296 



THE GARDENER'S MONTHLY 



[October, 



where.* 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 oflf 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 i)lant8 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 re- 

 spect similar, were tested simultaneously, in 

 order to note the variations produced by'grow- 

 ing jilants. In the window of one of the rooms 

 were situated five thrifty plants, the other con- 

 tained none. 



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. Think- 

 ing that possibly this diflference of humidity 

 might not be owing solely to tlie presence of 

 plants, the conditions were varied, and further 

 observations made, with similar results. The 

 manner in which these investigations were car- 

 ried out cannot be here detailed. The following 

 conclusion should, however, be quoted : " Dur- 

 ing 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."t This point, 

 as will presently be seen, is of special importance 

 where houses are heated by dry-air furnaces. 



Although science cannot readily determme 

 the exact relative humidity most conducive to 



*See American Naturalist for March, 1879. 

 t" Beneficial Influence of Plants," American Naturalist, 

 December, 1879. 



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 de- 

 gree of humidity is generally below that standard 

 in this latitude. It was also found that air 

 warmed by an open fireplace, or bj' 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 tem- 

 perature. 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. Stills ob- 

 serves, "... 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 pul- 

 monary diseases, and, in a word, deterioration of 

 all the functions."* 



It is true that in special states of the system — 

 e. g., in chronic rheumatism — dry heat is bene- 

 ficial; but this is no argument against the bene- 

 fit 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 bj'^ 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 de- 

 lighting 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 



♦Therapeutics, vol. i. pp. 637, 638. 



