ON NETTLES, SENSITIVE PLANTS, ETC.
Stinging nettles at home and abroad--The use of the nettle--Sham
nettles--Sensitive plants--Mechanism--Plants alive, under
chloroform and ether--Telegraph plant--Woodsorrel--Have plants
nerves?--Electricity in the Polar regions--Plants under electric
shocks--Currents of electricity in plants--The singing of trees
to the electro-magnetic ear--Experiments--Electrocution of
vegetables.
The common nettle is one of our most interesting British plants. It
is exposed to great danger; one sees it growing not only in pastures
and parks, but in waste places, along roadsides, and near cultivated
ground. Yet it is very seldom either eaten or even touched. Cattle do
occasionally eat the young shoots. But this is exceptional, for even
in fields where there are plenty of cattle great clumps of nettle
luxuriate and increase in size every year.
The stinging hairs are hollow and shaped rather like a narrow bulb
or flask; the tip is slightly bent over and rounded (not sharp);
the hairs contain formic acid. If one grasps the nettle or strokes
it in a particular way (from below upwards) the hairs are pressed
flat against the stem or broken, so that no wound is made by them in
the skin and consequently they do no harm. But if the point of the
hair pierces the skin, the well-known irritation is set up. That is
because formic acid is poured into the wound. Besides the stinging
hairs which keep off all the larger animals (including man) there are
others, shorter and thickly set, which do not sting at all, but are
intended to keep off snails.[96]
[96] _Gard. Chronicle_, 32, 390.
The pain produced by our common nettle is, however, a very trifling
matter compared with that produced by some of the foreign species.
One of the Indian kinds was used to excite and irritate bulls when
they were intended to fight with tigers in the games which used to
be held at some Indian Courts. Another found in Timor is called the
Devil's Leaf; the effect of its sting may last for twelve months and
may even produce death. But a still more dangerous stinging plant
is a handsome tree (_Laportea moroides_) found in Australia. It is
often 120-140 feet high, and has fine dark-green leaves often one
foot in length. The sting is so powerful that even horses are killed
by touching its leaves. The sting of _Jatropha urens_ is so strong
that people become unconscious. In Java also the sting of _Urtica
stimulans_ continues to smart for twenty-four hours, and may produce
a fever which is very difficult to shake off.[97]
[97] Lindley, _l.c._; Ludwig, _l.c._
Yet our common nettle is the favourite food-plant of the caterpillars
of the Small Tortoiseshell, Red Admiral, Peacock, Camberwell Beauty,
and other butterflies.[98] These caterpillars are possibly more
intelligent than many of our country folk, who do not know that the
nettle is a very useful plant, as the following statements most
clearly prove. Its young leaves make an excellent spinach, and it
was, according to Sir Walter Scott, formerly cultivated in Scotland
as a pot-herb. Pigs, turkeys, geese, and fowls like the leaves when
they are chopped up. It is said that the dried leaves and seeds will
make hens lay in winter time. The seeds, under pressure, yield quite
a good oil. A yellow dye can be obtained by boiling the roots with
alum. An excellent string can also be made from the inner bark of the
stems, which has, in fact, been used to make twine and even clothing.
The nettle is also valuable as an external stimulant in cases of
paralysis.
[98] _Memories of the Month_, First Series, p. 73.
A plant with so many wonderful properties would not be so common as
it is, or so little disturbed, if it were not for its powerful stings.
There are one or two plants which are extremely like the nettle
at first sight. Lord Avebury has an illustration in his excellent
little book[99] in which it is most difficult to tell which are White
Deadnettles and which are stinging nettles. No doubt the harmless
deadnettle is helped to escape injury by this resemblance. The Hemp
Deadnettle and some Campanulas are also very like it when growing.
These also are sham nettles and may escape in the same way.
[99] _Flowers, Fruit, and Leaves._
There are several common greenhouse Primulas which also produce
irritation of the skin. When handled by gardeners a painful smart is
set up which lasts for some time. _Primula obconica_ is the worst
of these, but _P. sinensis_, _P. cortusoides_, and _P. Sieboldii_
sometimes have the same effect. In all these cases it is due to a
peculiar secretion of certain glandular hairs.[100]
[100] Nestler, _Sitz. d. K. Akad. d. Wiss. Wien_, vol. 3, p. 27.
