# Popular Science Monthly/Volume 54/February 1899/Vegetation a Remedy for the Summer Heat of Cities

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APPLETONS’
POPULAR SCIENCE
MONTHLY.
FEBRUARY, 1899.
* VEGETATION A REMEDY FOR THE SUMMER HEAT OF CITIES.
A PLEA FOR THE CULTIVATION OF TREES, SHRUBS, PLANTS, VINES, AND GRASSES IN THE STREETS OF NEW YORK FOR THE IMPROVEMENT OF THE PUBLIC HEALTH, FOR THE COMFORT OF SUMMER RESIDENTS, AND FOR ORNAMENTATION.
By STEPHEN SMITH, M. D., LL. D.
ONE of the most prolific sources of a high sickness and death rate in the city of New York is developed during the summer quarter. It has been estimated that from three to five thousand persons die and sixty to one hundred thousand cases of sickness occur annually in this city, from causes which are engendered during the months of June, July, August, and September. An examination of the records of the Health Department for any year reveals the important fact that certain diseases are not only more frequent during the summer quarter than at any other time, but that they are far​—more fatal, especially in the months of July and August, than during any other period of the year. These are the "zymotic diseases," or those depending upon some form of germ life. The following table illustrates the course of mortality from those diseases in one year:
| Month. | Deaths. | Month. | Deaths. |
| January | 641 | July | 1,433 |
| February | 475 | August | 1,126 |
| March | 476 | September | 791 |
| April | 554 | October | 522 |
| May | 584 | November | 460 |
| June | 798 | December | 504 |
It appears that during eight months of the year, excluding June, July, August, and September, the average monthly mortality from "zymotic diseases" was 452. Had the same average continued during the remaining four months the total mortality from those diseases for that year would have been 4,424; but the actual mortality was 7,764, which proves that 3,340 persons were sacrificed during those four fatal months to conditions which exist in the city only at that period of the year. Still more startling is the estimate of the sickness rate caused by the unhealthful conditions created in the summer months in New York city. If we estimate that there are twenty cases of sickness for every death by a zymotic disease there were 66,800 more cases of sickness in the year above referred to than there would have been had the sickness rate been the same in the summer as in the other months of that year.
One of the saddest features of this high sickness and death rate appears when we notice the ages of those who are especially the victims of these fatal diseases. During the week ending July 9th last there were 399 deaths from diarrhœal diseases, of which number 382 were children under five years of age. The following table taken from the records of the Health Department show in a very striking manner how fatal to child life are the conditions peculiar to our summer season:
| Month. | deaths from diarhœal diseases. | deaths from diarhœal diseases. | deaths from diarhœal diseases. | deaths from diarhœal diseases. |
| Month. | Under one year. | Under two years. | Under five years. | All ages. |
| January | 50 | 55 | 58 | 82 |
| February | 47 | 51 | 58 | 75 |
| March | 75 | 80 | 83 | 96 |
| April | 82 | 91 | 97 | 108 |
| May | 101 | 117 | 121 | 104 |
| June | 387 | 430 | 436 | 467 |
| July | 809 | 990 | 1,020 | 1,100 |
| August | 464 | 565 | 697 | 762 |
| September | 267 | 394 | 409 | 462 |
| October | 114 | 148 | 154 | 190 |
| November | 59 | 70 | 72 | 89 |
| December | 57 | 62 | 64 | 82 |
​These statistics demonstrate the extreme unhealthfulness of New York during the summer, and the vast proportion of children who perish from the fatal agencies which are then brought into activity. It is a matter of great public concern to determine the nature of the unhygienic conditions on which this excessive mortality depends, and thus discover the proper remedial measures.
