Monday, 4 February 2019

Survival of Plants

Survival of Plants

After discovering all this about the way plants grow and what they need for life, we wonder more than ever how it is possible for our vacant-lot gardens to exist. How do the plants get there in the first place? How do they manage to survive in see mingy impossible situations? These are really tremendous questions, but city dwellers generally take them for granted, when they notice the wild plants at all. People who may be trying desperately to keep a shrub alive or make a houseplant bloom in the adverse conditions of the city seldom stop to think that the success of the uncared for wild things is almost miraculous.

In the vacant spot left by a torn-down city building, there are usually no living seeds in the newly bared earth. The place may be separated from other green areas by several blocks of tall buildings and paved streets. And yet the wild seeds quickly arrive. They root and grow in a place that would seem to lack even the most basic necessities for life.

In nature, the vegetation of any area is the result of many factors working together  soil, air, rain, light, temperature, animal life. The soil must contain the water and minerals necessary for the manufacture of food.. It must be loose enough that water and air and roots can penetrate it. It is normally fertilized by creatures that eat and digest plant material and return its minerals to the soil in excretion and in death. It must contain the bacteria, molds, and other fungi that break down dead animal and plant organisms into their original components, for the green plants to use again in a new cycle.

And yet our city wild flowers grow in earth that is packed hard, with few spaces for the vital water film to collect around soil particles. Few earthworms tunnel to loosen the soil. It is "fertilized" by the discarded products of city life rather than the discarded bodies of living things. Rainwater falling on the hard-packed surface is likely to run off too quickly to penetrate to root level, or else it stands in badly drained puddles and threatens to drown the roots. The plants live in a constant fall of soot. This and the oily black film that coats the leaves and smudges our hands when we pick a bouquet damage the plant by clogging some of its tiny breathing pores and by cutting down the sunlight that reaches the chlorophyll. In fact, the heavy canopy of smog that hangs over many cities has already shut out a good portion of the natural sunlight. The air is full of poisonous chemicals poured out by car motors, incinerators, and factory chemical poisons that burn the leaves and break down their chlorophyll. Some of them also make the soil so acid that the kill the bacteria and fungi that normally act on dead matter.

And in the canyons of the city, many of the insects that pollinate wild flowers are missing. Honeybees and bumblebees are by far the most important pollinators in the country, but they rarely venture into the cement-and-brick parts of our cities. They are found only m the more open lots and in parks and botanic gardens, where they often live in hives specially provided by man.

Most plants can not live at all under these conditions, so our city weeds certainly do not have to fight for growing space. But they do have to fight for almost everything else, and it is important to find out what traits give some plants the ability to do so well in spite of so many difficulties. In the first place, how did they arrive? A growing plant, of course, spends its life where it is rooted, but its seeds can make their way out into the world for long distances. This is the reason for all the parachutes and wings and barbs that we have found on so many city seeds. The flying seeds of milkweed, dandelion, and goldenrod, thus are carried on the wind by the thousands even by the millions. They lodge m all kinds of places, and many of them can germinate and grow in the most unlikely spots  in the chink of a brick wall or in a small handful of dust in the corner of a roof as well as in any earth man opens up for them.

Seeds equipped with spines, like those of beggar-ticks, or enclosed in bristly pods, like those of burdock, catch in the hair of animals or the clothing of people and hitch a ride to a new home. The small seeds of some plants, like pepper grass, are provided with a glue which, when moistened by dew, can fasten them to the feet of birds. Some plants the tumbleweeds of the West which evolved in dry open country, break from their roots when they have reached maturity. The whole plant rolls along before the wind, scattering seeds as it goes. Sometimes these plants can be found growing near Eastern railroad yards, carried thousands of miles from home on the wheels of trains. Many seeds are not especially equipped for travel. They drop directly down at the roots of their parent plant and soon increase the population in that particular spot. But they also, of course, manage to get to new places, carried perhaps in the mud on a pigeon's feet or in his gizzard. Many seeds can be eaten by birds and pass unharmed through their digestive systems, to be dropped eventually at some distant place.

Once settled in a new location, many of our wild flowers can produce hundreds of new plants without seeds. In some species, like thistle, milkweed, and toad flax, the roots send out runners from which new plant lets sprout. In others, stems may lie flat on the ground or travel underground, root at the leaf nodes, and send up a new plant at each node. This is true of mung wort, yarrow, and many grasses. Once a plant of this sort becomes established, it will soon be the center of a colony that grows ever denser as the years pass.

Most of the plants that thrive in a city lot are the ones that even in the country do not need a fertile soil. There are, of course, some useful materials in city earth. Its minerals are renewed to a certain extent by the plants that die each year, fall to the ground, and decay there. Members of the pea family clover, melilot, alfalfa  are especially valuable to the plant community because their roots are able to capture a rich store of nitrogen that is returned to the soil at their death. And a few animal bodies also return their minerals to the earth a rat, cat, a bird, many kinds of insects.

All this may explain the plants' ability to survive, but their pollination still presents us with questions. Part of the answer is found in the fact that a great number of our city plants are wind-pollinated. These include the grasses, ragweed, lamb's quarters, worm seed, dock, and the plaintains all of which grow in great profusion.

But the many insect-pollinated flowers are another matter Evening primrose should be pollinated by night-flying moths. Other plants, like milkweed, burdock, and red clover, would in the country be pollinated by bees or butterflies; and the have had to find other pollinators in places where no bees come at all.  There are a few butterflies, even in the heart of the city occasionally a surprising and beautiful flock of migrating monarchs. And great numbers of wasps and flies of all kinds are everywhere. These presumably transfer the pollen, though the often seem to be the wrong size and shape for the job. There are also a few beetles, but, at best, beetles are poor pollinators, and they usually eat the flowers and are enemies rather than friends.

However, many of these flowers can pollinate themselves when other means fail, and some species, including a large number of our weeds, do this habitually. There are even some plants, like the dandelion, that can produce seeds without any pollination at all.

Whatever the method, the plants do get pollinated, and many of them produce seeds in enormous numbers. A large tumbling pig weed was found to have more than six million seeds to shed as it rolled before the wind. This is one of the secrets of the success of weeds. A great number of seeds, well equipped to travel and to survive, may be more important than the toughness of the plant itself.

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