The methods of protection against grazing animals so far described,
such as stinging hairs, thorns, spines, etc. (see page 190), are
obvious enough, but perhaps the most ingenious system of defence is
that exhibited by the Sensitive Plant and a few others.
When man or any heavy animal is approaching certain Indian plants,
their leaves suddenly drop, and the leaflets close together. The mere
shaking of the ground or of the air produces these extraordinary
movements in the sensitive Woodsorrel (_Oxalis sensitiva_), in two
Leguminous plants (_Smithia sensitiva_ and _Aeschynomene indica_),
and in several Mimosas.
When one leaf-tip of _Mimosa pudica_, the Sensitive Plant (_par
excellence_), is touched or injured, a series of changes begin. All
the little leaflets shut up one after the other; then the secondary
stalks drop; after this the main stalk of the leaf suddenly droops
downwards. After a short interval, the next leaf above goes through
identically the same movements. If the shaking or injury is severe,
every leaf from below upwards moves in the same way.
One probable advantage of these movements can be understood from the
behaviour of flies, which alight upon the leaves and make them drop.
The flies are startled and go away. Grazing animals will consider
such behaviour in a vegetable as very uncanny, and will probably go
to some other less ingeniously protected plant.
Of course such extraordinary behaviour has been a challenge to the
botanical world, and there is an overwhelming mass of speculation,
and observations about the Sensitive Plant.
It has been proved that the movements are caused by the thickened
part at the base of the main stalk of the leaf. This is swollen, and
full of water, and much thicker than the stalk itself. It is by this
thickened portion that the leaf is kept at its proper angle. When the
tip of the leaf is shaken or injured, the cells on the under side of
this swollen part allow their water to exude into the spaces between
them, and in consequence down comes the leaf-stalk.
This is not, by any means, a full or even a sufficient explanation.
There is certainly some peculiar sending of messages from the tip of
the leaf to the swollen part itself. It is not safe to say that it is
a nerve message, but the process resembles the way in which messages
are sent by the nerves in animals. Not only so, but the contraction
of the under side and a corresponding expansion on the upper side,
resembles the muscular movements of contraction and expansion in
animals.
It must always be remembered that plants are alive; their living
matter is not in any way (so far as we know) essentially different
from that of animals or of man. Their living matter (protoplasm) in
leaf-stalks and leaves is cut up into boxes or cells, each enclosed
in a case or wall of its own. Yet these are not entirely independent
and unconnected, for thin living threads run from cell to cell, so
that there is an uninterrupted chain of protoplasm all along the
leaf, leaf-stalk, and stem.
In this particular case of the Sensitive Plant, the leaves at night
regularly take up the position which they adopt when injured or
shaken during the daytime.
The easiest way to produce the shrinking of the leaves is, as has
been mentioned, to hold a lighted match a little below the leaf-tip.
Severe shaking, a strong electric shock, or a railway journey will
also produce closing of the leaves.
Under chloroform or ether, or if the atmospheric pressure is suddenly
diminished, the leaves will also fall. In some respects they are very
lifelike, for if too often stimulated they become "fatigued," and
will not react unless a sufficient interval of rest is allowed them.
The reaction occurs very soon if the plant is in good condition: in
less than one second it begins, and the leaf-stalk may fall in two
to five seconds, but the recovery is very slow.
Vivisection is a cruel sort of proceeding, although it may sometimes
be necessary. The most curious vivisections have been performed on
Mimosa. When the leaflets are cut off, it is possible, on a stimulus
being applied, to see water oozing out of the cut surface of the
stalk. This would go to show that it is the water being discharged
from the leaf-base that produces the movement.
There are, however, many points in the behaviour of the Sensitive
Plant which have not yet been explained.
Possibly the curious Semaphore or Telegraph Plants, whose leaflets
suddenly and without any obvious reason move with a jerk through an
angle of several degrees, may also be protected from animals by this
uncanny and unusual behaviour.
