1140. _What is the difference between an animal, a plant, and a
mineral?_
The great naturalist, Linnæus, used to say that animals _grow_, _live_,
and _feel_; plants _grow_ and _live_; and minerals _grow_.
Animals are here defined to enjoy _three_ conditions of existence;
plants _two_ conditions; and minerals _one_ condition.
This definition has, in latter days, been held to be unsatisfactory,
since there _are a few plants_ that are _supposed to feel_, and _a
few animals_ that are supposed to have even _less feeling_ than the
_sensitive plants_ alluded to.
The concise definition by Linnæus, nevertheless, is true, as far as
regards a _vast majority_ of the bodies constituting the three great
kingdoms of nature. And it may be sufficient to say that
_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:"--
[Verse: "The heavens declare the glory of God: and the firmament
showeth his handy work. Day unto day uttereth speech, and night unto
night showeth knowledge."--PSALM XIX.]
"The power of decomposing carbonic acid is a vital function
which belongs to the leaves and bark. It has been stated, on the
authority of Leibig, that during the night the plant acts only
as a mere bundle of fibres--that it allows of the circulation of
carbonic acid and its evaporation, unchanged. In his eagerness to
support his chemical hypothesis of respiration, the able chemist
neglected to enquire if this was absolutely correct. The healthy
plant never ceases to decompose carbonic acid during one moment of
its existence; but during the night, when the excitement of light
is removed, and the plant reposes, its vital powers are at their
minimum of action, and a much less quantity is decomposed than
when a stimulating sun, by the action of its rays, is compelling
the exertion of every vital function."
In hot, swampy countries, where vegetation is very rapid, and
the soil loaded with decomposing carbonic matter, the plants
absorb more carbonic acid than they require, and they _then_
evolve carbonic acid gas from their leaves. Hence such climates
as the West Indies are injurious to _life_, though favourable to
_vegetation_.
1147. _How do plants obtain hydrogen?_
They obtain _hydrogen_ in combination with _oxygen_ in water, and with
_nitrogen_, in the form of _ammonia_, as which it exists in animal
manures.
1148. _How do plants obtain nitrogen?_
From the _atmospheric air_, and from the _soil_, in which it is
combined with other elements.
1149. _How do plants apply these elements to the formation of their own
structures?_
When those substances which form the food of plants are absorbed,
either by their leaves or their roots, they are converted, with the aid
of water, into a _nutritive sap_, which answers the same purposes in
_plants_ as _blood_ does in _animals_.
1150. _How is the nutritive sap applied to the growth and enlargement
of the plant?_
Every seed contains a small amount of nutrition, sufficient for the
sustentation of the _germ of the plant_, until those vessels are
formed, by which the nutritive elements can be absorbed and used for
the further development of the living structure.
The earth, penetrated by the sun's rays, warms the sleeping germ, and
quickens it into life. For a short time the germ lives upon the seed,
which, moistened and warmed by the soil, yields a kind of glutinous
sap, out of which the first members of the plant are formed. And
then the tender leaf, looking up to the sky, and the slender rootlet
penetrating the soil, begin to draw their sustenance from the vast
stores of nature.
[Verse: "He causeth the grass to grow for the cattle, and herb
for the service of man: that he may bring forth food out of the
earth."--PSALM CIV.]
1151. _Of what do vegetable structures consist?_
Of _membranes_, or thin tissues, which, being variously arranged, form
cells, tubes, air passages, &c. Of _fibres_, which form a stronger
kind of membrane, and which is variously applied to the production of
the organs of the plants. And of _organs_, formed by those elementary
substances, by which the plants absorb, secrete, and grow, and fulfil
the conditions of their existence.
1152. _Why are seeds generally enveloped in hard cases?_
Because the covering of the seed, like the shell of an egg, is designed
_to preserve the germ_ within from the influence of external agencies,
until the time for development has arrived, and the conditions of
germination are fulfilled.
1153. _Why does a seed throw out a root, before it forms a leaf?_
Because moisture, which the root absorbs from the earth, is necessary
to enable the germ _to use the nutrition which the seed itself
contains_, and out of which the leaf must be eliminated. Moisture forms
a kind of gluten, in which the starch of the seed is dissolved, and
converted into sugar, the sugar into carbonaceous sap, and the sap into
cellular tissue and woody fibre, as the leaves present themselves to
the influence of the air and light.
1154. _Why does a plant grow?_
Because, as soon as membranes and vessels are organised in the young
germ, the nutritive fluid, formed by its first organs, _begins to move
through the fine structures_, and from that time the plant commences
to incorporate with its own substance the elements with which it is
surrounded, that are suitable to its development.
[Verse: "Can the rush grow up without mire? can the flag grow
without water? Whilst it is yet in his greenness, and not cut down, it
withereth before any other herb."--JOB VIII.]