Step 1: Understanding the Concept:
The development of the male gametophyte in angiosperms begins within the microsporangium of the anther.
Microsporogenesis leads to the formation of haploid microspores from diploid pollen mother cells.
Each microspore is the first cell of the male gametophyte generation and must undergo maturation before it is released as a pollen grain.
This maturation involves specific mitotic divisions and cellular differentiation that prepare the pollen for its journey to the stigma.
Step 2: Detailed Explanation:
When a microspore matures into a pollen grain, its nucleus undergoes an asymmetric or unequal mitotic division.
Before this division, the nucleus moves to one side of the cell as large vacuoles develop in the cytoplasm.
The unequal division results in two cells of very different sizes and functions:
1. The Vegetative Cell (Tube Cell): This is the larger cell that contains a vast amount of cytoplasm and abundant food reserves in the form of starch and proteins.
It possesses a large, irregularly shaped nucleus.
The primary role of the vegetative cell is to govern the growth of the pollen tube once the pollen grain lands on a compatible stigma.
2. The Generative Cell: This is a small, spindle-shaped cell that initially adheres to the pollen wall but later detaches and floats freely within the cytoplasm of the vegetative cell.
It has dense cytoplasm and a prominent, often spherical nucleus.
The generative cell is destined to divide and produce the two male gametes required for double fertilization.
In approximately 60% of all angiosperm species, the pollen grain is shed from the anther at this specific two-celled stage.
In the remaining 40%, the generative cell divides into two male gametes before shedding, resulting in a three-celled stage.
However, the question asks for the "general" condition, which refers to the majority of species.
Thus, the most common state at the time of anther dehiscence is the two-celled stage.
Step 3: Final Answer:
The pollen grain is generally shed at the two-celled stage, consisting of one vegetative cell and one generative cell.