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INSECTIVOROUS PLANTS OF THE BOG-LANDS
One
bright morning in late July I decided to climb to the top of the
Graigellachie, that precipitous spur of the Monadhliaths standing like a
rampart behind Aviemore, and explore the surroundings of a mountain tarn
and its associated bog-land, situated in a depression beyond the summit.
The day
was warm and sunny, with a gentle wind that whispered softly through the
trembling Birches that clothed the craggy and scree encumbered slopes of
this great grey pile.
I climbed
up to the barren plateau that forms the summit of Craigellachie where the
green carpets of the Bearberry covered the grey slabs of stone which were
separated by patches of black bog.
From here
a magnificent view presented itself as one looked out across the wide
expanse of Speyside towards the Cairngorms. Below lay the great black
mass of Rothiemurchus Forest through which the Spey wound like a silver
serpent, while beyond the lochs lay like diamonds amid the blackness of
the Pines. And beyond all, like a great wall stretched the deeply cut
massif on the Cairngorms. From here Cairngorm, Ben Macdhui, Braeriach and
Cairn Toul were plainly visible and the deep glens of the Lairig Ghru,
Glen Einich and Glen Feshie were easily discernible.
It was a
scene of indescribable beauty bathed in sunshine, an ever-changing
kaleidoscope of colour, the purple moors, the black forest of pines, the
brown mountain sides, the silver lochs and green pastures forming a
perfect symphony in colour.
Leaving
this beautiful picture, I descended 100 feet through the heather toward
the little tar, lying behind Craigellachie. Around its edges grew small
mountain willows, for the rest the heather and needle whin were the chief
vegetation. Along the lower edge of the loch lay a large patch of mossy
bog, yellow with the Bog Asphodel Moving cautiously I flushed a curlew
who had been busy searching in the mud along the shore and flew off with a
mournful cry as if the spirit of the loch had taken flight at my approach.
The bog
was a little floral garden, where I found the beautiful innocent blooms of
the Cloudberry, the fragrant Bog Myrtle, the purple Butterwort and the
Round-leaved Sundew and its long-leaved relative along with numerous
sedges and bog grasses.
Here I
spent a delightful day alone with the sun, the gentle breeze and the
sparkling mountain air filled with the honeyed perfume of the heather.
I studied
the divers inhabitants of this interesting spot, and especially the
insectivorous plants.
It will
come as a surprise to many people to know that in the plant world there
are certain plants which trap and digest insects, in much same way as a
spider catches a fly and eats it. We are so used to the insect visiting
glower to steal their pollen and nectar, but that plants should actually
live on an insect diet is almost unbelievable.
The plant
I am about to describe are some of the most interesting in the world and
have been the sourced of much speculation and research since Darwin’s
classical experiments with them.
In the
bogs we can find two types of insectivorous plants occur, such as the
Bladderworts, but I shall describe them in another chapter as they are
actually aquatic plants.
 SUNDEWS
In
Britain we have three Sundews. They are the Round-leaved Sundew (Drosera
rotund folia), which is the common species; the Long-leaved Sundew (D.
long folia) and the English Sundew (D. anglica) are frequent
although less common than the first one. All three species are found in
the Highlands in bogs from sea-level up to 2,000 or 3,000 feet.
They are
among the strangest plants to be found in Britain, for they have reversed
the order of Nature and, like carnivorous animals, trap and digest flesh
in the form of small insects such as flies and mosquitoes.
The three
species are very similar and close study of D. rotundifolia will
apply equally to the other two.
If we
examine a single plant we shall find that it has a very small, weak root
system which is very small in proportion to the plant that it is supposed
to nourish.
The
rootstock is crowned by a rosette of almost round leaves on long slender
stalks, which are covered by long, reddish glandular hairs. The upper
surface of the leaf is also covered by many red hairs, each with a
glistening drop at its extremity which shines like a jewel in bright
sunshine. The leaves are quite different from those we are usually
accustomed to in the plant world, and one may be well at a loss to
understand their purpose.
If we
examine a number of these plants we shall find that a fly or gnat,
attracted by the glistening drops, alights upon the leaf and commences to
the lick the drop. Immediately we shall notice that the hairs nearest to
the insect move down and touch it, and then the hairs further away begin
to move also. The fly struggles to escape, but the honey-like drops that
tempted it are stickier than glue and it cannot move. Its struggles
excite the rest of the hairs which, exuding more and more sticky fluid,
enclose the fly like the tentacles of an octopus. They move under the
same principle that makes the tendrils of a clematis twine around a
support. Gradually the breathing pores of the insect are choked by the
sticky fluid and death arrives to still its sufferings.
When the
leaf has completely closed around the insect, special glands upon the
surface exude a fluid which is exactly similar to the gastric juices of
the stomach of an animal. The insect is slowly digested, the juices
entering the leaf by specialized glands. As soon as the fly is completely
digested the leaf opens again, ejects the indigestible portion such as
wings and is ready for the next victim. Thus the fly actually nourishes
the plant.
