Pollen grain size is significant (see following figure). If a pollen grain is too large, it may not disperse well; however, if it is too small, there will be insufficient resources to produce a pollen tube long enough to reach the ovules. Corn pollen averages 120 μm in diameter, which is muchlarger than either horse chestnut or oak pollen. Unlike either of these tree pollens, corn has  to support the growth of a pollen tube up to 15 cm long.

Western ragweed pollen averages 21 μm in diameter and produces a pollen tube length of about 1 cm. Consider the differences in pollen production by corn (4 million grains per plant) and western ragweed (75 million pollen grains per plant). (Western ragweed pollen per stem was calculated from data presented in Wan et al., 2000, Response of an allergenic species, Ambrosia psilostachya (asteraceae), to experimental warming and clipping: Implications for public health. American Journal of Botany 89(11): 1843–1846.) Speculate how the characteristics of each plant’s pollen benefit the reproduction of its species.