Generally pollengrain has two cells one vegetative and the other generative
so in many of the angiosperms about 60% of them shed off from anther at this two celled stage
and in remaining 40% of angiosperms the generative cell divides into two cells called as male gamets when present in the anther and at this three celled stage of the pollen grain they shed out..... three cells means one vegetative ,two male gametes

Pollen grains are the male gametophytes in flowering plants (angiosperms). They are responsible for the transfer of male gametes (sperm cells) to the female reproductive organs (pistils) for fertilization. The development of pollen grains involves several stages, and they are shed (released) from the anthers of the flower at a specific stage of development.

Pollen Development Stages:
1. Microsporogenesis: This is the first stage of pollen development, where the microspore mother cell undergoes meiosis to produce four haploid microspores.
2. Microgametogenesis: The microspore undergoes mitosis and develops into a mature pollen grain. This stage involves several sub-stages:
a) Pollen grain formation: The microspore undergoes mitosis to form a two-celled pollen grain, consisting of a generative cell and a tube cell.
b) Pollen maturation: The pollen grain undergoes further development and becomes a mature pollen grain. At this stage, the generative cell divides to form two sperm cells.
c) Dehydration and desiccation: The mature pollen grain becomes dehydrated and desiccated to protect it during dispersal.

Shedding of Pollen Grains:
Pollen grains are shed from the anthers when they reach the two-celled stage (2-celled stage). This occurs when the pollen grains are fully developed and ready for dispersal. The anthers open up, allowing the pollen grains to be released into the environment. The release of pollen grains is a crucial step in plant reproduction as it allows for the transfer of male gametes to the female reproductive structures of the flower.

Importance of Shedding Pollen at the 2-celled Stage:
Shedding pollen at the two-celled stage ensures that the pollen grains are mature and capable of successful fertilization. At this stage, the generative cell has divided into two sperm cells, which are necessary for double fertilization in angiosperms. One sperm cell fertilizes the egg to form the zygote, while the other sperm cell fuses with the central cell to form the endosperm, which provides nourishment to the developing embryo.

In conclusion, pollen grains in most angiosperms are shed at the two-celled stage of development. This ensures that the pollen grains are fully mature and capable of successful fertilization.