Where do plants keep their sperm?
In modern flowering plants (angiosperms), it’s located in pollen.
For those of you who suffer allergies, that yellow stuff splattering your eyes, nose, and mouth, tormenting your existence, is literally plant jizz!
Pollen is technically a male gametophyte, meaning an organism that makes “gametes”, which in this case is sperm. In its simplest form, pollen has three cells: one vegetative cell enclosing two sperm cells.
So we’re all covered in plant sperm. Neat! But how does the sperm get to the egg?
A tube-like male organ suddenly squirts sperm-filled sex juice. (That’s all I learned from Indiana sex ed. It’s literally true for plants. Human sex is apparently more complicated…)
When pollen lands on the tip of the pistil of a flower, its vegetative cell grows a “pollen tube” that extends down the whole pistil into the ovary, targeting an ovule. The pollen tube then explodes, rocket-launching the two sperm into the plant ovule. Cowabunga!
This fluorescence microscopy image shows the pistil of a tomato flower. The dots at the upper tip of the pistil are pollen grains, and the threads extending downwards are pollen tubes.
Remember, the vegetative cell that makes the pollen tube is still just one single cell. In corn, pollen tubes have to grow through the entire length of the “silk” before they can launch their load. A single-celled corn pollen tube grows 12 inches in just one day! That’s like 2000 times the length of the original pollen grain.
Corn does all that just to have sex. And you thought you were thirsty.
I am both frightened and aroused. How did that system evolve?
Glad you asked!
Back in the good old Precambrian days when all plants were unicellular and lived in the ocean (Make Laurentia Plant-less Again!), plant sperm could just swim straight to an egg. Bryophytes, such as mosses and liverworts, still use this strategy, and only reproduce when it’s wet out.
Here are some present-day liverwort sperm swimming around. These lil’ racers are designed for speed, with a sleek corkscrew shape and two flagella.
This method of reproduction requires a lot of moisture, and it is inefficient. Most of these guys will tragically never make it to an egg.
Ginkgos and cycad sperm have slightly better odds of getting it on than bryophyte sperm.
The main innovation is that ginkgo and cycads make pollen (unlike bryophytes). In the ginkgo/cycad system, wind carries the pollen to a drop of water on the tip of the ovule’s protective shell. The drop gets sucked inside the ovule. The pollen then grows a rudimentary tube towards the ovule’s “pollen chamber” and waits there for months as eggs develop in the ovule. Then the pollen tube explodes and the sperm suddenly have to swim through the water-filled pollen chamber to the eggs.
Ginkgo and cycads are the only seed plants with motile sperm. Their strategy falls somewhere in between free-swimming bryophyte sperm and pollen-tube-riding flowering plant sperm (as discussed earlier in this post). The sperm are delivered to the right location, but still have to swim for the gold.
Ginkgo sperm has a tiny twirly propellor. It’s powered by a continuous spiral of flagella.
Cycad sperm is even more spirally.
Japan has known this stuff for over a century, by the way. Sakugoro Hirase and Seiichiro Ikeno described the sperm of ginkgo trees and cycads, respectively, in the last decade of the 1800s. Below is one of Ikeno’s illustrations. Pretty accurate, no?
Flowering plants (angiosperms, the ones we talked about at the start of this post) developed their fertilization strategy last, when the dinosaurs already roamed the Earth, but they really stepped it up. Their secret? Flowers! The anatomy of a flower provides a place to grow a pollen tube, allowing sperm to get straight to the ovule without waiting in a holding chamber or swimming in circles. Flowers also allow for “pollinators”, insects and birds that bring pollen straight from one plant to another. Flowers’ bright colors are essentially a neon sign advertising free food in exchange for sex work. This efficient method of reproduction is one of the reasons for flowering plants’ overwhelming success from the Cretaceous up through the present.
Here’s a pollen tube smoothly conveying the sperm to the ovule (not shown).
And here’s a grainy video of the final step.
Yeah!! Squirt that sex juice!
Source of GIFs:
The Sea and the Seed. Scientifically supervised by Hori Terumitsu, directed by Murray Weston. Tokyo Cinema Inc., 2000.
Other Sources & Further Reading:
Ogura, Y. 1967. History of Discovery of Spermatozoids in Ginkgo biloba and Cycas revoluta. Phytomorphology 17: 109-114. Retrieved from Biological Classics in the Internet, msu.edu. (The featured image for this post is an illustration of Cycas revoluta sperm by Seiichiro Ikeno, 1898.)
McCormick, S. 2013. Pollen. Current Biology 23.22: R988-990. (A brief, accessible article by Sheila McCormick, a famous pollen researcher.)
Leydon, A.R. Tsukamoto, T., Dunatunga, D., Qin, Y., Johnson, M.A., Palanivelu, R. 2015. Pollen tube discharge completes the process of synergid degeneration that is initiated by pollen tube – synergid interaction in Arabidopsis. Plant Physiology 169.1: 485-496.
PLANT HUMOR 