The stigma, usually found at the tip of the style, is the portion of the carpel that receives pollen. It is commonly sticky or feathery to capture pollen. The style is a pillar-like stalk through which pollen tubes grow to reach the ovary. The style and stigma of the flower are involved in most types of self incompatibility reactions. Self-incompatibility, if present, prevents fertilization by pollen from the same plant or from genetically similar plants, and ensures outcrossing. Carpels are the building blocks of the gynoecium. Carpels begin as small primordia on a floral apical meristem, forming later than, and closer to the apex than sepal, petal and stamen primordial.