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Spring has moved on from blooming wildflowers to trees in bloom, and so I’ve been trying to catch native trees and shrubs in flower. While observing American Holly flowers I was reminded that some trees do things differently. Several species like the holly produce flowers of one sex only, and so there are literally boy and girl trees, which is not common in the plant world.

Most trees in the forest produce flowers that contain both the male and female sex organs. Recalling your high school botany, the stamens are the male part and often look like a circle of small stems that hold up an enlarged contraption on the end that produces pollen (the anther). The female organ is the pistil and looks like a small stalk extending out of the center of the flower. This contains the ovaries that are pollinated by the stamens and does the circle of life thing.  Plants that produce flowers with both male and female flower parts are called monoecious, which in Latin means “two houses”. 

Some species however produce only male flowers on one tree and female flowers on a separate tree and are called dioecious (“two houses”). If you look at them closely, the male flower has very pronounced stamens but only a rudimentary, non-functioning pistil. The female flower has a very pronounced pistil but small non-functioning stamens. If you know what to look for, the difference between the flowers is striking. I mentioned that American Holly is dioecious with separate sexed trees, but there are others in our area:  Persimmon, Ash, Mulberry, Honey-Locust, Willow,, and Spicebush are also dioecious.

At this point you might be saying okay…. why? The best answer I came up with is that monecious flowers with both sex organs can self-pollinate, but because the DNA is near identical in both the pollen and the ovary, the resulting fruit is poor and seeds not viable, which is what this sex thing is all about.  With male and female flowers on separate trees however, cross pollination is assured, with good fruit produced that contains seeds able to germinate, and genetic diversity is assured for a healthy plant population.

With everything there are pluses and minuses, and the disadvantage of dioecious trees is that the male and female trees must be in range of each other for their pollinators (mostly bees) to be able to fly the distance, which in the randomness of nature does not always happen. 

The male holly flower has very pronounced pollen producing stamens (yellow).

The female holly flower has a very pronounced ovary producing pistil (green).