As we have seen, the plants that survive best in the city are those of the natural environment, because, in general, the plants of the city are expected to maintain themselves. Even in the modern practice of horticultural plantings in pots around public buildings, the assumption is that the plant can survive on its own.

For the most part, the requirements of plants are simple. They require a few minerals (the amount in relation to the weight of the mature plant is a mere 3/10ths of 10), they need water, they require light in varying intensities, and they need carbon dioxide and the atmospheric water of the soil. Plants have a requirement for oxygen, but it is much less than that required by animals. The mature plants of the city probably do not depend very much upon surface water for their survival. In order to survive, those plants which are surrounded by concrete paving must receive their water supply from a permanent water table. It is almost certain, therefore, that a seedling will not survive unless its taproot reaches a permanent water supply, or at least a reliable water supply.

In paving cities and installing sewer lines, we have drastically altered the water table, run-off, and percolation from rainfall. The trees that we see along the main streets of our cities and admire so much probably gained access to a permanent water table before the streets were paved, since most of this paving occurred after the advent of the automobile. Street trees that are planted now, after all the concrete is in place, may never make it to a permanent water table. The ultimate goal for engineering plants into the city in those areas where a natural water table has been eliminated by the intrusion of subways, powerlines, cables, and the foundations of buildings may well be to install hydroponic facilities.

Meanwhile, American cities are discharging great quantities of nutrient-rich water into rivers and lakes. This material from the sewage system of cities is polluting our rivers and lakes when it could be fertilizing our city vegetation. Many experts believe that in the next 30-35 years the shortage of fresh drinking water will reach a crisis for many American cities. Yet we continue to pollute the streams and fresh water sources that are adjacent to our great cities. it should be possible to recycle great quantities of this nutrient-rich water into a second water system which could be used to nurture the growth and development of city vegetation.

Nor is it unreasonable to suggest piping such materials into agricultural areas to fertilize and enrich the soil for agricultural crops. The real obstacle to be overcome is not one of engineering but of public health. The public's fear that such a process might foster disease epidemics in heavily populated areas is well founded. But we should remember that we are not without expertise in the field of infectious diseases, their causes, and the principles underlying epidemics. The present state of our knowledge suggests that the public health aspect is easily surmountable. In fact, we have the technology to engineer a system that could safely utilize the waste water now being dumped into our freshwater streams and lakes, causing their pollution.