As high temperature is the distinguishing feature of the summer months, we very naturally conclude that excessive heat is a most important factor, if not the sole cause, of the diseases so fatal to human life at this period. A close comparison of the temperature and mortality records of any summer in this city demonstrates the direct relation
| Week ending | Total diarrhœal diseases. | Diarrhœal diseases un- der five yrs. | Mean tempera- ture (Fahren- heit). | Maximum temperature (Fahrenheit). | Minimum temperature (Fahrenheit). |
| May 7th | 10 | 8 | 52.4° | 42° | 47° |
| May 14th | 20 | 17 | 55.5° | 77° | 40° |
| May 21st | 14 | 12 | 63.3° | 86° | 52° |
| May 28th | 22 | 19 | 60.9° | 70° | 56° |
| June 4th | 18 | 16 | 65.8° | 76° | 54° |
| June 11th | 26 | 20 | 71.6° | 86° | 58° |
| June 18th | 36 | 32 | 73.0° | 89° | 59° |
| June 25th | 74 | 69 | 69.3° | 94° | 54° |
| July 2d | 170 | 164 | 78.6° | 94° | 67° |
| July 9th | 399 | 382 | 77.4° | 100° | 61° |
| July 16th | 330 | 321 | 71.1° | 91° | 57° |
| July 23d | 388 | 356 | 77.4° | 91° | 67° |
| July 30th | 380 | 353 | 78.5° | 95° | 70° |
| August 6th | 380 | 353 | 78.8° | 92° | 67° |
| August 13th | 342 | 306 | 73.9° | 90° | 65° |
| August 20th | 290 | 261 | 74.8° | 89° | 64° |
| August 27th | 268 | 246 | 76.6° | 93° | 63° |
| September 3d | 289 | 256 | 79.0° | 93° | 59° |
| September 10th | 283 | 255 | 74.0° | 92° | 58° |
| September 17th | 179 | 158 | 67.3° | 85° | 52° |
| September 24th | 193 | 167 | 68.7° | 90° | 52° |
| October 1st | 132 | 117 | 66.5° | 80° | 54° |
| October 8th | 90 | 78 | 69.6° | 81° | 53° |
| October 15th | 71 | 58 | 60.1° | 74° | 49° |
| October 22d | 54 | 42 | 55.9° | 71° | 44° |
| October 29th | 39 | 32 | 53.9° | 67° | 41° |
of the former to the latter. For illustration, we will take the records of the Health Department during the past summer, selecting diarrhœal diseases for comparison, as they prevail and are most fatal at that season of the year. The table gives the total mortality from these diseases and the mortality from those diseases of children under five years of age. To the four months, June, July, August, and September, are added May and October, for the purpose of showing the gradual increase of the mortality from these diseases as the hot weather approaches and its decline as the hot weather abates.
Again, if we compare the temperature and mortality records for a series of days instead of months, it will be noticed that the mortality ​record follows the fluctuations of the heat record with as much precision as effect follows cause. The summer heat generally begins about the 20th of June and continues with varying intensity until the 15th of September. Within that period we can select many examples which strikingly illustrate the relations of temperature to mortality. For example, the first heated term of the year before us began on the 19th of June and lasted until the 26th of that month. The two records are as follows:
| Day. | Temperature. | Mortality. |
| 19th | 78° | 83 |
| 20th | 80 | 100 |
| 21st | 82 | 122 |
| 22d | 80 | 116 |
| 23d | 77 | 104 |
| 24th | 68 | 119 |
| 25th | 65 | 88 |
On the 28th of June a second heated term began, when the temperature rose to 80°, and continued above that figure until July 5th, a period of eight days. The following is the record, including the temperature in the sun:
| Day. | temperature. | temperature. | temperature. |
| Day. | In shade. | In sun. | Mortality. |
| June 28th | 80° | 118° | 118 |
| June 29th | 84 | 120 | 163 |
| June 30th | 85 | 124 | 191 |
| July 1st | 88 | 125 | 247 |
| July 2d | 87 | 128 | 351 |
| July 3d | 82 | 120 | 238 |
| July 4th | 84 | 122 | 227 |
| July 5th | 80 | 121 | 184 |
It will be noticed that during the last heated period there was a more prolonged high temperature than during the first, and that the mortality of the second was higher for the same temperature than that of the first. These facts are in accord with the history of our summer months. The range of temperature increases as the season advances, and the rate of mortality rises, owing to the diminished resisting power to the effects of high heat on the part of the people, especially of the children, the aged, and those already enfeebled by disease.
In order to fully understand the influence of heat and its effects upon the public health, we must first notice the conditions regulating the temperature of the body in health and disease.
The temperature of animals in a state of health is not a fixed quantity, but has a limited range which depends upon internal and ​external conditions not incompatible with health. In man the range of temperature in health is fixed at 97.25° F. to 99.5° F. Any temperature above or below these extremes, unless explained by special circumstances not affecting the normal condition of the person, is an indication of disease. This comparatively fixed temperature in health is a remarkable feature of the living animal. When subjected to a temperature above or below the extremes here given it will still maintain its equilibrium. This fixed temperature under varying conditions of heat and cold is due to a "heat-regulating power," inherent in the constitution of every animal, by which it imparts heat when the temperature of the air is high and conserves heat when the latter is low. The heat escapes from the body—1, by radiation from the surface; 2, by transmission to other bodies; 3, by evaporation; and 4, by the conversion of heat into motion. The surface of the body furnishes the principal medium for the loss of heat by the first three methods—viz., radiation, transmission, and evaporation. It is estimated that 93.07 per cent of the heat produced escapes by the processes of radiation, evaporation, conduction, and mechanical work. The remaining heat units are lost by warming inspired air and the foods and drinks taken. There are apparently other subtile influences, so-called "regulators of heat," at work to preserve an equilibrium of temperature in the animal body, but they are not well known. The result of the operation of these forces is this—viz., if, by any means, the heat of the body is increased, compensative losses of heat quickly occur, and the normal temperature is soon restored; and if, on the contrary, the loss of heat is unusually increased, the compensative production of heat of the body at once follows, and the equilibrium is at once restored. The important fact to remember is this—viz., the production and loss of heat in the human organism when in health and not subjected to too violent disturbing causes are so nicely balanced that the temperature is always maintained at an average of 98.6° F., the extremes being 97.25° F. and 99.5° F. "So beautifully is this balance preserved," Parkes remarks, "that the stability of the animal temperature in all countries has always been a subject of marvel." If, however, anything prevents the operation of the processes of cooling—viz., radiation, evaporation, and conduction—the bodily temperature rises by the accumulation of heat, and death is the result from combustion. In experiments in ovens a man has been able to bear a temperature of 260° F. for a short period, provided the air was dry so that evaporation could be carried on rapidly. But if the air is very moist, and perspiration is impeded, the temperature of the body rises rapidly, and the person soon succumbs to the excessive heat. Another important fact is this, viz., the normal temperature of the young and ​of the very old is higher than the middle-aged. The infant at birth has a temperature of 99° F. to 100° F., and it maintains a temperature of 99° F. and upward for several days. The variations of temperature from other causes are much greater in children than in adults, as also the normal daily variations of temperature. About the sixtieth year the average temperature of man begins to rise, and approximates that of the infant. In the young and old the "heat-regulating power" is more readily exhausted, and hence continued high temperature is far more fatal to these classes.
