds are dispersed along with faeces of the bird.
Ovules develop into seeds after fertilization. A seed is composed of a seed coat and an embryo. The embryo is composed of a radicle, an embryonal axis and one or two cotyledons.
Benefits of Seedds: Formation of seed is a big evolutionary step for plants. A seed can remain dormant for many years. A seed germinates only under favourable condition. This ensures proliferation of seed bearing plants all over the world.
The outermost covering of the seed is called seed coat. The seed coat is composed of two layers. The outer layer is called testa and the inner layer is called tegmen.
There is a scar on the seed coat through which the developing seed was attached to the fruit. This scar is called hilum. There is a small pore above the hilum, called micropyle.
The embryo lies within the seed coat. It is composed of an embryonal axis and two cotyledons. Cotyledons are usually fleshy and contain reserve food materials. Radicle and plumule are present at the two ends of the embryonal axis.
Endosperm is present in some seeds. It is important to recall that endosperm is formed from the Primary Endosperm Nucleus (PEN), and it supplies food for the growing embryo. Seeds with endosperm are called endospermous. When endosperm is not present in mature seeds, the seeds are called non-endospermous. Most of the monocot seeds are endospermous, while only some of the dicot seeds are endospermous, e.g. castor seed.
Monocotyledonous seeds are usually endospermic, but some are non-endospermic, e.g. orchids.
In the seeds of cereals, the seed coat is membranous and is usually fused with the fruit wall.
The endosperm is bulky and stores food. The outer covering of endosperm separates the embryo by a proteinous layer. This layer is called aleurone layer.
The embryo of monocot seed is small. It is situated at one end of the endosperm. It consists of one large and shield-shaped cotyledon which is known as scutellum. There is a short embryonal axis, with radicle and plumule.
The plumule and radicle are enclosed in sheaths. The sheath near the plumule is called coleoptile, and the one near the radicle is called coleorrhiza.
The semi-technical description of a typical flowering plant should be brief. It should be written in simple and scientific language. It should be in proper sequence. The description begins with habit and habitat, vegetative characters and then floral characters, inflorescence and floral parts.
Floral Formula: This is a short description of a particular flowering plant which shows key characters through various symbols. Various symbols used in floral formula are as follows:
K (for calyx)
C (for corolla)
P (for perianth)
A (for androecium)
and G(for gynoecium)
G (with a line under it) is used for superior ovary and G (with a line above it) is used for inferior ovary, ♂ is used for male flower, ♀ is used for female flower, ⚥ is used for bisexual flower, ⊕ is used for actinomorphic flower and % is used for zygomorphic flower.
Fusion of floral part is indicated by enclosing the figure within bracket. Adhesion of floral parts is indicated by drawing a line above the symbols of the floral parts. A floral diagram gives the information about the number of different parts of a flower, their arrangement and their relation with one another. The position of the mother axis is shown by a dot on top of the floral diagram. Different whorls of the flower are drawn in correct sequence.
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