1876.] Hygiene of House Plants. 669 
and overturn the soil as by an earthquake. We are learning to 
look for power in its fullest development, not so much in the 
more noisy phenomena that all observe, as in those unobtrusive, 
noiseless processes, unseen save by the eye of science, that go on 
all about us. In raindrop and snowflake, in forming leaf and 
opening bud, we are taught to look for force greater than we can 
know. In the rush of a landslide, as it crushes and overturns 
everything in its way, all recognize vast power; but all do not 
know that in the growth of every tree that lies crushed in the 
pathway of those rock-masses more force was expended than 
would be needed to hurl them whence they came. It is well 
known that the leaves of plants exhale moisture, but it is not 
so well known how much various plants give to the air. The 
amount varies almost constantly, being affected by temperature, 
dryness of air, amount of light, and condition of the plant. But 
the amount of water given to the air during a season by plants, 
ìs very considerable. A French botanist found that an oak ex- 
haled in one season eight and a half times as much water as fell 
in rain over an area equal to that of the leaves. And other simi- 
lar experiments give similar results. 
The well-known process of taking carbonic acid from the air 
and returning oxygen to it, fixing the carbon in the tissues of the 
Plant, has been shown by Bernard and others to be a true digest- 
We and assimilative process, while all the time a true respiratory 
process is carried on by which oxygen is taken and carbonic acid 
Siven out. During the day, when the leaves are subject to the 
action of light, both these processes go on, but the assimilative 
| Process is vastly the more energetic and conceals the other proc- 
8 wholly, When light is withdrawn the respiratory process 
comes into prominence, because of the almost or entire cessation 
| ‘the other, so that the action of plants by night a 
the reverse of that by day, and so it is practically ; but itis to be 
i ‘Temembered that the most vigorous and important action of the 
a Plant, that which alone exerts any very marked influence upon 
— Ve surrounding air, is that by which carbonic acid is taken from 
arand oxygen given back. The re 
: wo processes Ois at different times of the year, as ar T 
Winder has shown that when the leaves expand they contain a 
ee Proportion of nitrogenous matter, which decreases gradu- 
I J until autumn, while as the leaves become 
e oa naceous matter increases, at first rapidly ’ 
; uo after a time it remains fixed until towards autumn, 
is said to be 
lative activity of these 
fully developed the 
then more slowly, 
when it 
