794 | The Beneficial Influence of Plants. { December, 
being the abode of spirits, some of which were held sacred while 
others were supposed to be demonic. Trees were also supposed 
to be sentient beings and even possessed of souls. Strange as it 
may seem in this age of enlightenment, some relics of these 
ancient superstitions still linger in certain quarters of the globe. 
Prof. Pettenkofer! has lately discussed the question of the 
hygienic relations of plants from a new standpoint, and has 
doubtless thrown new light upon it. He has, to his own satisfac- 
tion, demonstrated that three of the great functions in plants, 
namely, the giving off of oxygen, the absorption of carbonic acid 
and generation of ozone, really have no hygienic value whatever. 
The proof of his argument rests largely upon the solid basis of 
experimental researches conducted by himself and other noted 
investigators. It is but fair to say, however, that Prof. Petten- 
kofer does not deny a// hygienic influence of vegetation, but 
attributes its influence to other circumstances rather than to the 
variation in the amount of the gases; and yet, in setting forth 
what he believes to be the sanitary operations of plants, he omits 
making any allusion to the process of transpiration as affecting 
the sanitary conditions of the air. This is not so surprising when 
we reflect how very imperfect our knowledge of this function has 
been up to a very recent date. In a paper on this subject we 
have presumed to attach more importance to this function. 
We shall now make the proposition—a deduction from actual 
experiment—that the hygienic conditions of the air are both 
directly and indirectly affected by plant transpiration. It will be 
seen that the statement ventured contains two distinct elements, 
one implying the direct effect of transpiration on the air, the 
other the indirect; and it has been deemed best to discuss 
these elements separately in order to render the subject easier of 
comprehension. 
The direct effect of transpiration might be formulated thus: 
In all atmospheres in which the proportion of aqueous vapor is 
less than the healthiest standard (about seven-eighths of what 
the air can contain at a given temperature), the beneficial influence 
-~ of transpiration must be in proportion to the amount of aqueous 
vapor exhaled. In this connection the question naturally arises, 
_ What is the rate at which watery vapor is given off from plants i 
Here it will, perhaps, be pardonable to refer to the author's pre- 
_ + Popular Science Monthly for February, 1878. 
