The male and female gametes in the flowering plants are produced in the pollen grains and embryo sac. The gametes produced by them are non-motile so they require agents for their movement. When they move, only then they will be brought together for fertilization. The process of pollination is responsible for this. By the process of pollination, the pollen grains are transferred to the stigma. Not just seeds, but there are fruits dispersed by water and also plants dispersed by water. We will study and learn about the characteristics of seeds dispersed by water. Abiotic and biotic factors are the two agents of pollination.
There are Three Types of Pollination, They are:
Autogamy: In this type of pollination, the anther is transferred to the stigma of the same flower. The plants have made many adaptations to ensure self-pollination. When both the whorls are present in the same flower, it is known as bisexuality. Homogamy is the time when both the anther and stigma matures at the same time. This ensures that both the male and female gamete mature at the same time. And the pollen grains and the stigma are matured in a synchronized manner. In cleistogamy, the flower does not open. This ensures that the anther gets fallen to the stigma. This is also very useful in the absence of any pollinating agents. Thus, it eliminates the pollinators from the process of fertilization.
Geitonogamy: In this, the pollens are transferred from the anther of one flower to the stigma of another flower but of the same plant species. Agents of pollination are required in this process. It can be termed as cross-pollination functionally and genetically it is similar to autogamy.
Xenogamy: In this, the pollen grains are transferred from anther of one flower to the stigma of another flower of different flower species. By this technique only, there are genetically different types of plants present.
The agents of pollination are of two types. They are abiotic and biotic agents. Wind and water come under abiotic agents of pollination whereas biotic factors include animals. Anemophily is pollination with the help of wind and hydrophily is pollination by water.
This is a technique of seed dispersal by water. Pollination or dispersal by water is seen in water lily seed dispersal. This pollination with the help of water is quite rare in flowering plants. It is limited to only thirty genera and they are mostly monocot plants. Examples are seed dispersal by water lotus, Hydrilla, marine water plant like Zostera. The characteristics of seeds dispersed by water are:
The pollen grains are light in nature and are generally unwettable.
The pollen grains are surrounded by a mucilaginous covering.
This covering protects them from being wet.
The stigma of such plants is long and sticky.
These characteristics of seeds dispersed by water give an advantage to the water plants to carry out the process of water pollination with ease.
The Technique of Seed Dispersal by Water Can Occur at Two Places. They are:
Epihydrophily: This process of dispersal by water occurs on the surface of the water. It is seen in the Vallisneria plant. In this process, the female part of the plant has a very long pedicel. This pedicel helps the plant to reach the surface of the water. The male flowers are found floating on the surface of the water. These male flowers then release the pollen grains on the surface of the water. These pollen are carried away by water. This is known as passive transport as no energy is used and this pollen goes with the flow of the water and not against the flow of water. Out of these, some pollen reaches the surface of the water and thus the stigma of the flower and then the process of fertilization takes place.
Hypohydrophily: This process is seen in Zostera. It is also known as seagrass. It is a marine water plant. In this, the female flowers are submerged inside the water. The pollen grains have a ribbon-like structure. Here, these pollen grains are carried by water. They are carried passively with the flow of water currents and not against the concentration gradient. Due to this, some of the pollen then reaches the stigma and then they achieve pollination.
The flowers in the majority of the plants that are pollinated by water are present above the level of water. These water plants can be thus pollinated with the help of wind and insects.
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The flowers that are pollinated by abiotic agents have developed special characteristics. These flowers are not very colourful. They also lack nectar. Flowers that are bright in colour, can easily attract insects and thus these insects can pollinate them. The nectar serves as a reward for them, for doing this pollination. The pollination that is carried out by abiotic agents is a kind of chance pollination. It is just a chance factor that the pollen grain may come in contact with the stigma of the flower. As it is a chance factor, these flowers produce pollen grains in very enormous quantities. The ovules are less as compared to the number of pollen grains. This gives the pollen grains a high chance of succeeding.
1. What do You Understand by Adhesion and Cohesion of Stamens?
Answer: Adhesion means the attachment of the stamen. In this condition, the stamen may get attached to the other floral organs such as petals and sepals. When they are attached to the petal, they are called epipetalous. When the stamen is attached to the perianth, it is called epiphyllous. The cohesion of stamens means that they may be free or united. Polyandrous is the name given to them when they are free. When they are united in a single bundle, they are called monadelphous. When they get united and form two bundles, they are known as diadelphous. Polyadelphous is the condition when they get united and form more than one bundle.
2. Describe the Structure of Pollen Grain.
Answer: The pollen grains are spherical structures and are about 25-50 micrometre in diameter. Sporoderm is the name given to their cell wall. This sporoderm consists of two layers that are the exine and the intine. The exine is the hard outer layer that is made up of sporopollenin. The sporopollenin is one of the most resistant organic materials. This layer can withstand high temperatures and even strong acids and alkalis. Till now, no enzyme that can degrade sporopollenin is known. This strong layer also helps in fossilization. This means that the pollen grains can be well preserved. This layer also protects the seed from various biotic and abiotic stresses. For the taxonomic significance, this layer also exhibits various patterns and designs.
Germ pore is the area where the sporopollenin is not present.