The first noticeable fact in regard to bodily temperature in disease is that there are daily fluctuations as in health, but much more extreme. In general, the remission of temperature in disease occurs in the morning, and the exacerbation in the afternoon and evening; the minimum is reached between six and nine o'clock in the morning, and the maximum between three and six o'clock in the evening. In many diseases the minimum temperature is not below 100° F., and usually it is one or two degrees above that point, while the maximum has no definite limit and may reach the dangerous height of 107° F. It should be noticed that the highest daily temperature in disease, as in health, occurs in the afternoon, when the temperature of the air in summer is the greatest.
The conditions affecting the temperature of the body other than those due to physiological conditions are very numerous. First and most obvious is the temperature of the surrounding atmosphere. It is a well-established fact that an average temperature of the air of 54° F. is best adapted to the public health, for at that temperature the decomposition of animal and vegetable matter is slight, and normal temperature is most easily maintained. Every degree of temperature above or below that point requires a more or less effort of the heat-regulating power to maintain the proper equilibrium. Even more potent in elevating the bodily temperature is the introduction into the blood, whether by respiration or by direct injection, of putrid fluids and the gases of decomposing matters. If this injection is repeated at short intervals, death will occur with a high temperature. The air of cities contains emanations, in hot weather, from a vast number of sources of animal and vegetable decomposition, and the inhalation of air so vitiated brings in contact with the blood these deleterious products in a highly divided state which cause a fatal elevation of temperature in the young, old, and enfeebled. The same effect is produced by the air in close and heated places, as in tenement houses, workshops, schoolhouses, hospital wards, and other rooms where many persons congregate for hours. Air thus charged with poisonous gases becomes more dangerous if ​the temperature of the place is raised, as happens almost daily in the summer months in cities.
From the preceding facts we may conclude that, as long as the body continues in health, the "heat-regulating power," which constantly tends to preserve an equilibrium of temperature, is capable of resisting the ordinary agencies that, operating externally or internally, exaggerate the heat-producing conditions, and thus destroy the individual. But if the person is suffering from a disease which weakens the "heat-regulating power" these deleterious agencies, which the healthy person may resist, will readily overpower the already quite exhausted heat-regulating forces, and he perishes by combustion. It is very evident that in an organism having complicated functions, like that of man, and subject to such a multitude of adverse influences, the balance between health and disease must be very nicely adjusted. Too great an elevation or too great a depression of temperature may destroy the "heat-regulating power," and disease or death will be the consequence. Or this "heat-regulating power" may be weakened or destroyed by causes generated within the body, or received from without, and the heat-producing agencies are then under influences which may prove to be powerfully destructive forces.
It will not now be difficult to understand in what manner high temperature affects the public health of large cities. Evidently in the direct action of heat upon the human body we have the most powerful agency in the production of our great summer mortality. While sunstroke represents the maximum direct effect of solar heat upon the human subject, the large increase of deaths from wasting chronic diseases and diarrhœal affections., of children under one year of age and persons upward of seventy years of age, shows the terrible effects of the prevailing intense heat of summer upon all who are debilitated by disease or age and thereby have their "heat-regulating power" diminished. The fact has been established by repeated experiment that when solar or artificial heat is continually applied to the animal the temperature of its body will gradually rise until all of the compensating or heat-regulating agencies fail to preserve the equilibrium, and the temperature reaches a point at which death takes place from actual combustion. In general, a temperature of 107° F. in man would be regarded as indicating an unfavorable termination of any disease. In persons suffering from sunstroke the temperature often ranges from 106° F. to 110° F., the higher temperature appearing just before a fatal termination.
