"Flower in the crannied wall,
I pluck you out of the crannies;
Hold you here, root and all, in my hand,
Little flower; but if I could understand
What you are, root and all, and all in all,
I should know what God and man is."
We cannot learn all about this little flower, but we can learn
enough to understand that it has a real separate life of its
own, well worth knowing. For a plant is born, breathes, sleeps,
feeds, and digests just as truly as an animal does, though in a
different way. It works hard both for itself to get its food,
and for others in making the air pure and fit for animals to
breathe. It often lays by provision for the winter. It sends
young plants out, as parents send their children, to fight for
themselves in the world; and then, after living sometimes to a
good old age, it dies, and leaves its place to others.
We will try to follow out something of this life to-day; and
first, we will begin with the seed.
I have here a packet of primrose-seeds, but they are so small
that we cannot examine them; so I have also had given to each
one of you an almond-kernel, which is the seed of the almond-
tree, and which has been soaked, so that it splits in half
easily. From this we can learn about seeds in general, and then
apply it to the primrose.
If you peel the two skins off your almond-seed (the
thick, brown, outside skin, and the thin, transparent one under
it), the two halves of the almond will slip apart quite easily.
One of these halves will have a small dent at the pointed end,
while in the other half you will see a little lump, which fitted
into the dent when the two halves were joined. This little lump
(a b, Fig. 37) is a young plant, and the two halves of the
almond are the seed leaves which hold the plantlet, and feed it
till it can feed itself. The rounded end of the plantlet (b)
sticking out of the almond, is the beginning of the root, while
the other end (a) will in time become the stem. If you look
carefully, you will see two little points at this end, which are
the tips of future leaves. Only think how minute this plantlet
must be in a primrose, where the whole seed is scarcely larger
than a grain of sand! Yet in this tiny plantlet lies hid the
life of the future plant.
When a seed falls into the ground, so long as the earth is cold
and dry, it lies like a person in a trance, as if it were dead;
but as soon as the warm, damp spring comes, and the busy little
sun-waves pierce down into the earth, they wake up the plantlet
and make it bestir itself. They agitate to and fro the particles
of matter in this tiny body, and cause them to seek out for
other particles to seize and join to themselves.
But these new particles cannot come in at the roots,
for the seed has none; nor through the leaves, for they have not
yet grown up; and so the plantlet begins by helping itself to
the store of food laid up in the thick seed-leaves in which it
is buried. Here it finds starch, oils, sugar, and substances
called albuminoids, -- the sticky matter which you notice in
wheat-grains when you chew them is one of the albuminoids. This
food is all ready for the plantlet to use, and it sucks it in,
and works itself into a young plant with tiny roots at one end,
and a growing shoot, with leaves, at the other.
But how does it grow? What makes it become larger? To answer this
you must look at the second thing I asked you to bring - a piece
of orange. If you take the skin off a piece of orange, you will
see inside a number of long-shaped transparent bags, full of
juice. These we call cells, and the flesh of all plants and
animals is made up of cells like these, only of various shapes.
In the pith of elder they are round, large, and easily seen (a,
Fig. 39); in the stalks of plants they are long, and lap over
each other (b, Fig. 39), so as to give the stalk strength to
stand upright. Sometimes many cells growing one on the top of
the other break into one tube and make vessels. But whether
large or small, they are all bags growing one against the other.
In the orange-pulp these cells contain only sweet juice, but in
other parts of the orange-tree or any other plant
they contain a sticky substance with little grains in it. This
substance is called "protoplasm," or the first form of life, for
it is alive and active, and under a microscope you may see in a
living plant streams of the little grains moving about in the
cells.
Now we are prepared to explain how our plant grows. Imagine the
tiny primrose plantlet to be made up of cells filled with active
living protoplasm, which drinks in starch and other food from
the seed-leaves. In this way each cell will grow too full for
its skin, and then the protoplasm divides into two parts and
builds up a wall between them, and so one cell becomes two. Each
of these two cells again breaks up into two more, and so the
plant grows larger and larger, till by the time it has used up
all the food in the seed-leaves, it has sent roots covered with
fine hairs downwards into the earth, and a shoot with beginnings
of leaves up into the air.
