Read before the Alumni of the Auxiliary Department of Medicine, University of Pennsylvania, February 6, 1880.

The old question of the effects of living plants on the air of houses is one of considerable interest. The family doctor is ofttimes confronted with the query, "How do plants in rooms affect the health of the inmates?" Formerly, it was the universal opinion that they were injurious to health, particularly in the sleeping-room and sick-chamber. Unfortunately, this still continues to be a popular impression. To review the various views on this topic down to the present would be foreign to the scope of this article and quite out of place. The discussion will necessarily be confined to the present state of our knowledge concerning the subject, and especially such of its bearings as are interesting from a medical point of view.

Three of the chief functions in plant life are the absorption of carbonic acid, the exhalation of oxygen, and the generation of ozone. Now, it has been conclusively shown* that variations in the amount of these gases from the presence of any number of plants have no appreciable effect on the air of an apartment, the absorption and exhalation of these substances being carried on too slowly either to improve or to vitiate the air.

There is, however, yet another process in plants, which in this connection is of far greater importance, viz., that of transpiration. By this term is meant the exhalation of moisture by the leaves. About this function very little was known until recently. Careful investigations of the subject have been made by the writer, to which brief reference can only be made here, for they have formed the basis of a paper elsewhere.* It may suffice to say that the average rate of transpiration for plants having soft, thin leaves, as the geranium, lantana, etc., is one and a half ounces (by weight) of watery vapor per square foot of leaf-surface for twelve diurnal hours of clear weather. In order to convey some notion of the great activity of this function, it might be stated that at the above rate the Washington elm, at Cambridge, Massachusetts with its two hundred thousand square feet of leaf-surface, would give off seven and three-quarter tons of water in twelve hours. In the twenty-four hours an indoor plant will transpire more than half as much as one in the open air. It would appear to follow naturally from these facts that growing plants would be capable of raising the proportion of aqueous vapor of the air of closed apartments.

And this suggestion prompted the writer to make observations with the view of establishing this fact experimentally. By means of the hydrometer, the atmosphere of two rooms at the Episcopal Hospital, in which the conditions and dimensions were in every respect similar, were tested simultaneously, in order to note the variations produced by growing plants. In the window of one of the rooms were situated five thrifty plants, the other contained none.

* Pettenkofer, Pop. Science Monthly for February, J878.

For eighteen consecutive days the dew-point of the room containing plants gave an average complement one and a half degrees lower than the room in which there were no plants. Thinking that possibly this difference of humidity might not be owing solely to the presence of plants, the conditions were varied, and further observations made, with similar results. The manner in which these investigations were carried out cannot be here detailed. The following conclusion should, however, be quoted: " During the summer months, when the windows are thrown widely open and the doors kept ajar, the influence of transpiration is quite inconsiderable; on the other hand, when the interchange of air is not too rapid, a sufficient number of plants, well watered, have the effect (if the air be not already saturated) of increasing the amount of moisture to a considerable extent." This point, as will presently be seen, is of special importance where houses are heated by dry-air furnaces.

Although science cannot readily determine the exact relative humidity most conducive to health, still, according to the best authorities on the subject, it is considered that about seven-eighths of what the air will contain at a given temperature is the proper standard. By repeated testing the writer has recently found that the degree of humidity is generally below that standard in this latitude. It was also found that air warmed by an open fireplace, or by air heated by steam, gave a complement of the dew-point from two to four degrees Fahrenheit greater than the external air, and in the case of rooms heated by a dry-air furnace the complement was from five to seven degrees greater at the same temperature. From this exhibit it will be seen that the atmosphere of a room warmed by dry air contains far too little moisture to be healthful. The peculiar effects of dry air on human beings are well known to the progressive practitioner. With respect to this question Prof. A. Stille" observes, "... A great demand is made upon the system to supply the air with moisture ; the skin and pulmonary mucous membrane are dried, and a condition is induced which is expressed in irritability of the nervous system, paleness and susceptibility of the skin to cold, liability to pulmonary diseases, and, in a word, deterioration of all the functions."*

*See American Naturalist for March. 1879. †" Beneficial Influence of Plants," American Naturalist, December, 1879.

It is true that in special states of the system- e. g., in chronic rheumatism - dry heat is beneficial; but this is no argument against the benefit ordinarily derived from a proper amount of moisture in the atmosphere. On the other hand, if the presence of a certain number of thrifty plants in an occupied room warmed by dry air would have the effect of raising the relative humidity to the extent indicated, it is clear that we possess in them one of the readiest means of obviating these evil consequences. In all in-| stances, then, in which artificial heat is used, but particularly in the case of dry air, as that fur-nished by furnaces, plants become, under proper regulations, hygienic agents of special value.

Were this article intended for popular reading, much might also be said in favor of keeping house-plants for the benefit they confer in delighting our senses and ministering to our aesthetic tastes ; but we are discussing the ques-tion from a strictly medical point of view, and such matter would be somewhat irrelevant.

Since it is well established that certain maladies are benefited by a moderately moist and warm atmosphere, and since plants can, as has been shown, furnish this moisture to the warm air of rooms, they might with propriety be classed as therapeutic agents; but to draw lines of distinction between their hygienic and their therapeutic application would be an unnecessary refinement of terms.

* Therapeutics, vol. i. pp. 637, 638.

Of course it is chiefly in diseases of a chronic nature, and particularly those affecting the lungs and air-passages, that we should expect to derive good results from such a measure as stocking the sick-room with growing plants, for it is in such cases that dry heat does the most harm. Still, they would prove beneficial also, in a less degree, in acute diseases, especially the continued fevers, and, perhaps, membranous croup, where moisture in the air is so desirable. House-plants have, however, a sphere of usefulness which is independent of atmospheric humidity. In nervous disorders of the functional class, such as melancholia and chlorosis, in diseases of the mind proper, and in other allied conditions (excessive grief, ennui, etc.), where it is necessary to divert the mind or relieve tension, nothing is more efficient than the pleasing occupation of studying and caring for plants.

But it is in that sweeping disease phthisis that plants offer the best hope of success as therapeutic agents. The importance of this point demands that it should receive careful attention.

(To be continued).