Although pollen has covered cars for weeks and allergy sufferers have been sneezing, we think of sex as being the realm of animals. But plant sex can be quite interesting, especially in species that can have male or female flowers. More than 90 percent of flowering plant species combine both sexes in one plant. In the less than 10 percent of species where female and male flowers exist on separate plants, they typically remain female or male throughout their lifetime. But it isn't always this simple. In a new study, National Science Foundation-funded researchers found that striped maple trees can change sex from year to year. A tree may be male one year and female the next, and while male trees grow more, female trees are more likely to die. The study found that 54 percent of striped maple trees changed sexes over a four-year period, with some switching at least twice. Male trees usually outnumber female trees by more than three to one. Since the study started in 2014, 75 percent of trees that died were female. Since only female trees can make seeds, changes in the relative numbers of males and females might lead to reduced populations.
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