Sometimes the seed-leaves themselves come above the ground, as in
the mustard-plant, and sometimes they are left empty behind,
while the plantlet shoots through them.
And now the plant can no longer afford to be idle and
live on prepared food. It must work for itself. Until now it has
been taking in the same kind of food that you and I do; for we
too find many seeds very pleasant to eat and useful to nourish
us. But now this store is exhausted. Upon what then is the plant
to live? It is cleverer than we are in this, for while we cannot
live unless we have food which has once been alive, plants can
feed upon gases and water and mineral matter only. Think over the
substances you can eat or drink, and you will find they are
nearly all made of things which have been alive: meat,
vegetables, bread, beer, wine, milk; all these are made from
living matter, and though you do take in such things as water
and salt, and even iron and phosphorus, these would be quite
useless if you did not eat and drink prepared food which your
body can work into living matter.
But the plant as soon as it has roots and leaves begins to make
living matter out of matter that has never been alive. Through
all the little hairs of its roots it sucks in water, and in this
water are dissolved more or less of the salts of ammonia,
phosphorus, sulphur, iron, lime, magnesia, and even silica, or
flint. In all kinds of earth there is some iron, and we shall see
presently that this is very important to the plant.
Suppose, then, that our primrose has begun to drink in water at
its roots. How is it to get this water up into the stem and
leaves, seeing that the whole plant is made of closed bags or
cells? It does it in a very curious way, which you can prove for
yourselves. Whenever two fluids, one thicker than the other,
such as treacle and water for example, are only separated by a
skin or any porous substance, they will always mix, the thinner
one oozing through the skin into the thicker one. If you tie a
piece of bladder over a glass tube, fill the tube half-full of
treacle, and then let the covered end rest in a bottle of water,
in a few hours the water will get in to the treacle and the
mixture will rise up in the tube till it flows over the top. Now,
the saps and juices of plants are thicker than water, so, directly
the water enters the cells at the root it oozes up into the cells
above, and mixes with the sap. Then the matter in those cells
becomes thinner than in the cells above, so it too oozes up, and
in this way cell by cell the water is pumped up into the leaves.
When it gets there it finds our old friends the sun-beams hard at
work. If you have ever tried to grow a plant in a cellar, you
will know that in the dark its leaves remain white and sickly.
It is only in the sunlight that a beautiful delicate green tint
is given to them, and you will remember from Lecture II. that
this green tint shows that the leaf has used all the sun-waves
except those which make you see green; but why should it do this
only when it has grown up in the sunshine?
The reason is this: when the sunbeam darts into the leaf and sets
all its particles quivering, it divides the protoplasm into two
kinds, collected into different cells. One of these remains
white, but the other kind, near the surface, is altered by the
sunlight and by the help of the iron brought in by the water.
This particular kind of protoplasm, which is called "chlorophyll,"
will have nothing to do with the green waves and throws them back,
so that every little grain of this protoplasm looks green and
gives the leaf its green colour.
It is these little green cells that by the help of the sun-waves
digest the food of the plant and turn the water and gases into
useful sap and juices. We saw in Lecture III. that when we
breathe-in air, we use up the oxygen in it and send back out of
our mouths carbonic acid, which is a gas made of oxygen and
carbon.
Now, every living things wants carbon to feed upon, but plants
cannot take it in by itself, because carbon is solid (the
blacklead in your pencils is pure carbon), and a plant cannot
eat, it can only drink-in fluids and gases. Here the little
green cells help it out of its difficulty. They take in or
absorb out of the air carbonic acid gas which we have given out
of our mouths and then by the help of the sun-waves they tear
the carbon and oxygen apart. Most of the oxygen they throw back
into the air for us to use, but the carbon they keep.
If you will take some fresh laurel-leaves and put them into a
tumbler of water turned upside-down in a saucer of water, and
set the tumbler in the sunshine, you will soon see little bright
bubbles rising up and clinging to the glass. These are bubbles
of oxygen gas, and they tell you that they have been set free by
the green cells which have torn from them the carbon of the
carbonic acid in the water.