The indirect effects of heat appear in the production of poisonous gases which vitiate the air and render it more or less prejudicial to health. Decomposition of all forms of refuse animal and ​vegetable matter proceeds with far greater rapidity during the summer quarter than during other months of the year. Among the early results of summer heat is the damage to food. Milk retailed through the city, the sole or chief diet of thousands of hand-fed infants, undergoes such changes as to render it not only less nutritious but also hurtful to the digestive organs. The vegetables and fruits in the markets rapidly deteriorate and become unfit for food. Meats and fish quickly take on putrefactive changes which render them more or less indigestible. The effect of this increase of temperature upon the refuse and filth of the streets, courts, and alleys, upon the air in close places, in the tenement houses, and upon the tenants themselves is soon perceptible. The foul gases of decomposition fill the atmosphere of the city and render the air of close and unventilated places stifling; while languor, depression, and debility fall upon the population like a widespread epidemic. The physician now recognizes the fact that a new element has entered into the medical constitution of the season. The sickly young, the enfeebled old, those exhausted from wasting diseases, whose native energies were just sufficient to maintain their tenure of life, are the first to succumb to this pressure upon their vital resources. Diarrhœal diseases of every form next appear and assume a fatal intensity, and finally the occurrence of sunstroke (or heat-stroke) determines the maximum effects of heat upon the public health. The sickness records of dispensaries and the mortality records of the Health Department show that a new and most destructive force is now operating, not only in the diseases above mentioned, but in nearly all of the diseases of the period. Fevers, inflammatory diseases, and others of a similar nature run a more rapid course, and are far less amenable to treatment. This is due, in the opinion of eminent medical authority, to the addition of the heat of the air to the heat of the body. Indeed, the only safety is in flight from the city to the country and to cool localities, as the seashore or the mountains. The immediate improvement of those suffering from affections of the city when transferred to the country is often marvelous, and shows conclusively how fatal is the element of heat in its direct and indirect effects upon the residents of the city.
Let us next consider the causes of high temperature in the city of New York. It is a well-established fact that the temperature of large and densely populated towns is far higher than the surrounding country. This is due to a variety of causes, the chief of which are the absence of vegetation; the drainage and hence the dryness of the soil; the covering of the earth with stone, bricks, and mortar; the aggregation of population to surface area; the massing together of buildings; and the artificial heat of workshops and manufactories. ​The difference between the mean temperature of the city at Cooper Institute and at the Arsenal, Central Park, for a single month, illustrates this fact. Another striking difference between the temperature of these two points of observation is that the range is much greater at Central Park than at Cooper Institute, the temperature falling at night more at the former than at the latter place. The effect of vegetation is to lower the temperature at night, while brick and stone retain the heat and prevent any considerable fall of temperature during the twenty-four hours. It may be said of New York that it has all the conditions of increased temperature above given in an intensified form. It has a southern exposure; all of its broad avenues run north and south; the surface is covered with stone, brick, and asphalt; it is destitute of vegetation except in its parks, which have a very limited area compared with the needs of the city; its buildings are irregularly arranged and crowded together so as to give the largest amount of elevation with the least superficial area; ventilation of courts, areas, and living rooms is sacrificed; its ill-constructed and overcrowded tenement houses, especially of certain districts, have the largest population to surface area of any city in the civilized world. To these natural and structural unfavorable sanitary conditions must be added the enormous production of artificial heat in dwellings. When the summer temperature begins to rise the solar heat is constantly added to the artificial heat already existing. The temperature of the whole vast mass of stones, bricks, mortar, and asphalt gradually increases, with no other mitigation or modification than that caused by the inconstant winds and occasional rainstorms. And the evils of high temperature are yearly increasing as the area of brick, stone, and asphalt extends. . The records of sunstroke during the past few years is appalling, both on account of the number of cases and their comparative increase. If no adequate remedy is discovered and applied, the day would not seem to be distant when the resident, especially if he is a laborer, will remain in the city and pursue his work during the summer at the constant risk of his life.
Turning now to consider the question of the measures which are best adapted to protect the present and future population of New York from the effects of high summer temperatures, we are met by many suggestions of more or less value. The more important methods proposed are: a large supply of public baths; the daily flushing of the streets with an immense volume of river water; recreation piers; excursions to the seashore; temporary residence in the country, etc. But these are for the most part temporary expedients, applicable to individuals, and are but accessory to some more radical measure which aims to so change the atmospheric conditions that ​excessive heat can not occur. The real problem to be solved may be thus stated: How can the temperature of the city of New York be so modified during the summer months as to prevent that extreme degree of heat on which the enormous sickness and death rate of the people depend? Discussing the subject broadly from this standpoint, it becomes at once evident that w T e must employ those agencies which in the wide field of Nature are designed to mitigate heat and purify the air and thus create permanent climatic conditions favorable for the habitation of man.