But though the Sensitive Plant is certainly protected from grazing
animals by these movements, other advantages may be derived. Heavy
rain, for instance, such as occurs in the tropics, will not injure
its delicate leaves. Dust-storms will not damage it, and at night
there will be no loss of heat by radiation. The "shrunk" or folded
condition of the leaflets will decrease any chance of injury by
raindrops, for the rain will not fall on the broad surface of the
leaflets. A nearly vertical leaf also will not suffer the loss of
heat which a horizontal one would endure.
Besides the plants mentioned above, there are several others in which
by a rather severe shaking the leaves can be made to fold up. This is
the case with the common Woodsorrel (_Oxalis acetosella_), with the
False Acacia (_Robinia_), and a few others.
The former has a peculiarly delicate leaf. In cold, wet weather its
leaflets hang limp and numb from the leaf-stalk all day. In fine
weather they are spread out horizontally. On a fine sunny afternoon
its leaflets may sometimes take a mid-day sleep, for they hang
loosely down in the same way that they do in cold, wet weather or at
night.
But in the Woodsorrel these movements are not for protection against
grazing animals.
There are other examples amongst plants of a distinct sudden movement
which begins whenever part of the plant is touched. The movements of
tendrils have been already referred to. The Venus' Fly Trap and the
Sundew will be mentioned when we are discussing Insectivorous Plants.
There are also several flowers in which the stamens suddenly spring
up when they are touched by an insect (Barberry, Centaurea, and
Sparmannia), and in Mimulus the style-flaps close when touched (see
p. 70).
All these cases seem to involve some sort of mechanism which replaces
the nervous system of animals.
No very definite laws have yet been discovered as to the way in which
plants are affected by electricity, but enough is known to show that
there are many interesting discoveries in prospect.
Professor Lemström has made some interesting experiments in the Polar
regions which go to show that the rich development of plant life in
that desolate region may be connected with the peculiar electrical
conditions of the Polar atmosphere; the aurora borealis, which is a
common phenomenon there, being also produced by those conditions.
Several writers have claimed that slight electric shocks given at
frequent intervals help the growth of plants and especially quicken
the germination of seeds, but it can scarcely be said that this has
been proved.
When a branch or leaf-stalk is wounded or injured by being
tightly clamped in a vice, then it will be found that a current
of electricity passes from the injured spot to the part that is
untouched, and then in the reverse direction.
Changes of current are also produced when a leaf is suddenly exposed
to light for a short time and then shaded.
One of the most interesting observations is that made by Major
Squiers near Lorin Station, in America, where the California Gas
and Electric Corporation of San Francisco has a long-distance
transmission telegraph line. The power is transmitted at a voltage
of 56,000 with a frequency of sixty cycles per second (three-phase).
Major Squiers, from previous experiments, thought that a note
corresponding to this frequency might be heard in a telephone
receiver. The following was the result:--
"Upon connecting the telephone between two nails driven in any
growing tree along the route of the line, and at a reasonable
distance therefrom, the telephone responded to this note with great
clearness, and when the distance was not more than 100 feet, the
sound was very loud. For this experiment no microphone need be used,
nor any source of electromotive force other than that induced in the
tree itself, the telephone being connected directly between two nails
driven into the tree....
"Several kinds of trees of various sizes and forms were examined
along this power transmission line, and all were found to be singing
with a loud voice the fundamental note characteristic of the line
current. Indeed, the strip of vegetation along this line has thus
been singing continuously, day and night, for several years, since
the operation of the line began; it needed only the electro-magnetic
ear to make the sound apparent....
"The general appearance of vegetation along this route is certainly
vigorous."[101]
[101] Squiers, "On the Absorption of Electro-magnetic Waves by
Living Vegetable Organisms," December 3, 1904.
An interesting little experiment was carried out by the author in
Glasgow, with the kind help of Professor Blyth, at the Glasgow and
West of Scotland Technical College. By attaching one wire to the
upper part of the stem of a young pot-plant whilst the other wire
was inserted in the base of the stem, it was easy to show that an
electric current was passing--at any rate, during the daytime. In the
evening, however, this was not at all distinct. That such currents do
occur in living trees seems to be admitted. A similar current was not
found in a stick of dead-wood. The mere passage of the water through
the plant in transpiration might, however, cause such a current, for
the water is evaporated at the leaves.
A strong electric shock may of course _electrocute_ a plant by
killing the cells. It is possible to cause the Mimosa leaves to close
by means of an electric shock.