SEEDS AND SEEDLINGS
If we wish to follow the whole life of a plant, we cannot do better
than begin by watching the baby plant “hatching” out from its seed at
the beginning of its active life.
There are many seeds which would be good to begin work on, any kind
would be interesting, but it is best to use some nice big ones which
allow us to see the parts easily. Good ones to choose would be broad
beans or peas. Notice first the size and shape of the dry seed of the
bean, make a drawing of it, and then place it in water. After a few
hours you will see that the outside skin wrinkles up; this is because
the skin absorbs water and increases in size, and so becomes too big
for the rest of the seed (~see~ fig. 3, A, B). After the water has
soaked right into the substance of the seed you will find that the
outer skin fits again and is once more smooth, and that the whole seed
is larger than it was before it was soaked (~see~ fig. 3, C).
[Illustration: Fig. 3. A single Bean seed, A dry; B half soaked, when
the skin wrinkles; C fully soaked and swollen.]
Take one of these soaked beans and examine its structure. Notice the
black mark where it was attached to the parent pod, and the little
triangular ridge pointing towards it (~see~ fig. 4, A). Now carefully
peel off the skin, noticing that there are two skins, an outer thick
one and an inner thin one, which protect the parts within. When you
have removed the skin, you will find that the inner portions of the
seed split very readily into two thick fleshy parts, and that lying
between them is a tiny young plant. Notice how this young plant is
connected on either side with the fleshy parts, so that to separate
them you must tear one side or the other as in fig. 4 B, where at (~a~)
we see the scar left where the tiny plant (~p~) was torn from the side.
The two big fleshy parts are really portions of the young plant, and
are in fact its two first leaves, but they are very different from
ordinary leaves, and are packed with food substances, and are called
the ~cotyledons~, or “nurse-leaves.” Notice also the tiny root of the
baby plant or ~embryo~, as it is called; it bends a little to the outer
side, and fits into a kind of pocket in the skin of the coat. You can
see the shape of the root even from the outside of the dry bean (~see~
fig. 4, A (~r~)). You will find in the pea, cucumber, and many other
seeds, that there is also the tiny embryo with its two nurse leaves,
the whole being protected by strong coats. The differences between the
bean, pea, and cucumber seeds are only in the details of shape and
colour, not in the actual ~parts~ of the seed.
[Illustration: Fig. 4. A, outside of Bean; (~h~) black scar showing
where the bean was attached to the pod; (~r~) ridge made by young root;
B, bean split open; (~n~) nurse leaves; (~p~) baby plant; (~a~) scar
where the baby plant was separated from the nurse leaf on that side.]
In the case of maize and corn, however, you will find that the seed
does not split into two equal parts like the bean, but that the young
plant lies at one side of the seed, and a solid white mass fills the
rest of the space (~see~ fig. 5). There are also differences in the
seedlings which you will notice when they begin to grow.
[Illustration: Fig. 5. A, outside of Maize fruit, showing the embryo
(~e~) on one side; B, sprouting plant, showing the root (~r~) and shoot
(~s~); C, the same further grown.]
Now that you have examined some seeds, you should start a number
growing, so as to have plenty to watch. They will grow more quickly
if you soak them in water for a night before you plant them in damp
sawdust, and keep them moist and fairly warm all the time. You should
have a number of seeds of each kind planted together to provide enough
for you to dig up one of them every day and examine it fully, inside
as well as out. Make a drawing of each one so that you will have a
complete series of drawings showing how the young plants grow. This
will kill them, so that you must leave at least one seedling which is
never touched, and which you can watch all through its life.
[Illustration: Fig. 6. Growth of Bean seedling: A, the root only
showing; B, the root lengthening and shoot appearing.]
As the young plant grows, notice how it breaks away from the protection
of its nurse leaves; first the root comes out and bends downwards into
the sawdust (~see~ fig. 6 A), then the little shoot which bends up into
the air.
Whichever way you plant the seeds you will find this is always the
case, for even if you start with the root pointing up, it will bend
round and grow downwards while the shoot bends up (~see~ p. 41).
As the plant gets bigger, side roots grow out from the main one, and
the little leaves of the shoot begin to open out--the whole plant is
growing (~see~ fig. 7).
[Illustration: Fig. 7. Later stage in the growth of Bean seedling; side
roots developed, and the shoot enlarged.]
=Now we may perhaps begin to find out something about the question
of feeding in plants.= What are the nurse-leaves doing all the time
the plant is growing? You will find in the bean that the seed coats
may split open a little, but that on the whole the cotyledons remain
all the time enclosed in them, and attached to the young shoot (~see~
fig. 7). Examine the nurse leaves of seedlings of different ages, and
you will see that they are much less thick and fleshy in the older
seedlings. As the plant gets bigger the nurse-leaves get thinner and
less until they become merely dry shrivelled remnants.
Now, ~what use could the cotyledons be if they only shrivel away~?
Take a freshly soaked seed and cut a thin slice of the nurse-leaves
and drop it into a little solution of iodine;[1] the tissue will go a
violet blue colour. Then drop iodine on a piece of bread, a piece of
potato, and some boiled rice, and you will find that they also go blue,
or almost black. The food in the nurse-leaves is in some ways the same
as that in bread, potato, and rice, and in many other things we eat.
[1] Get a chemist to make a solution of iodine and potassium iodide,
which should be a bright, clear, orange colour.
The part of the food which goes blue with iodine is ~starch~, and this
blue colouring of starch with iodine is an easy and safe test for it.
You will see the same colour if you take some ordinary laundry starch
and stir it up with hot water and a little iodine. Look now at the corn
seed; the white solid mass in the seed contains starch, as you can
prove with iodine, and although it is not in the cotyledon, yet it is
quite near the young plant, which can get at it easily.
We have found, therefore, that ~young plants have a store of food in
their nurse-leaves, or near them in the seed~, and that this food is
the same as very much of our own food, that is, it is starch. There
are other food substances present, too, but they are more difficult to
find. The seed, therefore, contains not only the young living plant
itself, but also a storehouse of food for its use, ~and as the plant
grows we see this store getting less and less~ in the shrivelling
cotyledons. This shows that the young plant uses up this food in the
course of its growth.
But you must not forget that, although we find the young plants
provided with food in this way, we have not yet settled the question
of the food supply for all plants. As we see, the cotyledons shrivel
up and are emptied of their store long before the plant is full grown.
Remember that baby calves have milk for food, while old cows have
grass. And when the store of food supplied in the seed is finished the
older plants must find new supplies for themselves.
In growing seedlings you must always keep them well supplied with
water, the soil or sawdust in which they grow ~must~ be kept moist. If
you take one out of the sawdust and try to grow it only in the air, you
will find that it soon dies. Even for the seedling the storehouse of
food is not enough; it requires to have water too.
You can keep seedlings growing quite well, however, if you place them
in glass jars so that their roots are in water, or even in closed glass
jars standing over water, so that the air is thoroughly moist. You will
then be able to see very well numbers of fine white hairs on the roots
(~see~ fig. 8). These hairs are very important and absorb the water
which keeps the whole plant moist.
[Illustration: Fig. 8. Maize seedlings growing enclosed in damp air,
supported on a wire stand over dish of water so that their roots do not
touch it, but grow in the air. Notice the “root hairs” growing out from
the roots.]
You have now seen that seedlings require ~water~ for their life just as
animals do; and also that young plants are provided by their parents
with a store of ~food~ which is largely ~starch~, and which they use up
during their early growth.