It requires but little knowledge of the physical forces which modify the climate of large areas of the earth's surface to recognize the fact that vegetation plays a most important part. And of the different forms of vegetation, trees, as compared with shrubs, plants, vines, and grasses, are undoubtedly the most efficient. This is due to the vast area of surface which their leaves present to the air on a very limited ground space. The sanitary value of trees has hitherto been practically unrecognized by man. With the most ruthless hand he has everywhere and at all times sacrificed this most important factor in the conservation of a healthful and temperate climate. He has found, too late, however, that by this waste of the forests he has by no means improved his own condition. The winters have become colder, the summers hotter; the living springs have ceased to flow perpetually; the fertilizing streams have disappeared; the earth is deeply frozen in winter and parched in summer; and, finally, new and grave diseases have appeared where formerly they were unknown.
It is well understood that the temperature in a forest, a grove, or even a clump of trees, is cooler in summer and warmer in winter than the surrounding country. Man and animals alike seek the shade of groves and trees during the heat of the day, and are greatly refreshed and revived by the cool atmosphere. The difference between the temperature of the air under and among the branches of a single tree, densely leaved, and the surrounding air, on a hot day, is instantly realized by the laborer or traveler who seeks the shade. The thermometer in the sun and shade shows a difference of twenty, thirty, and forty degrees, and in the soil a difference of ten to eleven degrees. The reverse is true in winter. The laborer and traveler exposed to the cold of the open country find in the forest a degree of warmth quite as great as in a building but imperfectly inclosed. Railroad engineers inform us that they have occasion to use far less fuel in passing through forests in winter than in traversing the same distance in the open country. When the ground in the fields is frozen two or three feet deep, its temperature in the forest is found above the freezing point. ​Forests and even single trees have, therefore, a marked influence upon the surrounding atmosphere, especially during the summer, and they evidently tend to equalize temperature, preventing extremes both in summer and winter. Hence they become of immense value as sanitary agencies in preserving equality of climatic conditions.
It is believed by some vegetable physiologists that trees exert this power through their own inherent warmth, which always remains at a fixed standard both in summer and winter. "Observation shows," says Meguscher, "that the wood of a living tree maintains a temperature of from 54° to 56° F., when the temperature stands from 37° to 47° F. above zero, and that the internal warmth does not rise and fall in proportion to that of the atmosphere. So long as the latter is below 67° F., that of the tree is always highest; but, if the temperature of the air rises to 67° F., that of the vegetable growth is the lowest." Since, then, trees maintain at all seasons a constant mean temperature of 54° F., it is easy to see why the air in contact with the forest must be warmer in winter and cooler in summer than in situations where it is deprived of that influence.
Again, the shade of trees protects the earth from the direct rays of the sun, and prevents solar irradiation from the earth. This effect is of immense importance in cities where the paved streets become excessively heated, and radiation creates one of the most dangerous sources of heat. Whoever has walked in the streets of New York, on a hot summer's day, protected from the direct rays of a midday sun by his umbrella, has found the reflected heat of the pavement intolerable. If for a moment he passed into the dense shade of a tree, he at once experienced a marked sense of relief. This relief is not due so much to the shade as to the cooling effect of the vaporization from the leaves of the tree.
Trees also have a cutaneous transpiration by their leaves. And although they absorb largely the vapor of the surrounding air, and also the water of the soil, they nevertheless exhale constantly large volumes into the air. This vaporization of liquids is a frigorific or cooling process, and when most rapid the frigorific effect reaches its maximum. The amount of fluid exhaled by vegetation has been, at various times, estimated with-more or less accuracy. Hales states that a sunflower, with a surface of 5.616 square inches, throws off at the rate of twenty to twenty-four ounces avoirdupois every twelve hours; a vine, with twelve square feet of foliage, exhales at the rate of five or six ounces daily. Bishop Watson, in his experiments ​on grasses, estimated that an acre of grass emits into the atmosphere 6.400 quarts of water in twenty-four hours.
It is evident, therefore, that vegetation tends powerfully to cool the atmosphere during a summer day, and this effect increases in proportion to the increase of the temperature. The influence of trees heavily leaved, in a district where there is no other vegetation, in moderating and equalizing the temperature, can not be overestimated. The amount of superficial surface exposed by the foliage of a single tree is immense. For example, "the Washington elm, of Cambridge, Mass., a tree of moderate size, was estimated several years since to produce a crop of seven million leaves, exposing a surface of two hundred thousand square feet, or about five acres of foliage."