Darwin
made an exhaustive study of these plants feeding them artificially with
raw meat. He gave them indigestion by over-feeding them and poisoned them
with poisonous objects. He found that inanimate things such as leaves,
dirt or wood excited the nearest hairs which closed round the object only
to unfold when they found that the object was not fit to eat. In nature
this must often happen as the wind blows particles of sand, et., on to the
sticky hairs, but by this amazing selective capacity no vital juice is
wasted upon these useless substances.
One will
probably ask why it is that the Sundew has evolved this amazing way of
life. The reason is not difficult to find. As we have already shown, the
soil of the bogs is poor in certain vital elements, the chief of which is
nitrogen. Now this element is of great importance to the well-being of
plants and a certain quantity is necessary for healthy growth. Animal
life is rich in nitrogen and the flies and insects trapped by these plants
supply the nitrogen that is deficient in the soil.
The
flowers of the Sundew are small and white and arranged in a raceme at the
summit of a very fine flower stalk. The raceme is rolled up in a curve
when young, but gradually uncurves as the flowers expand.
The
flowers have a tiny calyx of five green sepals, within which are found the
five rather long petals which only open in sunshine. They are fertilized
by small bees and flies.
The
Long-leaved Sundew is distinguished from the preceding by the leaves being
more erect and long and narrow, gradually tapering into the stalk. It is
a less common plant than the former, although often found with it.
The
English Sundew is much like the long-leaved species, but the leaves are
longer and narrower, often over one inch long without the stalk, and the
flowers are rather larger than in either of the preceding species.
 THE
BUTTERWORTS
In
Britain we have four species of Butterworts, of which three are found in
the Highlands, one species, the Alpine Butterwort, not being found beyond
the limits of the western Highlands.
[Electric Scotland Note: We have
been informed that the Alpine butterwort has been extinct in the British
Isles since 1919. Source: an email from Faith Anstey.]
The three
species are found in similar situations to the Sundews and include the
Common Butterwort (Pinguicula vulgaris), a very common species
throughout the Highlands from sea-level to 2,500 feet or more; the Pale
Butterwort (P. Lusitanica), a small species confined to the western
Highlands although found elsewhere in Britain, and the Alpine Butterwort (P.
alpina), found only in Skye, Ross and Sutherland.
As in the
case of the Sundews a detailed description of P. vulgaris will
suffice for the other two species.
The
Common Butterwort belongs to the Lentibulariaceae, a family
composed of insectivorous plants, including the Bladderworts.
The
leaves, which are broad, rounded, rather succulent and arranged in a close
rosette, are of a light grayish-green colour and are covered with numerous
crystalline points which give them a wet and clammy appearance.
The
leaves are very interesting and well worth a close examination, the upper
surface being covered in a sticky fluid which is secreted by certain
specialized glands and is the medium by which the insects are trapped.
If the
surface of the leaf is scrutinized under a powerful lens one will find
that there are numerous glands scattered over it, some with stalks and
some without. Several thousands of these glands have been counted upon a
single leaf. They supply the digestive juices and the sticky fluid, which
shines in the sunlight and attracts insects who arrive in the hope of
finding nectar. As soon as they alight upon the treacherous surface they
are trapped.
The
glands then supply a ferment which gradually reduces the insect to a
liquid, only the hard portions, such as wings and legs, being left. Very
often the leaves curl up to form an improvised stomach. Other glands
absorb the liquid as it is formed. In this way the plant obtains
foodstuff which is absent from the poor, acid soil. The undigested
portions remain upon the leaves until they are washed away by the rain,
and the trap is then rest for fresh victims.
The
flower of the Butterwort is very handsome and of a deep purplish-blue, and
terminates a tall leafless stalk. The calyx, like that of the Snapdragon,
is lipped and consists of five segments. The corolla is also lipped and
is situated at right angles to the stalk, the lower lip consisting of
three broad lobes, the upper of two shorter segments. The throat of the
corolla is bell-shaped and is extended backwards to form a long, straight
spur at the base of which are situated the nectarines.
Two
stamens are situated near the roof of the throat while the stigma projects
towards the entrance.
An insect
alighting upon the lower lip must touch the projecting stigma with its
head and thus covers it with pollen. The stigma immediately springs up so
that when the insect withdraws its head it will not deposit any pollen
from the flower’s own stamens upon it, and thus self-fertilization is
avoided.
Butterflies and bumble-bees are the chief visitors as their tongues alone
are long enough to reach the nectarines at the base of the long spur.
The
Alpine Butterwort is very similar, but rather smaller in all its parts,
the flowers being of a pale yellow colour and having a very short obtuse
spur, whilst the middle lobe of the lower lip is long and broad. Its
insect visitors, as the short spur indicates, are short tongued and
include hone-bees and flies.
The third
species, the Pale Butterwort, is much smaller than the Common Butterwort.
The leaves are very similar, but the flowers, which are smaller than those
of the Alpine Butterwort, are pale yellow in colour, tinged with lilac,
and have very slender stalks, whilst the spur is short and slightly
curved. Honey-bees and small butterflies are the chief benefactors.
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