A flower of Lilium lancifolium, syn. L. tigrinum, the Tiger lily. The picture shows the recurved petals ( orange with dark spots), and below, six curving stamens (to left and right), each terminating in an anther, and (centre to bottom) the stigmatic surface at the tip of a curving stalk - the style. The anther locules have opened to expose the pollen grains (deep orange). When a pollen grain is transferred to the surface of the stigma, it germinates to produce a pollen tube that penetrates the surface and grows down the style, conveying the male gametes to the egg cells (ovules) at its base, where fertilisation takes place. Many plants have mechanisms to prevent self-pollination. One of these is known as the incompatibility response. This is genetically controlled; if the pollen grain is incompatible with the stigma, it does not germinate to produce a pollen tube