But what becomes of the carbon? And what use is made of the water
which we have kept waiting all this time in the leaves? Water,
you already know, is made of hydrogen and oxygen, but perhaps
you will be surprised when I tell you that starch, sugar, and
oil, which we get from plants, are nothing more than hydrogen
and oxygen in different quantities joined to carbon.
It is very difficult at first to picture such a black thing as
carbon making part of delicate leaves and beautiful flowers, and
still more of pure white sugar. But we can make an experiment by
which we can draw the hydrogen and oxygen out of common loaf
sugar, and then you will see the carbon stand out in all its
blackness. I have here a plate with a heap of white sugar in it.
I pour upon it first some hot water to melt and warm it, and then
some strong sulphuric acid. This acid does nothing more than
simply draw the hydrogen and oxygen out. See! in a few moments a
black mass of carbon begins to rise, all of which has come out of
the white sugar you saw just now. *(The common dilute sulphuric
acid of commerce is not strong enough for this experiment, but
pure sulphuric acid can be secured from any chemist. Great care
must be taken in using it, as it burns everything it touches.) You
see, then, that from the whitest substance in plants we can get
this black carbon; and in truth, one-half of the dry part of every
plant is composed of it.
Now look at my plant again, and tell me if we have not already
found a curious history? Fancy that you see the water creeping
in at the roots, oozing up from cell to cell till it reaches the
leaves, and there meeting the carbon which has just come out of
the air, and being worked up with it by the sun-waves into
starch, or sugar, or oils.
But meanwhile, how is new protoplasm to be formed? for without
this active substance none of the work can go on. Here comes
into use a lazy gas we spoke of in Lecture III. There we thought
that nitrogen was of no use except to float oxygen in the air,
but here we shall find it very useful. So far, as we know,
plants cannot take up nitrogen out of the air, but they can get
it out of the ammonia which the water brings in at their roots.
Ammonia, you will remember, is a strong-smelling gas, made of
hydrogen and nitrogen, and which is often almost stifling near a
manure-heap. When you manure a plant you help it to get this
ammonia, but at any time it gets some from the soil and also
from the rain-drops which bring it down in the air. Out of this
ammonia the plant takes the nitrogen and works it up with the
three elements, carbon, oxygen, and hydrogen, to make the
substances called albuminoids, which form a large part of the
food of the plant, and it is these albuminoids which go to make
protoplasm. You will notice that while the starch and other
substances are only made of three elements, the active protoplasm
is made of these three added to a fourth, nitrogen, and it also
contains phosphorus and sulphur.
And so hour after hour and day after day our primrose goes on
pumping up water and ammonia from its roots to its leaves,
drinking in carbonic acid from the air, and using the sun-waves
to work them all up into food to be sent to all parts of its
body. In this way these leaves act, you see, as the stomach of
the plant, and digest its food.
Sometimes more water is drawn up into the leaves than can be
used, and then the leaf opens thousands of little mouths in the
skin of its under surface, which let the drops out just as drops
of perspiration ooze through our skin when we are overheated.
These little mouths, which are called stomates (a, Fig. 42) are
made of two flattened cells, fitting against each other. When
the air is damp and the plant has too much water these lie open
and let it out, but when the air is dry, and the plant wants to
keep as much water as it can, then they are closely shut. There
are as many as a hundred thousand of these mouths under one
apple-leaf, so you may imagine how small they often are.
Plants which only live one year, such as mignonette, the sweet
pea, and the poppy, take in just enough food to supply their
daily wants and to make the seeds we shall speak of presently.
Then, as soon as their seeds are ripe their roots begin to
shrivel, and water is no longer carried up. The green cells can
no longer get food to digest, and they themselves are broken up by
the sunbeams and turn yellow, and the plant dies.
But many plants are more industrious than the stock and
mignonette, and lay by store for another year, and our primrose
is one of these. Look at this thick solid mass below the primrose
leaves, out of which the roots spring. (See the plant in the
foreground of the heading of the lecture.) This is really the
stem of the primrose hidden underground, and all the starch,
albuminoids, &c., which the plant can spare as it grows, are
sent down into this underground stem and stored up there, to lie
quietly in the ground through the long winter, and then when the
warm spring comes this stem begins to send out leaves for a new
plant.