Trees regulate the humidity of the air by the process of absorption and transpiration. They absorb the moisture contained in the air, and again return to the air, in the form of vapor, the water which they have absorbed from the earth and the air. The flow of sap in trees for the most part ceases at night, the stimulus of light and heat being necessary to the function of absorption and evaporation. During the heated portions of the day, therefore, when there is the most need of agencies to equalize both temperature and humidity, trees perform their peculiar functions most actively. Moisture is rapidly absorbed from the air by the leaves, and from the earth by the roots, and is again all returned to the air and earth by transpiration or exudation. The effect of this process upon temperature and humidity is thus stated by Marsh: "The evaporation of the juices of the plant by whatever process effected, takes up atmospheric heat and produces refrigeration. This effect is not less real, though much less sensible in the forest than in meadow and pasture land, and it can not be doubted that the local temperature is considerably affected by it. But the evaporation that cools the air diffuses through it, at the same time, a medium which powerfully resists the escape of heat from the earth by radiation. Visible vapor or clouds, it is well known, prevent frosts by obstructing radiation, or rather by reflecting back again the heat radiated by the earth, just as any mechanical screen would do. On the other hand, clouds intercept the rays of the sun also, and hinder its heat from reaching the earth." Again, he says, upon the whole, their general effect "seems to be to mitigate extremes of atmospheric heat and cold, moisture and drought. They serve as equalizers of temperature and humidity."
Again, let us notice the effects of trees upon malarial emanations. The power of trees, when in leaf, to render harmless the poisonous emanations from the earth has long been an established fact. Man may live in close proximity to marshes from which arise ​the most dangerous malaria with the utmost impunity, provided a grove intervene between his home and the marsh. This function of trees was known to the Romans, who enacted laws requiring the planting of trees in places made uninhabitable by the diffusion of malaria, and placed groves serving such purposes under the protection of some divinity to insure their protection. It is a rule of the British army in India to select an encampment having a grove between the camp and any low, wet soil.
Finally, trees purify the atmosphere. The process of vegetable nutrition consists in the appropriation by the plant or tree of carbon. This element it receives from the air in the form principally of carbonic acid, and in the process of digestion the oxygen is liberated and again restored to the air, while the carbon becomes fixed as an element of the woody fiber. Man and animals, on the contrary, require oxygen for their nutrition, and the supply is in the air they breathe. Carbon is a waste product of the animal system, and, uniting with the oxygen, is expired as carbonic acid, a powerful animal poison. A slight increase of the normal quantity of carbonic acid in the air renders it poisonous to man, and continued respiration of such air, or a considerable increase of the carbonic acid, will prove fatal. The animal and vegetable world, therefore, complement each other, and the one furnishes the conditions and forces by which the other maintains life and health. "Plants," says Schacht, "imbibe from the air carbonic acid and other gaseous or volatile products exhaled by animals, developed by the natural phenomena of decomposition. On the other hand, the vegetable pours into the atmosphere oxygen, which is taken up by animals and appropriated by them. The tree, by means of its leaves and its young herbaceous twigs, pre-sents a considerable surface for absorption and evaporation; it abstracts the carbon of carbonic acid, and solidifies it in wood fecula, and a multitude of other compounds. The result is that a forest withdraws from the air, by its great absorbent surface, much more gas than meadows or cultivated fields, and exhales proportionally a considerably greater quantity of oxygen. The influence of the forests on the chemical composition of the atmosphere is, in a word, of the highest importance."
In large cities, where animal and vegetable decomposition goes on rapidly during the summer, the atmosphere is, as already stated, at times saturated with deleterious gases. At the period of the day when malaria and mephitic gases are emitted in the greatest quantity and activity, this function of absorption by vegetation is most active and powerful. Carbonic acid, ammoniacal compounds, and other ​gases, products of putrefaction, so actively poisonous to man, are absorbed, and in the process of vegetable digestion the deleterious portion is separated and appropriated by the plant, while oxygen, the element essential to animal life, is returned to the air. Trees, therefore, in cities, are of immense value, owing to their power to destroy or neutralize malaria, and to absorb the poisonous elements of gaseous compounds, while they render the air more respirable by emitting oxygen.
The conclusion from the foregoing facts is inevitable that one of the great and pressing sanitary wants of New York city is an ample supply of trees. It is, in effect, destitute of trees; for the unsightly shrubs which are planted by citizens are, in no proper sense, adequate to the purpose which we contemplate. Its long avenues, running north and south, without a shade tree, and exposed to the full effect of the sun, are all but impassable at noonday in the summer months. The pedestrian who ventures out at such an hour finds no protection from an umbrella, on account of the radiation of the intense heat from the paved surface. Animals and man alike suffer from exposure in the glowing heat. Nothing mitigates its intensity but the winds or an occasional rainstorm. And when evening comes on, the cooling of the atmosphere produced by vegetation does not occur, and unless partially relieved by favoring winds or a shower the heat continues, but little abated, and the atmosphere remains charged with noxious and irrespirable gases. It is evident that shade trees, of proper kinds, and suitably arranged, supply the conditions necessary to counteract the evils of excessive heat. They protect the paved streets and the buildings largely from the direct rays of the sun; they cool the lower stratum of air by evaporation from their immense surfaces of leaves; they absorb at once the malarious emanations and gases of decomposition, and abstract their poisonous properties for their own consumption; they withdraw from the air the carbonic acid thrown off from the animal system as a poison, and decomposing it, appropriate the element dangerous to man, and give back to the atmosphere the element essential to his health and even life.
