What is Fertilization?
Fertilization is the fundamental step to initiate the reproductive system in plants. In this process, the female and the male gamete merge to produce a diploid zygote. A series of post-fertilization events take place in the zygote, and a seed develops.
Angiosperm is the most developed species of plant that consists of flowers, stems, leaves, and roots. Flower acts as a carrier of seeds. The formation of the seed occurs in the ovule a fruit develops, inside an ovary that forms in flowers.
In the reproductive system of a plant, after fertilization, several changes happen that cause the development of a fruit. Here are the four developmental steps to describe the post-fertilization changes in a flower -
Endosperm
Embryogeny
Seed
Fruit
Development of Endosperm
The endosperm, a tissue, present in the seed during the fertilization, precedes the development of an embryo. The development of endosperm can be categorised into three types -
Nuclear formation
Successive nuclear divisions occur to the primary endosperm nucleus and create a wall free nuclear endosperm.
Cellular formation
Subsequently, from the periphery, the creation of a cell wall also starts, and cellular endosperm occurs. Example: rice, coconut, maize, etc.
Helobial formation
In between nuclear and cellular endosperm formation, another intermediate endosperm formation happens, and that is known as helobial formation.
Objective of Endosperm
Endosperm tissue cells reserve starch as food and provide metabolic support to the embryo.
The endosperm also mechanically supports the embryo and helps in its development. Contrastingly, during seed germination, the endosperm is utilised by the seed.
Development of Embryo
The development process of an embryo from the zygote is known as embryogeny. This is the second post fertilization event. Mostly, after some endosperm formation, the zygote starts dividing. At first, the zygote develops to a pre-embryo and then it matures to a complete embryo.
The stages of embryogeny are similar for dicot and monocot plants. However, they have different structures.
Dicot Embryo
The embryo in a dicot plant has one embryonal axis and two cotyledons. The part of the embryonal axis above the cotyledon level is called epicotyl and hypocotyl, which is situated below the cotyledon level. The hypocotyls end in the root tip and the cover of the tip is known as root cap.
Monocot Embryo
Monocot plants have one cotyledon, called scutellum. Coleorhiza, an undifferentiated sheath, covers the root cap. Above the scutellum, the part of the embryonal axis is known as epicotyl. A shoot apex and some leaf primordia called coleoptiles are present in epicotyl.
Development of Seed
Usually, a Seed has Three Body Parts
Seed Coat
The seed coat is double-layered and made by integuments. Testa is the hard outer part of the seed coat. Tegmen is the inner part of the seed coat. A small opening on the seed coat, called micropyle, helps in water and oxygen movement in the seed.
Cotyledon
There are one or two cotyledons present in a seed. These cotyledons are swollen and thick due to the preservation of foods for the seed.
Embryo Axis
The embryonic root and shoot are together called the embryo axis. The part of this axis above cotyledon grows as a shoot, and the lower part becomes the root.
Various Types of Seeds
There are mainly two types of seeds in existence in the plant kingdom, these are - Monocotyledonous Seed and Dicotyledonous Seed.
Monocotyledonous Seed: as the name suggests, these are the seeds with only one cotyledon. Seeds of cereals are its examples
Dicotyledonous Seed: they have two cotyledons. Castor seeds are its example
Development of Fruit
A fruit develops through differentiation and cell division process in the ovary. The ovary wall matures into pericarp (which is the fruit wall). In some fruits like guava, cucumber, etc. the pericarp is fleshy. However, in pea, mustard, etc. a leathery pericarp is found.
The primary function of the pericarp is to protect the seed from any damage. It also helps the seed to disperse.
Types of Fruit
True Fruit
Without the association of the non-capillary part, when a fruit derives from the ovary of a flower, it is called true fruit.
Example: Mango, Guava, Tomato, etc.
False Fruit
Other than an ovary, when a fruit derives with the help of other accessory parts of a flower, it is called false fruit.
Example: Strawberry, Cashew Nut, etc.
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FAQs on Post Fertilization Events in Plants
1. What are the major post-fertilization events that occur in a flowering plant?
Following successful double fertilization, a series of changes known as post-fertilization events transform the ovule and ovary. The four primary events are:
- Endosperm Development: The Primary Endosperm Nucleus (PEN) develops into the endosperm, a nutritive tissue.
