Scanning electron micrograph of a stamen of chickweed, Stellaria media. The picture shows the tip of the stamen after release of pollen (yellow) by the anthers. The polygonal grains - 30 microns wide - have a germ pore on each face. The pore is the site of emergence of the pollen tube that, following pollination, grows through the stigma down to the ovaries to effect fertilisation. S. media flowers are variable in structure; there may (rarely) be up to 10 stamens, but most commonly there are 3 to 7; some may develop sterile anthers that cannot produce pollen. The flower is protandrous - the anthers shed pollen shortly before the female stigma is receptive. In many flowers this behaviour limits self-pollination, but not in chickweed. As a unusually variable plant, it does not depend on cross-pollination for its success as a common and widespread annual weed in many global habitats.