_Morning-Glory_. A seed covering. Some albumen. Two cotyledons. A
caulicle.
_Sunflower_. An outer covering.[1] An inner covering. Two cotyledons. A
caulicle.[2]
[Footnote 1: The so-called seed of Sunflower is really a fruit. The outer
covering is the wall of the ovary, the inner the seed-coat. Such closed,
one-seeded fruits are called akenes.]
[Footnote 2: The plumule is sometimes visible in the embryo of the
Sunflower.]
_Bean_. A seed covering. Two cotyledons. A caulicle. A plumule.
_Pea_. The same as the Bean.
They have also learned how the first leaves in the last three differ from
those of the Morning-Glory, being considerably thicker in the Sunflower,
and very much thicker in the Bean and Pea. Why should the Morning-Glory
have this jelly that the others have not? Why do the first leaves of the
Sunflower change so much as the seedling grows? What becomes of their
substance? Why do those of the Bean shrivel and finally drop off? By this
time some bright pupil will have discovered that the baby-plant needs food
and that this is stored around it in the Morning-Glory, and in the leaves
themselves in the others. It is nourished upon this prepared food, until
it has roots and leaves and can make its own living. The food of the
Morning-Glory is called _albumen_; it does not differ from the others in
kind, but only in its manner of storage.[1]
[Footnote 1: Reader in Botany. III. Seed-Food.]
Also the questions have brought out the fact that the Bean and Pea
have the plumule ready formed in the seed, while the Morning-Glory and
Sunflower have not. Why should this be? It is because there is so much
food stored in the first two that the plumule can develop before a root is
formed, while in the others there is only nourishment sufficient to enable
the plantlet to form its roots. These must make the second leaves by their
own labor.
3. _Comparison with other Dicotyledons_.--The pupils should now have other
seeds to compare with these four. Let them arrange Flax, Four o-clock,
Horsechestnut, Almond, Nasturtium, Maple-seeds, etc., under two heads.
_Seeds with the Food stored _Seeds with the Food stored
outside the plantlet in the embryo itself
(Albuminous)_. (Exalbuminous)_.
Flax. Four-o'clock. Acorn. Horsechestnut. Almond.
Morning-Glory. Maple. Sunflower. Squash.
Bean. Pea. Nasturtium.
They may also be divided into those with and without the plumule.
_Without Plumule_. _With Plumule_.
Flax. Maple. Sunflower. Acorn. Horsechestnut.
Four-o'clock. Almond. Bean. Pea.
Morning-Glory. Squash. Nasturtium.
Those with plumules will be seen to have the most abundant nourishment. In
many cases this is made use of by man.
These last can be again divided into those in which the cotyledons come up
into the air and those where they remain in the ground.
_In the Air_. _In the Ground_.
Bean. Almond. Squash. Acorn. Horsechestnut.
Pea. Nasturtium.
In the latter the cotyledons are so heavily gorged with nourishment that
they never become of any use as leaves. As Darwin points out, they have
a better chance of escaping destruction by animals by remaining in the
ground.
The cotyledons are very good illustrations of the different uses to which
a single organ may be put, and the thorough understanding of it will
prepare the pupils' minds for other metamorphoses, and for the theory that
all the various parts of a plant are modified forms of a very few members.
4. _Nature of the Caulicle_.--Probably some of the pupils will have called
the caulicle the root. It is, however, of the nature of stem. The root
grows only at the end, from a point just behind the tip; the stem
elongates throughout its whole length. This can be shown by marking the
stem and roots of a young seedling with ink. India ink must be used, as
common ink injures the plants. Dip a needle in the ink and prick a row
of spots at equal distances on a young root. Corn is very good for this
purpose, but Morning-Glory or Bean is better for experiments on the
stem. The plants should then be carefully watched and the changes in
the relative distance of the spots noted. The experiment is very easily
conducted with the seedlings growing on sponge, with their roots in the
moist air of the tumbler, as before described.
Dr. Goodale says of this experiment,--"Let a young seedling of corn be
grown on damp paper in the manner described in No. 1,[1] and when the
longest root is a few centimetres long let it be marked very carefully by
means of India ink, or purple ink, put on with a delicate camel's-hair
pencil just one centimetre apart. Plants thus marked are to be kept under
favorable conditions with respect to moisture and warmth, so that growth
will be as rapid as possible. The marks on the older part of the root
will not change their relative distance, but the mark at the tip will be
carried away from the one next it, showing that the growth has taken place
only at this point. Such experiments as the one described are perfectly
practicable for all classes of pupils except the very youngest. How far
the details of these experiments should be suggested to the pupils, or
rather how far they should be left to work out the problem for themselves,
is a question to be settled by the teacher in each case. The better plan
generally is to bring the problem in a very clear form before the whole
class, or before the whole school, and ask whether anybody can think of a
way in which it can be solved; for instance, in this case how can it be
found out whether roots grow only at their tip or throughout their whole
length. If the way is thought out by even a single pupil the rest will be
interested in seeing whether the plan will work successfully."