And we may add that cultivated shade trees in New York would be an artistic and attractive feature of the streets. Every citizen enjoys trees, as is evident from the efforts made to cultivate them throughout the city.
It is frequently alleged that trees can not be successfully cultivated in cities on account of the gases in the soil. There are ample proofs to the contrary. The city of Paris strikingly illustrates the ​possibility of cultivating a large variety of trees in the streets and public places of large cities when the planting and cultivation is placed under competent authority. In our own country the cities of New Haven and Washington are examples of the successful cultivation of trees to an extent sufficient to greatly modify the summer temperature. Authorities on landscape gardening and forestry sustain the view that under proper supervision by competent and skilled persons a great variety of trees, shrubs, plants, and vines can be cultivated in the streets and public places of this city. Mr. Frederick Law Olmstead, to whom the city is so much indebted for his intelligent supervision of Central Park in its early period, warmly supported a movement to cultivate trees, shrubs, plants, and vines in the streets of New York. Dr. J. T. Rothrock, the very able and experienced Commissioner of Forestry of Pennsylvania, under date of October 10, 1898, speaking of the proposed plan of securing the cultivating trees in the streets of this city, remarks: "I think it an excellent measure, and I am sure that during the torrid season the more tree shade you have the fewer will be your cases of heat exhaustion. It is idle to say, as is often said in this country, that trees can not be made to grow in our cities. Under existing conditions the wonder is, not that trees look unhealthy in most cities, but that any of them manage to live at all. It is perfectly well known that the city of Paris has thousands of trees growing vigorously under such surroundings as the American gardener would think impossible. Two things are necessary to success—viz., first, the kinds of trees to endure city life must be found; and, second, select from among them such as are adapted by their size and shape to each special place."
Mr. Gilford Pinchot, of the Division of Forestry, Department of Agriculture, Washington, writes under date of December 2, 1898: "Street trees are successfully planted in great numbers in all of the most beautiful cities of the world. Washington and Paris are conspicuous examples. That such trees succeed is largely due to the great care taken in setting them out. The attractiveness of cities has come to be reckoned among their business advantages, and nothing adds to it more than well-selected, well-planted, and well-cared-for trees. On the score of public health trees in the streets of cities are equally desirable. They become objectionable only when badly selected and badly maintained."
In a recent paper on Tree Planting in the Streets of Washington, Mr. W. P. Richards, surveyor of the District of Columbia, remarks that, under the plan adopted, "tree planting has never been at an experimental stage" in that city. Washington was a city of young trees during the seventies, and in the spring of 1875 more than six ​thousand trees were planted, consisting of silver maples, Norway maples, American elms, American and European lindens, sugar maples, tulip trees, American white ash, scarlet maples, various poplars, and ash-leaved maples. . . . A careful count was made of the trees in 1887, and by comparing this with the number of trees since planted and those removed, there is found to be more than seventy-eight thousand trees, which if placed thirty feet apart would line both sides of a boulevard between Washington and New York. These consist of more than thirty varieties." Mr. Richards adds: "The planting and care of trees in Washington grows from year to year, and the future will probably demand more skill and judgment than in years past. About twenty thousand dollars is spent annually, most of it in the care of old trees. From one to three thousand young trees are planted during the spring and fall of each year. The nursery has several thousand of the best varieties ready for planting."
The opinions of these authorities and the success of the work in Washington, now extending over a quarter of a century, determine beyond all question the feasibility and practicability of successfully cultivating trees in the streets of cities. And if any one doubts the power of trees cultivated in the streets to change the temperature of a city let him calculate the amount of foliage which the seventy-eight thousand trees, when full-grown, will furnish the city of Washington, taking as his basis the fact that a single tree, the Washington elm, at Cambridge, Massachusetts, when in full leafage, equals five acres of foliage, and that one acre of grass emits into the atmosphere 6.400 quarts of water in twenty-four hours, a powerfully cooling process.