- Embryo Development (Embryogeny): The diploid zygote develops into an embryo.
- Seed Formation: The ovules mature into seeds.
- Fruit Formation: The ovary matures into a fruit, which encloses the seed(s).
2. What is the main function of the endosperm in a seed?
The primary function of the endosperm is to provide nutrition to the developing embryo. It is a tissue rich in reserve food materials like starch, oils, and proteins. In some seeds, like castor and coconut, the endosperm persists in the mature seed (albuminous seeds) and is used during germination. In others, like pea and bean, the embryo completely consumes the endosperm during its development (ex-albuminous seeds).
3. How does the structure of a dicot embryo differ from that of a monocot embryo?
The primary difference lies in the number of cotyledons and protective sheaths.
- A dicot embryo possesses an embryonal axis and two cotyledons. The portion of the axis above the cotyledons is the epicotyl (terminates in plumule), and the portion below is the hypocotyl (terminates in radicle).
- A monocot embryo has only one large, shield-shaped cotyledon called the scutellum. The radicle is protected by a sheath called the coleorhiza, and the plumule is protected by a sheath called the coleoptile.
4. What is the difference between a true fruit and a false fruit? Please provide examples.
The distinction is based on which floral parts contribute to fruit formation.
- A true fruit develops exclusively from the maturation of the ovary after fertilization. Examples include mango, tomato, and guava.
- A false fruit (or pseudocarp) develops from the ovary along with other accessory floral parts, such as the thalamus. Examples include apple, strawberry, and cashew nut, where the main edible part is the fleshy thalamus.
5. Why does endosperm development typically precede embryo development in angiosperms?
This sequence is a crucial evolutionary adaptation to ensure the viability of the offspring. Endosperm development begins first to create and store a reliable food supply. The plant invests energy in forming the embryo only after this nutritive tissue is established. This strategy prevents the wastage of resources on developing an embryo that might starve and die if a food source were not guaranteed, thus maximizing reproductive success.
6. What are parthenocarpic fruits and how are they different from normal fruits?
Parthenocarpic fruits are fruits that develop without the process of fertilization. Consequently, they are typically seedless. This process can occur naturally or be induced by applying growth hormones. A common example is the banana. In contrast, normal fruits are the result of successful fertilization, leading to the development of an ovary into a fruit and ovules into seeds.
7. After fertilization is complete, what is the fate of the other floral parts like petals, sepals, and stamens?
Once fertilization has occurred, the primary reproductive function of the flower is complete. The plant then redirects its energy towards developing the seed and fruit. As a result, non-essential parts like the petals, stamens, style, and stigma typically wither and fall off. In some plants, the sepals may persist and remain attached to the fruit, as seen in tomatoes and brinjals.
8. What are the key biological advantages of seed formation for a plant?
Seed formation offers several significant advantages for plant survival and propagation:
- Protection: The hard seed coat protects the dormant embryo from mechanical damage and harsh environmental conditions.
- Nourishment: Seeds contain stored food reserves (endosperm or cotyledons) to nourish the seedling during its initial growth.
- Dispersal: Seeds are adapted for dispersal to new locations by wind, water, or animals, preventing overcrowding and enabling colonization of new habitats.
- Dormancy: Seeds can enter a state of dormancy, allowing them to survive unfavorable conditions and germinate only when conditions are suitable.
9. What is polyembryony and how does it occur in plants like citrus?
Polyembryony is the phenomenon of having more than one embryo within a single seed. While one embryo develops from the zygote (sexual reproduction), the additional embryos in citrus fruits are often formed asexually. Cells of the nucellus or integument surrounding the embryo sac can become stimulated to divide and develop into embryos. These are called adventive embryos and are genetically identical to the parent plant.
10. What are the three main parts of a typical dicotyledonous seed?
A typical dicotyledonous seed consists of three principal parts:
- Seed Coat: The protective outer covering, which is differentiated into an outer, hard testa and a thin, inner tegmen.
- Cotyledons: These are two fleshy structures that typically store food reserves for the embryo.
- Embryonal Axis: This is the main axis of the embryo, which contains the plumule (future shoot) and the radicle (future root).
