1) 2 Celled stage . ( 1 vegetative and 1 generative )

The Stage at Which Pollen Grains Shed in Angiosperms

In over 60% of angiosperms, pollen grains are shed at the 3-celled stage. This stage is known as the mature or tricellular stage of pollen development. Let's delve into the details to understand why this stage is crucial for the dispersal of pollen in angiosperms.

1. Introduction to Pollen Development
Pollen development, also known as microsporogenesis, occurs within the anthers of angiosperms. It is a complex process that involves the formation of microspores, which eventually develop into mature pollen grains. This process can be divided into several stages.

2. Early Stages of Pollen Development
During the early stages of pollen development, a microspore mother cell undergoes meiosis, resulting in the formation of four haploid cells called microspores. Each of these microspores contains a single nucleus.

3. Formation of the Pollen Grain
The microspore undergoes further development, and the nucleus within it divides mitotically to produce two nuclei. One of these nuclei becomes the generative nucleus, while the other becomes the vegetative nucleus. The generative nucleus is responsible for fertilization, while the vegetative nucleus plays a role in pollen tube formation.

4. The Mature or Tricellular Stage
At the mature or tricellular stage, the pollen grain consists of three cells - two sperm cells and one vegetative cell. The vegetative cell surrounds the two sperm cells. This stage is crucial for pollen dispersal and fertilization.

5. Pollen Shedding
In angiosperms, the anthers mature and undergo dehiscence, allowing the pollen grains to be released into the environment. At this point, the pollen grains are ready for pollination. The release of pollen occurs through various mechanisms, such as wind, insects, birds, or other animals.

6. Significance of the 3-Celled Stage
The 3-celled stage is advantageous for angiosperms because it ensures the delivery of two sperm cells to the ovule for double fertilization. One sperm cell fertilizes the egg cell to form the zygote, which develops into the embryo. The other sperm cell fuses with the polar nuclei to form the endosperm, which provides nourishment to the developing embryo.

Conclusion
In over 60% of angiosperms, pollen grains are shed at the 3-celled stage. This stage is crucial for successful pollination and double fertilization in angiosperms. The 3-celled stage ensures the delivery of two sperm cells to the ovule, leading to the formation of both the embryo and the endosperm. By shedding pollen at this stage, angiosperms have evolved an effective strategy for reproduction and propagation.