News & Politics » Field & Street

Field & Street

by

comment

By Jill Riddell

One hundred twenty-three trees were killed last Sunday, and I'm confessing to the crime. I freely admit I single-handedly pulled a crop of tender young maples out by the roots and sent their corpses to the landfill.

After a few days of steady rain I'd gone to check on the herbs and lettuces I'd planted in boxes on the roof and found that a forest of small trees had taken root among them. They stood pertly upright--hopeful, green, without sin of any sort. Perhaps they would have had a happy life. But they were in the way. My way. For starters, they were competing with the Bibb lettuce for space and nutrients. And if allowed to grow to maturity, 123 maple trees would be a bit heavy for a flat tar roof.

Like any diligent gardener, I started pulling up the trees just as I would any weed. But there were so many it made me pause to imagine the vast numbers of seeds that have to be sent out by the trees on the surrounding blocks just to make this many seedlings. These of course weren't the only bits of genetic material cast into the city winds by parent trees. About three or four hundred more seeds rested in a pile on another part of the roof. And lately as I've walked to work the streets are silly with seeds skittering over sidewalks and blowing into loose drifts in the gutter.

Seeds form inside the ovaries of the blossoms on a tree. After fertilization the zygote's cells divide and develop into a tiny embryo. The tree generates some food to go along with each embryo and encases both in a tough coating that helps the genetic material withstand travel and periods of dormancy. It's this whole structure--embryo, food source, and seed coat--that we call a seed. By the time the seed is ripe and ready to be released, the seed is dry and the embryo dormant.

Right now in Chicago the maple seeds are falling in profusion. They twirl to earth in pairs, accompanied by flat, thin structures that catch the wind. The seeds of ashes and elms will come soon. The ashes are like smaller versions of the maple but with single seeds and single wings. Elms have round seeds smaller than subway tokens and only slightly winged.

Then come the seeds of the cottonwood, the champion of wind dispersal. The cottonwood, a tough prairie tree, helps its genetic material travel by attaching the embryos to white, feathery structures that stay aloft for long periods.

Often in early June there's so much of the stuff it looks like snow. I recall an afternoon years ago when my college roommate and I skipped classes, believing that getting our legs tanned would do more to ensure short-term success than learning the history of China. We settled in on the lawn of a small Evanston park, rubbed on suntan oil, and fell asleep on beach towels. We woke to discover our sticky bodies had been plastered with the fluffy seeds, giving us the appearance of having been tarred and feathered.

Trees that depend primarily on the wind to transport their genes tend to release seeds early in the summer. Species that hope to attract birds, squirrels, and other animals to do the work for them have to build up greater amounts of food surrounding the embryo--not for the embryo, but as a reward for the animal. These form fruits, berries, and nuts. By mid-July mulberry trees will be dropping their messy, fleshy fruits onto the hoods of cars. In August crab apples will ripen, and by fall nuts and acorns will be in season.

Germination of a seed begins when it encounters moisture. Water greatly increases the volume of a seed, sometimes by as much as 200 percent. This increases the activity of enzymes and the metabolic rate of the embryo. The embryo grows and soon bursts out of its seed coat and turns into a real plant.

If you were to lay a seed between two sheets of damp paper towels to determine if it's viable, or if a sprouting seed finds itself in some other equally hopeless environment, this is as far as its life goes. But if the embryo is lucky enough to have landed somewhere more hospitable--like the bed of soil I hauled up two flights of narrow stairs in 50-pound bags--it fares slightly better. When it rains the seed takes the opportunity to come out of dormancy. Its hypocotyl, the first part to emerge from the seed coat, promptly turns down into the dirt, regardless of which way the seed is oriented. Tiny root hairs form almost immediately. By the time the epicotyl, the aboveground part, shoots up, the hypocotyl has already made a good start on a stable root system. If allowed to continue, the cells will multiply and elongate and grow into an entire tree, which sounds reasonable in science-speak, but when you think about it too much seems unbelievable.

In nature the odds are against survival. In fact, the odds are against ever being born in the first place. Of the genetic material a tree produces, a certain percentage won't germinate no matter how ideal conditions are. Out of the seeds that are viable a large number fall onto buildings, parking lots, and expressways. Only a few manage to land on real earth. Of these, many fall in positions that prohibit the seed from sinking into the dirt, and when it rains and they germinate, the hypocotyl has nowhere to go.

After a futile attempt, these unfortunates wither and die. Some embryos drop into perfect conditions, like the ones in my roof garden. These sprout, the hypocotyl thrusts eagerly down into the soil, the epicotyl emerges like clockwork above the soil--and someone like me comes along and picks it like the weed it is and throws it away. Others land in lawns and get mowed. It's an astonishing thing that any plants manage to grow.

It almost seems that death and nonexistence are the norm and life the anomaly. Yet as I look out my window at the thin strip of parkway next to the house I'm confronted with life. There are two mature green ash trees, a four-year-old silver maple, a dozen daylilies, 100 weeds, 45,000 grass plants. Inside the bark of the trees insects, spiders, mites, bacteria, and fungi thrive. In the soil worms, nematodes, mites, centipedes, and larvae crawl around, surrounded by millions of bacteria, algae, and protozoa. It's hard to think of a less hospitable ecosystem than this thin strip of earth between the sidewalk and street, and still it's rich with millions of life forms. Even given nature's prodigious waste of genetic material and the small chance that any individual plant or animal will survive to adulthood, life on the planet is wildly abundant. Thankfully, there's little I or anyone else can do to stop it.

Add a comment