 Sycamore trees are very variable across their wide range and have strategies to prevent self @-@ pollination , which is undesirable because it limits the genetic variation of the progeny and may depress their vigour . Most inflorescences are formed of a mixture of functionally male and functionally female flowers . On any one tree , one or other of these flower types opens first and the other type opens later . Some trees may be male @-@ starters in one year and female @-@ starters in another . The change from one sex to the other may take place on different dates in different parts of the crown , and different trees in any one population may come into bloom over the course of several weeks , so that cross @-@ pollination is encouraged , although self @-@ pollination may not be completely prevented . 
