Love Is In the Air or What's All Over My Car?

A Story about Live Oaks, Pollen, and Understanding

By Laurie Sheldon

Writing in a layer of pollen on a vehicle.
Source: www.sciencephoto.com/media/84104/enlarge

     Aah, spring… the sun is shining, the birds are singing, the oaks are leafing out and my neighborhood is plastered in yellow. No, we don’t have a Post-it® fetish. It’s pollen, and it’s earlier and thicker than usual this year. Don’t go hating on the oak trees, though - it’s not really their fault. They are simply responding to the climbing temperature. The fresh air and changes in wind which push out at least some of the pollen come from the cold fronts we normally have in early spring. These beneficial fronts have been m.i.a. this year, which has left us roosting in an inert yellow cloud.

     On the bright side, however, is the effect that this abundance of pollen has had on my curiosity. It occurred to me that, while fairly knowledgeable about zoophyly (pollination by animals including birds, bats, and insects), I knew little about anemophyly (wind pollination) other than the fact that it seemed like the plant version of throwing a “Hail Mary” pass. Since I had to blog anyway, it was a great opportunity to further educate myself and share that information in this article. So here goes…

Plants are Sexy

     For starters, I have often found that I forget plants are sexual creatures. Perhaps it’s because they never whistle at me. Who knows. The bottom line is that plants are wired to try to reproduce, and, among angiosperms, their flowers are what enable them to do so.

Male and female flowers don't always live together

  Some flowers have both male and female reproductive organs (stamens and pistils), and some have only one or the other. These are referred to as bisexual and unisexual, respectively. To make matters even more complex, sometimes the male and female flowers of the same species are borne on entirely different plants. This setup is known as being dioecious, a Greek-derived term for two houses. When a single plant has both male and female unisexual flowers it is called monoecious (one house).

 

Primping

      Now that I’ve broken down the basic reproductive components of flowers and the various living arrangements that unisexual flowers have, let’s consider the advantages and disadvantages of each. Bisexual flowers are generally much showier than unisexual flowers, but their brightly-colored petals and/or fragrances take a good bit of energy to create. Why do they bother? That’s how they attract animal pollinators, who will, in turn, carry pollen directly from the stamen of one flower to the pistil of another - usually without even realizing it. The work these flowers do to get all dolled up is balanced out by the lack of effort they have to put into pollination. As an added bonus, because the pollen is flying non-stop from stamen to pistil, the plants don’t have to produce pollen in excess of what’s required for fertilization. Bees are much more reliable than airlines for getting baggage to the appropriate destination.

Axial female bud, left; mixed male bud
with elongating catkins, right

     Oak flowers, by contrast, are unisexual (they have separate male and female flowers). This is fairly common among anemophilous (wind-pollinated) species. Male oak flowers (which contain the pollen) are apetalous (have no petals) and terminal (located at the ends of branches), which makes good sense if you think about it. There’s no point in putting energy into making the sort of flashy outfit needed to coax critters into transporting their pollen, and petals would act like a wind baffle, which would be completely counter-productive. Being at the outermost edge of a branch gives them prime wind exposure. In addition, the terminal buds that contain the male flowers are mixed - they contain leaves as well. These leaves wait until the flowers are done blooming before they unfurl, since they would only get in the way of pollen movement. Female oak flowers are  located in the angle formed between stem and leaf. This positioning is referred to as axillary.

The barely visible leaf within
a mixed terminal bud

Inbreeding - Plants Know Better

     Most of us will recognize that it’s probably a bad idea to have offspring with blood relatives. Even if it was socially acceptable, those children would not have much going for them as far as genetic variation is concerned. Apparently our oaks are aware of that too! Although they have both male and female flowers on the same plant, the stigma of the female flower is not receptive when the male flowers shed their pollen (like a plant version of the rhythm method). They have no choice but to pollinate and be pollinated by other oaks. Now, if you’re thinking, “hey, wait a second - isn’t it incestuous when live oaks pollinate each other since they’re in the same family? They even have identical first and last names (Quercus virginiana)!” The answer is no. You and I aren’t “kissing cousins” simply because we are both Homo sapiens, right? (Just nod your head here) The same rules apply for oaks.

Doing The Deed

Male flowers, swollen with pollen

     Now let’s discuss how oak pollination works. Let me set the scene: It’s the end of winter, and chilly evening temperatures have given way to warm spring days. This temperature swing prompts the production of auxins - hormones that force our oaks to increase their water intake. The oaks suck up water like nobody’s business, which causes the female flower buds to enlarge and the terminal buds to swell and burst open. Out roll the male flowers, shoulder to shoulder on a dangling stalk that looks like a cat’s tail. Not surprisingly, this flower grouping is called a catkin. The catkin dances in the breeze, continuing to elongate, each of its flowers becoming more robust with pollen (which is dividing and multiplying through meiosis). The female flowers open up and extend their stigmas to catch pollen from a nearby tree. Shortly afterward, the male flowers open up, show off their swollen anthers, then - BOOM - the anthers explode from all of the water pressure. The catkins sway to-and-fro, with anthers liberating an unthinkable number of microscopic pollen grains like confetti on New Year’s. The first grain to fall into the cradle of a female flower will stake its claim by growing a tube to bore into the female ovary, through which it will release sperm. Soon the female flower will have a “bun in the oven” which will eventually become an acorn. Incidentally, the word acorn is a concatenation of “ac” the old English for “oak,” and “cern,” Greek for grain.
 

Fallen catkin with anthers extended
and emptied

The Lonesome Losers
     Most grains will not make a meaningful connection, poor things. The male flowers they once inhabited will dry up and fall from their lofty perches, and the leaves they hung in front of will get the green light to grow. April showers will wash away all that remains of the grains that hoped to start something wonderful but ended up splattered across lawn furniture, patio screens, and driveways.
     So instead of getting irritated the next time you find yellow dust on your just-washed car or brown bits of catkin on the living room carpet, just stop. Breathe deeply. Sneeze if you must. Then proceed as follows:
(1) Consider how lucky you are that your destiny isn't pinned to an afternoon breeze
(2) Know that the pollen had hoped for more in life than to do a faceplant on your Honda
(3) Reflect on your new understanding of its unfulfilled potential
and (4) Remember that none of our majestic oaks would exist without the efforts a pollen grain forty microns long.

Left: Oak pollen grains as seen with a scanning electron microscope.
Source: www.sciencephoto.com/media/32769/enlarge
Right: The "Angel Oak," possibly the largest Quercus virginiana in the United States
Source: media.hostedfile.com/hostedfile/pdata/55939.jpg

All photos and illustrations by Laurie Sheldon unless noted otherwise