THE HYGIENIC INFLUENCE OF PLANTS. 421 



disposed to give up the idea that the air in rooms can be improved by- 

 plants, because, as is well known, every green leaf absorbs carbonic 

 acid and gives out oxygen under the influence of light. This idea 

 may seem the more justifiable, because, although the production of 

 carbonic acid is not perceptible in the greatest assemblages of human 

 beings in the open air, it is always observed in confined spaces, al- 

 though the actual production is but small. In the air of a closed 

 apartment, every person and every light burning makes a perceptible 

 difference in the increase of carbonic acid in the air. Must not, there- 

 fore, every plant in a pot, every spray, any plant with leaves, make a 

 perceptible difference in a room ? Every lover of flowers may be 

 pardoned for wishing to see this question answered in the affirmative. 

 Have not even medical men proposed to adorn school-rooms with 

 plants in pots instead of ventilating them better, in order that their 

 leaves and stems might absorb carbonic acid from the mouths of the 

 children, and give out oxygen in its stead? But hygiene cannot 

 agree even to this. Hygiene is a science of economics, and every 

 such science has to ask not only what exists and whether it exists, 

 but how much there is and whether enough. The power of twenty- 

 pots of plants would not be nearly sufficient to neutralize the carbonic 

 acid exhaled by a single child in a given time. If children were de- 

 pendent on the oxygen given off by flowers, they would soon be suf- 

 focated. It must not be forgotten what a slow process the production 

 of matter by plants is matter which the animal organism absorbs 

 and again decomposes in a very short time, whereby as much oxygen 

 is used up as has been set free in the production of it. It is for this 

 reason that such great extents of vegetation are required for the sus- 

 tenance of animals and man. The grass or hay consumed by a cow 

 in a cow-house grows upon a space of ground on which a thousand 

 head of cattle could stand. How slow is the process of the growth of 

 wheat before it can be eaten as bread, which a man will eat, digest, 

 and decompose, in twenty-four hours ! The animal and human organ- 

 ism consumes and decomposes food as quickly as a stove burns the 

 wood which took so many thousand times longer to grow in the 

 forest. 



It would scarcely be intelligible if I were to calculate how much 

 carbonic acid and oxygen a rose, a geranium, or a bignonia, would 

 absorb and give out in a room in a day, and to what extent the air 

 might be changed by it, taking into account the inevitable change of 

 air always going on. I will draw attention to a concrete case which 

 every one can understand : 



When the Royal Winter Garden in Munich was completed and in 

 use, it occurred to me to make experiments on the effect of the whole 

 garden on the air within it. There could not be a more favorable 

 opportunity for experimenting on the air in a space full of vegetation. 

 This green and blooming space was not exposed to the free currents 