We have, finally, to consider through what agency the proposed cultivation of trees in the city of New York can be accomplished most rapidly and successfully. Three methods may be suggested, viz.: 1. Encourage citizens each to plant and cultivate trees on his own premises. 2. Organize voluntary "tree-planting associations," which shall aid citizens or undertake to do the work at a minimum cost. 3. Place the work under the entire supervision and jurisdiction of public authority. The first method has been on trial from the foundation of the city, and its results are a few stunted apologies for trees which are useless for sanitary purposes and unsightly for ornamentation. The average citizen is entirely incompetent either to select the proper tree or to cultivate it when planted. Tree-planting associations have proved useful agencies in exciting a popular interest in the subject, and in aiding citizens in the selection of suitable trees and in cultivating them. The Tree-Planting and Fountain Society of Brooklyn, under the very able ​management of its accomplished secretary, Prof. Lewis Collins, is a model organization of the kind, and has accomplished a vast amount of good in this field in that city. But it may well be questioned if we have not reached a period of sanitary reform in cities when a work of the kind we contemplate in New York should not be undertaken by the strong arm of the city government, as a matter of public policy, and carried steadily forward to its completion. The growth of the greater city is far too rapid in every direction to await the slow movements of the people under the pressure of voluntary organizations. The best work can be done in those outlying districts where the streets are as yet but sparsely built upon, and the soil has been undisturbed. Again, it is of the utmost importance that a work of this kind, which will largely prove one of city ornamentation, should be under the exclusive direction of a skilled central authority having ample power and means to harmonize every feature of the work from the center of the city to its remotest limits. Finally, the successful cultivation of trees and other vegetation in our streets can be successfully carried on only by experts in the art of tree culture, who devote their entire time and energies to these duties, and are sustained by the power of the city government. Mr. Frederick Law Olmstead remarks, "Not one in a hundred of all that may have been planted in the streets of our American cities in the last fifty years has had such treatment that its species would come to be if properly planted and cared for." Mr. Richards, in the paper referred to on Tree Planting in the Streets of Washington, makes the following statement: "The selection, planting, and care of all trees in the streets of Washington are under the direction of the District authorities; individual preferences and private enterprises are not allowed to regulate this improvement, as is generally done in other cities. Moreover, the city has its own nursery, where seeds planted from its own trees grow and supply all the needed varieties."
It is apparent that to accomplish such a work as we propose the undertaking must be placed under the jurisdiction of a department of the city government, skilled in the performance of such duties, fully equipped with all needful appliances, and clothed with ample power and supplied with the-financial resources necessary to overcome every obstacle. Fortunately, we have in our Department of Parks an organized branch of the city administration endowed with every qualification for the performance of these duties. The charter provides as follows: "It shall be the duty of each commissioner. . . to maintain the beauty and utility of all such parks, squares, and public places as are situated within his jurisdiction, and to institute and execute all measures for the improvement thereof for ornamental purposes and for the beneficial uses of the people of the ​city, . . . and he shall have power to plant trees and to construct, erect, and establish seats, drinking fountains, statues, and works of art, when he may deem it tasteful or appropriate so to do." At the head of this service is "a landscape architect, skilled and expert, whose assent shall be requisite to all plans and works or changes thereof respecting the conformation, development, or ornamentation of any of the parks, squares, or public places of the city, to the end that the same may be uniform and symmetrical at all times."
The conclusion seems inevitable that public policy requires that, in the interests of the health of the people and the comfort and well-being of that large class of the poor who can not escape the summer heat by leaving the city, the jurisdiction of the Park Department should be extended to all trees, shrubs, plants, and vines now and hereafter planted and growing in the streets of New York, and that said department should be required to plant such additional trees, shrubs, etc., as it may from time to time deem necessary and expedient for the purpose of carrying out the intent and purpose of such act which should be declared to be to improve the public health, to render the city comfortable to its summer residents, and for ornamentation.
"He who plants a tree, he plants love; Tents of coolness, spreading out above Wayfarers, he may not live to see. {{float-center|Gifts that grow are best, Hands that bless are blest. Plant. Life does the rest."
* ↑ In 1872, while a Commissioner of Health, I had occasion to examine and report on the causes of the high death rate during the summer months in the city of New York. The chief cause was determined to be the excessive heat which characterizes those months. It was recommended in the report to the Board of Health that legislation be secured empowering and requiring the Department of Parks to plant and cultivate trees, shrubs, plants, and vines in all the streets, avenues, and public places in the city. A bill was drafted and introduced into the Legislature, but it did not become a law, and no further effort has been made to secure such legislation. Meantime, two tree-planting societies have been established, one in the Borough of Brooklyn and the other in the Borough of Manhattan, which are endeavoring to awaken public interest to the importance of planting a suitable number and variety of trees in the streets for purposes of ornamentation. The aim of this paper, which is largely based on the report of 1872, is to revive the project of giving the Department of Parks jurisdiction over the trees in the streets, and require it to plant and cultivate additional trees, shrubs, plants, and other forms of vegetation for the improvement of the public health and for the purpose of ornamentation.
* ↑ Man and Nature. G. P. Marsh, New York, 1872.
* ↑ It is interesting to notice, in this connection, the remark of Angus Smith, that a temperature of 54° F. is important in the decomposition of animal and vegetable matter.
* ↑ Public Parks. By John H. Rauch, M. D., Chicago, 1869.
* ↑ Les Arbres, quoted by Marsh.
* ↑ The late Dr. Francis remarked that he had noticed a marked increase in the fatality of diseases in sections of the city after the removal of trees and all vegetation.