Let us learn about Diversity in Fruits. After reading this article you will learn about: 1. Development of the Fruit 2. Fruit Ripening 3. Parthenocarpy of Fruit 4. Functions of Fruits.
Development of the Fruit:
With the result of fertilization the embryo develops and a series of other changes takes place in the ovule and as a result the seed is formed. The development of the embryo remains associated with a series of changes in the ovary wall and other floral parts.
These changes result in the development of the fruit from the ovary. In other words the mature or ripened ovary is known as a fruit. Whenever the fertilization does not take place, the ovary simply withers and falls off.
A fruit mainly consists of two parts:
(i) The pericarp developed from the ovary wall and
(ii) The seeds developed from the ovules. In certain cases (e.g., oranges, grapes bananas, etc.) the ovary may develop into the fruit without fertilization. Such fruit is seedless and known as parthenocarpic fruit.
In most of cases the pericarp consists of three parts:
(i) Epicarp – the outer part, which makes the skin of the fruit;
(ii) Mesocarp the middle part, which makes the pulpy part of the fruit, as in mango, peach, etc., and
(iii) Endocarp, the inner part which is very often thin and membranous, as in orange, or it may be hard and stony as in mango, plums, etc. In some cases the pericarp is not differentiated into three parts.
In the cases where only the ovary of the flower develops into the fruit, it is called the true fruit, but in other cases where the other floral parts such as the thalamus, receptacle or calyx also take part in the development of the fruit; the fruit is said to be false fruit, spurious fruit or pseudo-carp.
For example, in apple and pear the thalamus grows round the ovary and becomes fleshy. In Dillenia, the calyx is persistent and fleshy and makes the main part of the fruit.
In rose the enlarged thalamus bears small true fruits on its inner concave surface. In cashewnut the peduncle and thalamus develop into a false fruit or pseudocarp, whereas the true fruit is a nut, and develops from the ovary seated on the swollen peduncle.
The aggregate fruit of custard apple having number of small true fruits fused together, makes a spurious fruit. Lastly, the fruits which develop from an inflorescence as those of mulberry, pine apple, jack fruit, fig, banyan, peepal, etc., also known as spurious fruits.
In the development of a fruit both cell division and cell expansion are involved. The growth of the ovary after fertilization may be rapid. For example, the pumpkin ovary shows a 20 fold increase in about two weeks time.
All the flowers borne on a plant do not always mature into fruit. In mango for example, fruit- set is extremely low as compared to the total number of flowers produced. Shedding of flowers may occur before or after anthesis; even young fruits may drop.
Dropping of blossoms and young fruits is of advantage to the cultivator because it results in the production of fruits of large size. In certain orchard trees, such as apples, pears and plums, flowers thinning by hormonal spray is a regular practice.
As soon as the growth of the ovary wall due to cell division and cell enlargement stops, the fruit is said to be mature. This is followed by the final phase of fruit development called fruit ripening. During this period the conversion of starch into sugar, the reduction in the concentration of acids, the production of various esters, and the breakdown of chlorophyll lead to changes in colour, texture, taste, and flavour of the fruit.
A mature fruit of mango is hard and green and its edible portion white and sour.
On ripening, the mesocarp becomes yellow-orange, juicy and sweet. A characteristic feature of ripening of some fruit (like banana) is a sudden increase in respiration which is known as the climacteric. After this stage, the fruit starts decaying. Although from the point of view of the fruit, ripening leads to death, for man and other animals it is a stage which is most useful.
Parthenocarpy of Fruit:
Some plants are able to form fruits without fertilization. Such fruits are called parthenocarpic and the phenomenon is known as parthenocarpy. Parthenocarpic fruits are either seedless or contain empty or nonviable seeds. In these fruits, the “seed-factor” for fruit growth is provided by the tissue of the ovary wall itself.
Seedless varieties of grapes and oranges have been reported to have up to seven times as much auxin in the ovaries of un-pollinated flowers as seeded varieties.
This may explain the normal development of parthenocarpic fruits even in the absence of healthy seeds. Most commonly cultivated varieties of banana are parthenocarpic. Parthenocarpic oranges and watermelons are also very common. Even in those plants which normally bear seeded fruits, parthenocarpy could be induced by the application of low concentrations of axuin or gibberellin.
Parthenocarpy is of great commercial value. Parthenocarpic fruits are ideal for consumption as such, or in preparation of jams and fruit juices on a commercial scale. You can imagine the annoyance caused by seed while eating a watermelon or guava.
Biological Significance of Fruit Formation:
Fruits (of all categories) were used by the ancient man as his main food and even today they form an important part of man’s diet. However, we might ask the question: “why does a plant invest so much of its food in the production of fruit?” Certainly not for human consumption. The fruit serves the plant in various ways. It protects the immature seeds against hostile climatic conditions and animals.
The seeds remain enclosed in the fruit until they are ready to germinate or at least withstand the possible unfavourable environmental conditions. The function of seed protection by the fruit wall is also achieved through its colour. When young, most fruits are green and remain hidden in the green foliage.
As the seeds mature, the fruit acquires bright colours and attract the seed dispersing agents. Immature fruits also offer chemical defence against the animals as they often contain such unpalatable and repelling substances as astringents, tannins, bitter alkaloids and sour acids which disappear upon ripening.
Another important role played by the fruit tissues is in the dispersal of seeds to distant localities which is of great biological significance.
Functions of Fruits:
The fruits are meant for the protection of the seeds. They are the main instruments of seed dispersal. The fruit wall is variously modified in many fruits for dispersal by wind, animals and water. The fleshy fruits by their decay enrich the soil with inorganic acids and thus provide a good start to the young seeding’s which develop from the seed. Many fruits are edible and make the main food of human beings.