[Footnote 1: Concerning a Few Common Plants, page 25.]
I have been more successful in pricking the roots than in marking them
with a brush.
The caulicle can be proved by the manner of its growth to be of the nature
of stem, not root. The main root grows from its naked end. Roots can also
grow from the sides of the caulicle, as in Indian Corn. In this, it acts
precisely as does the stem of a cutting. It can be prettily shown with the
seedlings by breaking off a bean at the ground and putting the slip in
water. It will throw out roots and the pupil will readily understand that
the caulicle does the same thing.
Darwin has made very interesting experiments on the movements of
seedlings. If the teacher wishes to repeat some of the experiments he will
find the details very fully given in "The Power of Movement of Plants."[1]
The pupils can observe in their growing seedlings some of the points
mentioned and have already noticed a few in their answers. They have said
that the caulicle was the part to grow first, and have spoken of the
arched form of the young stem. Their attention should also be drawn to the
root-hairs, which are well seen in Corn, Wheat, and Oats. They absorb the
liquid food of the plants. A secondary office is to hold the seed firmly,
so that the caulicle can enter the ground. This is shown in Red Clover,
which may be sown on the surface of the ground. It puts out root-hairs,
which attach themselves to the particles of sand and hold the seed. These
hairs are treated more fully in the lessons on roots.
[Footnote 1: The Power of Movement in Plants. By Charles Darwin. London.
John Murray, 1880.]
[Footnote 1: Reader in Botany. IV. Movements of Seedlings.]
5. _Leaves of Seedlings_.--Coming now to the question as to the number of
leaves at each joint of the stem, the Morning-Glory, Sunflower, and Bean
will present no difficulty, but probably all the pupils will be puzzled by
the Pea. The stipules, so large and leaf-like, look like two leaves,
with a stem between, bearing other opposite leaves, and terminating in a
tendril, while in the upper part it could not be told by a beginner which
was the continuation of the main stem. For these reasons I left this out
in the questions on the Pea, but it should be taken up in the class. How
are we to tell what constitutes a single leaf? The answer to this question
is that buds come in the _axils_ of single leaves; that is, in the inner
angle which the leaf makes with the stem. If no bud can be seen in the
Pea, the experiment may be tried of cutting off the top of the seedling
plant. Buds will be developed in the axils of the nearest leaves, and it
will be shown that each is a compound leaf with two appendages at its
base, called stipules, and with a tendril at its apex. Buds can be forced
in the same way to grow from the axils of the lower scales, and even from
those of the cotyledons, and the lesson may be again impressed that organs
are capable of undergoing great modifications. The teacher may use his own
judgment as to whether he will tell them that the tendril is a modified
leaflet.
[Illustration: FIG. 9. 1. Grain of Indian Corn. 2. Vertical section,
dividing the embryo, _a_, caulicle: _b_, cotyledon; _c_, plumule. 3.
Vertical section, at right angles to the last.]
6. _Monocotyledons_.--These are more difficult. Perhaps it is not worth
while to attempt to make the pupils see the embryo in Wheat and Oats. But
the embryo of Indian Corn is larger and can be easily examined after long
soaking. Removing the seed-covering, we find the greater part of the seed
to be albumen. Closely applied to one side of this, so closely that it
is difficult to separate it perfectly, is the single cotyledon. This
completely surrounds the plumule and furnishes it with food from the
albumen. There is a line down the middle, and, if we carefully bend back
the edges of the cotyledon, it splits along this line, showing the
plumule and caulicle within. The plumule consists of successive layers of
rudimentary leaves, the outer enclosing the rest (Fig. 10, 1, _c_). The
latter is the first leaf and remains undeveloped as a scaly sheath (Fig.
10, 2, _c_). In Wheat and Oats the cotyledon can be easily seen in the
largest seedlings by pulling off the dry husk of the grain. The food will
he seen to have been used up.
[Illustration: FIG. 10. 1. Germination of Indian corn. 2. Same more
advanced. _a_, caulicle; _c_1, first leaf of the plumule, sheathing the
rest; _c_2, second leaf; _c_3, third leaf of the plumule; _d_, roots.]
The series of Corn seedlings, at least, should be drawn as before and
the parts marked, this time with their technical terms. The following
questions should then be prepared.