Once a pollen grain comes in contact with the stigma, a part of the pistil in female plants, a chemical reaction causes the pollen grain to swell and to grow a tube that reaches down to the ovule.
Mara chembe ya chavua inapogusana na unyanyapaa, sehemu ya pistil katika mimea ya kike, mmenyuko wa kemikali husababisha chembechembe za chavua kuvimba na kukuza mrija unaofika chini kwenye yai.