Flowers Without Petals: Wind Pollination is Derived in Angiosperms

Flowers without petals: While walking the dog this summer, I tried to take note of all the flowers that are in bloom but which don't have petals. I took pictures of birch, oak, and hickory flowers; last summer I managed pictures of the willow flowers too (and with the help of some students labeled male and female plants along a trail.) The general belief is that the catkins that these plants have are probably, in many cases, examples of convergent evolution. Furthermore, all these flowering plants probably evolved from insect-pollinated ancestors. Wind-pollination is a derived characteristic in flowering plants, but an ancestral characteristic in gymnosperms. Mixed pollination system are probably more common than people realize & the paper below makes me want to keep an eye out for those mixed systems! (New word: ambophily)

Culley, T. M., S. G. Weller, A. K. Sakai. 2002. The evolution of wind pollination in angiosperms. TRENDS in Ecology & Evolution 17:361-369.

I had my daughter tap on the male cones of a gymnosperm in the cemetery while I tried to take a picture. You can kindof see the pollen poofing out. 

The gymnosperms have been wind-pollinated for a long time! In flowering plants, wind pollination is more recent & has evolved repeatedly.

I hope your allergies aren't acting up.

Pollen Sculpturing

Pollen Sculpturing:

The exine or hard outer part of a pollen grain (and for that matter other spores) is often very pretty - some species are known to have distinctive textures, spikes, and indentations. Here's a link to a nice picture on Wikipedia:

https://upload.wikimedia.org/wikipedia/commons/a/a4/Misc_pollen.jpg

Why the fancy surface texture? One possible reason is that those crevices and spikes help chemicals "stick" to the pollen grain. What kinds of chemicals are needed on the surface of a pollen grain? In some cases, chemicals that prevent self-pollination. Today, I am thinking about another common sticky substance found on the outside of pollen grains called pollenkitt. I googled "kitt and German", and it appears that "kitt" is a German word for "putty". That seems right to me. Pollenkitt is a sticky putty that holds pollen grains together.

If a plant is wind-pollinated, then it is better if the pollen grains don't stick together & so wind-blown pollen is expected to have less pollenkitt than animal-carried pollen.

This summer, I found myself surprised by the fact that Forsythia plants and Ash trees are in the same plant family. Forsythia is insect-pollinated and Fraxinus americana (white ash) is wind-pollinated. I wondered if they differ in their pollenkit content as predicted by their pollination systems? I did in fact look at the pollen grains of both under a microscope and was delighted to see that Forsythia pollen had visible pollenkitt and Ash pollen did not. Here's are a few picture (my own pictures):

Possibly because it is important for pollen grains to be coated with chemicals, some plants have evolved very beautiful pollen grains with very interesting surfaces. Maybe this is a stretch, but I can't help but wonder if pollenkitt may be one evolutionary reason why pollen grains are so beautiful.

On a side note: A few years ago I found this link to the work of an artist whose passion for pollen sculpturing has led her to make pollen sculptures.

http://www.jogolesworthy.com/wpimages/wp7ff3213b_05_06.jpg

Supplementary reading:
Dobson, Heidi E. 1988. Survey of pollen and pollenkitt lipids-chemical cues to flower visitors? American Journal of Botany 75: 170-182.