The reason why : $b a careful collection of many hundreds of reasons for things which, though generally believed, are imperfectly understood. A book of condensed scientific knowledge for the million
John Stuart Mill · en
_Animals_--grow, live, feel, and move.
_Plants_--grow and live.
_Minerals_--grow, by the addition of particles of inorganic matter.
If we now state the few exceptions that are admitted to this
definition, we shall bring the explanation as near to the truth,
as the present state of knowledge will permit.
[Verse: "And God said, Behold, I have given you every herb bearing
seed, which is upon the face of all the earth, and every tree, in the
which is the fruit of a tree, yielding seed; to you it shall be for
meat."--GENESIS I.]
1141. _Why is it understood that some plants feel?_
Because the _sensitive plant_ closes its leaves on being touched; the
_Venus's fly trap_ closes its leaves upon flies that alight upon them;
others _close_ upon the approach of rain, and at sunset, and _open_ at
sunrise, and turn towards the sun during its daily transit.
1142. _Why is it understood that some plants move?_
Because certain _sea-weeds_ throw off undeveloped young plants, which
move through the water by the aid of fine _cilia_, or muscular hairs,
until they find a suitable place upon which to attach themselves.
The roots of plants will penetrate through the ground in the direction
of water, and of favourable soil.
1143. _Of what elementary substances are plants composed?_
Of carbon, oxygen, hydrogen, and nitrogen.
1144. _Whence do plants derive those substances?_
From the air, the earth, and water.
1145. _How do plants obtain carbon?_
They obtain it chiefly from the air, in the form of _carbonic acid
gas_. The carbon, of the carbonic acid gas, which is thrown out by
the breath of animals, and by other processes in nature, is _absorbed
by the leaves of plants_, and the _oxygen_ which had united with the
carbon to form the _carbonic acid gas_, is again set free for the use
of animals.
1146. _How do plants obtain oxygen?_
They obtain it from the _atmospheric air_. But as they do not require
a large amount of oxygen for their own use, _they throw off the amount
which is in excess_, after having separated it from the other elements
with which it was combined when taken up by them. From the humble blade
of grass, to the stately tree of the forest, plants operate to purify
the air, and to correct and counteract the corruption of the air, by
the myriads of animals inhabiting the earth.
It has been generally stated that plants in rooms purify the air
by absorbing carbonic acid _by day_, and releasing a part of the
oxygen; but that, as the presence of light is necessary to produce
this action, they do not restore oxygen to the air, by night, but,
on the contrary, give off carbonic acid gas. Therefore it has been
stated that plants in rooms by night are unhealthy. Mr. Robert
Hunt, one of the ablest chemists of the present time, makes the
following remarks upon this subject in his "Poetry of Science:"--