HOW FLOWERS ARE FERTILISED
We have now seen something of the contrivances of flowers to aid
in their fertilisation, and in this chapter we shall consider
the ingenious arrangement some flowers possess to assist their
fertilisation.
[Illustration: (_a_)]
[Illustration: (_b_)]
Let us first look at the primrose. Have you ever noticed that there are
two kinds of primrose flowers? From the outside perhaps they look very
similar, but if you look closely, or better still, cut them open, you
will find where they differ. Let us look at these sketches and we shall
see that the one kind (_a_) has a long style, which reaches nearly
to the top of the corolla. The other kind (_b_) has quite a short
style, so that instead of the stigma, or knob, being at the top of the
corolla, it is really half-way down. We notice, too, that the anthers,
or pollen bags, in the first kind (_a_) are placed half-way down the
corolla, and in the other flower (_b_) they are at the top. We might
think that Nature had made some mistake here, for it seems that if the
pollen bags belonging to flower (_a_) were placed in flower (_b_), or
_vice versa_, things would be more natural.
Let us suppose that a bee visits flower (_a_) and dips her tongue
down the corolla to collect the nectar. Half-way down the flower the
tongue has to pass the pollen bags, and in doing so gets dusted over
with pollen grains. The bee, having collected the nectar, flies to
another plant, which we will suppose bears flowers of the other kind.
She dips down her tongue, which touches the stigma just at the place
where it had been covered with pollen by the first flower. By this
means, therefore, the flower (_b_) is fertilised. But, you will ask,
what about flower (_a_)? While the fertilisation of flower (_b_) has
been going on, the pollen bags of (_b_) at the top of the corolla have
dusted the root of the bee’s tongue, so that when she goes to a flower
of the (_a_) type, the pollen dust at the root of her tongue touches
the stigma, and the flower is thus fertilised.
What a wonderful arrangement this is, for you will see that it is
almost impossible for the flowers of one primrose plant to fertilise
each other; the pollen must come from the flowers of a different plant.
Some flowers, if not fertilised by insects, have the power to fertilise
themselves, and to this class belongs the sweet-pea (Plate XXXVIII.).
This flower belongs to the _papilionaceous_ (butterfly) tribe, and when
a bee alights on the flower its weight presses down the underpart.
While the bee is taking the nectar, the pollen bags rise and touch her
on the underside of the thorax. Then she goes on to another flower
whose stigma is ripe. This time the stigma rises and touches the same
part of the bee’s body, and in this manner the flower is fertilised.
[Illustration: PLATE XXXVIII
Sweet Pea]
Some plants have wonderful arrangements for transferring their pollen
to other flowers, some of which are so peculiar and clever that we
might think they had been designed by some crafty scientist. One of
these is called the salvia, and it belongs to the same family as the
dead nettle. The anthers are mounted like a see-saw, and when the bee
makes its way into the flower it pushes one end of the see-saw up. This
causes the other end, on which the pollen bags are situated, to come
down thump on to the bee’s back. The pollen is thus scattered there,
and the bee also receives what may be called a pat on the back! As the
salvia flower grows old its pollen bags shrivel up, but at this time
the stigma is ripe. It grows longer and longer, and bends over till it
is like a letter J turned upside down: [Illustration: upside down J]
After a bee has visited some young flowers and had her back dusted with
pollen, she will, without doubt, visit some of the older ones too, and
it is quite easy to understand that when she enters these she rubs her
back against the overhanging stigma, and the pollen adheres to it.
Another interesting plant is the violet, the nectar of which is stored
at the end of the long spur, which you will have noticed. The pollen
bags fit closely round the stigma, and so when pollen drops from them
it does not fall out of the flower, for its passage is blocked by the
tight-fitting pollen bags. When the bee comes, she has to push her
tongue right up the spur, and in doing this she forces it past the
pollen bags. This causes the pollen to fall out on to her head, and so
it is carried to the next flower.