Planet Earth

#87: Speedy Sperm Explains Flower Power

The quickest out of the gate, angiosperms dominate the plant world.

By Jane BosveldDec 7, 2008 12:00 AM
Tubes from pollen, headed for an egg cell. | Image courtesy of Joseph Williams


Sign up for our email newsletter for the latest science news

There are an estimated 270,000 known species of angiosperms, or flowering plants, but only about 900 species of gymnosperms, which are nonflowering plants such as conifers, cycads, and ginkgoes. The secret to the dominance of angiosperms may lie in the mechanics of fertilization, according to a recent study by Joseph Williams, an evolutionary biologist at the University of Tennessee.

Williams analyzed the rate of growth of pollen tubes, which transport sperm from pollen into a plant’s egg cell. In gymnosperms, wind deposits a pollen grain on an exposed egg cell, and then the pollen tube—growing at the slow rate of 20 micrometers an hour—releases an enzyme that destroys any tissue blocking the sperm’s entrance into the egg cell.

In early angiosperms, a different and much faster mechanism evolved. Instead of eating away tissue, the pollen tubes secrete a sugar molecule called callose, which forms thin walls for the pollen tube. The thin walls let the pollen tube grow between cells, so the tubes can snake through tissue at speeds of 80 to 600 micrometers an hour. Williams contends that these innovations took hold at least 125 million years ago and made it possible for early angiosperms to evolve more flexible and sheltered modes of fertilization, including ovaries containing egg cells deep inside the plant.

1 free article left
Want More? Get unlimited access for as low as $1.99/month

Already a subscriber?

Register or Log In

1 free articleSubscribe
Discover Magazine Logo
Want more?

Keep reading for as low as $1.99!


Already a subscriber?

Register or Log In

More From Discover
Recommendations From Our Store
Shop Now
Stay Curious
Our List

Sign up for our weekly science updates.

To The Magazine

Save up to 70% off the cover price when you subscribe to Discover magazine.

Copyright © 2022 Kalmbach Media Co.