''2,4 - D widely used to kill dicotyledonous weeds , does not affect mature moocotyledonous plants. ''

  why does  it afffect oly dicotyledonous weeds and not monocotyledonous weed.?

2, 4-D (2, 4-dichlorophenoxyacetic acid) are synthetic auxins. It selectively kills dicot weeds, and  does not affects mature monocot weeds. Monocots responds differently to auxinic herbicides.This fact could be explained on the basis of various reasons. Monocots have different vascular anatomy than dicots.

Monocot weeds are not killed by 2, 4-D because of either limited translocation or rapid degradation of exogenous auxin in monocots.