POLLEN
Whilst it is true that plants cannot speak or walk about, yet they
live a separate life of their own. They breathe and sleep, feed and
digest just as animals do, but in a different manner. In order that
we may understand more about this and the use that bees are to them,
we must first learn a little about the construction of the flowers
themselves. Let us choose a daffodil about which to speak, for it is
both interesting and easily obtainable.
You will know that it is made up of “flower leaves,” and that there is
no calyx like that of a primrose, for instance. The corolla is a deep
yellow tube, and to it the flower leaves are joined. If now we cut the
flower in half, we find that there is a long rod, called the style,
at the end of which is a kind of sticky knob, called the stigma; this
you will see on Plate XXXVI. There are six smaller rods grouped round
the style, and these are called the stamens. They are thickened at the
end near the stigma, and the thickenings are called the anthers. The
anthers are the pollen-bearing parts of the flower, and though their
position often varies, you will find both anthers and stigma in nearly
every kind of flower. Below the corolla of the daffodil is the ovary,
and this is where the seeds are formed. If we look in the ovary of
our daffodil, we shall see several tiny round objects of a transparent
nature. These are called the ovules, and in time they may become seeds.
There is a remarkable difference between an ovule and a seed, for if
we planted one of the former, it would simply wither and decay in the
ground. If, however, we set a seed, sooner or later a plant, like that
from which the seed was taken, will spring up.
An ovule only becomes a seed after it has been fertilised, and this
is accomplished by some pollen being placed on the stigma. The style
is a kind of tube, and is connected with the ovary, and when grains
of pollen fall on the stigma they send out long shoots, called pollen
tubes. These pollen tubes grow down the style till they reach the
ovary. Each pollen tube then finds an ovule, forces its way in,
and pours in nutrition from the pollen grain on the stigma above.
The ovules then undergo certain important changes, and are turned
into seeds. Pollen grains are of all sizes and shapes, but they are
generally very tiny indeed. When I tell you that hundreds of grains
of the kind would take up no more room than a pin-head, you will
understand how very minute and wonderful are these tiny pollen tubes.
The change in the ovules, which we have just read about, is called
fertilisation, and we know that this is necessary to a plant if its
ovules are to be changed into seeds. We might imagine that there is
no difficulty about this in the cases of flowers where there are both
anthers and stigma, but it is a law of Nature that it is not desirable
for flowers to be fertilised by their own pollen. Why this should be
we do not know, but it certainly is an actual fact. By this I do not
mean to say that flowers cannot be fertilised by their own pollen, but
that they produce healthier and more numerous seeds when fertilised by
pollen from another plant. Pollen from another flower of the same plant
will not do, but it should be from another plant altogether. Of course
the two plants must be of the same kind, for it would not do to expect
the pollen of a sweet-pea to fertilise a wallflower.
Some flowers will not be fertilised at all by pollen from their own
plant, and one of these is clover. Mr. Darwin, a scientist who has
taught us a great deal about this subject, tried an experiment in which
he fertilised twenty heads of clover by the pollen of other clover
plants. They produced no less than 2290 seeds, but when another twenty
heads of clover were kept from being fertilised by any but their own
pollen, not a single seed was produced.
No doubt you will be wondering why a flower is not fertilised when
anthers covered with pollen surround the stigma. The explanation is
very simple, for the stigma has to become ripe before it can receive
any pollen. In some plants the stigma is ripe before the anthers give
off pollen, whilst in others all the pollen is given from off the
anthers before the stigma becomes ripe. Thus we see how Nature prevents
a flower from fertilising itself.