The style in a flower is a pillar-like stalk through which pollen tubes grow to reach the ovary. Styles are always tube-like — either long or short. The style can be open (containing few or no cells in the central portion) or closed (densely packed with cells throughout). Pollen tubes grow the length of the style to reach the ovules, and in some cases self-incompatibility reactions in the style prevent full growth of the pollen tubes. Studies have shown that in some species, at least, the pollen tube is directed to the micropyle of the ovule by the style.