Final answer:
Pollen grains remain in the 2-celled stage during the progamic phase to ensure efficient division of the generative cell into two viable male gametes for fertilization as the pollen tube grows towards the ovule.
Step-by-step explanation:
Pollen grains remain in the 2-celled stage during the progamic phase primarily due to the developmental timing and functionality required for successful fertilization. The progamic phase occurs after a pollen grain adheres to the stigma and the pollen tube cell begins to grow. During this phase, the generative cell, which is housed within the pollen tube cell, travels down the pollen tube and undergoes mitotic division to form two sperm cells. The reason that the pollen grain remains in the 2-celled stage and does not divide into the 3-celled stage until after the pollen grain has reached the stigma is that it allows the pollen tube to grow and extend toward the ovule without the added metabolic burden of additional cells.
Proteomic analysis has shown that the generative cell's division is necessary to produce two male gametes (sperm cells). However, this division is more efficient and occurs at a more opportune time when it takes place within the elongating pollen tube as it navigates the style. It ensures that the male gametes are freshly divided and viable by the time they reach the female gametophyte for double fertilization. The elongated pollen tube not only assists the non-motile sperm in reaching the ovule but also provides nutrients and a protective environment during their journey.