In 1875 appeared another great work from the master's pen,
"Insectivorous Plants," which was destined to place in a yet more
striking light the many-sidedness and fertility of his mind. As usual
Darwin tells us that this work dated from many years back. "During the
summer of 1860," he says, "I was surprised by finding how large a number
of insects were caught by the leaves of the common sun-dew (_Drosera
rotundifolia_) on a heath in Sussex. I had heard that insects were thus
caught, but knew nothing further on the subject. I gathered by chance a
dozen plants, bearing fifty-six fully expanded leaves, and on thirty-one
of these dead insects or remnants of them adhered." Here was the germ of
something, the discoverer scarcely knew what. It was evident to him that
the little sun-dew was excellently adapted for catching insects, and
that the number of them thus slaughtered annually must be enormous. What
bearing might this have upon the problem of the struggle for existence?
A masterly series of experiments was forthwith set on foot, with the
result of proving that sun-dews and a number of other plants obtain the
bulk of their nourishment by catching, killing, and digesting insects.
They may be called truly carnivorous plants. What an unexpected
reversal this was of the order of things hitherto believed to prevail
universally. Animals live on other animals or on plants. Here were
plants living on animals, and keeping down their number. Moreover,
without a nervous system, the action of the parts of a sun-dew leaf was
proved to be as apparently purposive as the combined action of the limbs
of an animal. Without a stomach, the sun-dew poured forth a digestive
fluid as effective in extracting and fitting the nutritious matter of
the insect for its own purposes as that of an animal. Without sensory
nerve-endings, there was a percipient power in the sun-dew which
recognised instinctively and at once the non-nutritious nature of
various objects, and which responded to the most delicate chemical
stimuli and to the minutest weights.
We cannot describe the little sun-dew better than in Darwin's own words:
"It bears from two or three to five or six leaves, generally extended
more or less horizontally, but sometimes standing vertically upwards.
The leaves are commonly a little broader than long. The whole upper
surface is covered with gland-bearing filaments, or tentacles as I shall
call them from their manner of acting. The glands were counted on
thirty-one leaves, but many of these were of unusually large size, and
the average number was 192; the greatest number being 260, and the least
130. The glands are each surrounded by large drops of extremely viscid
secretion, which, glittering in the sun, have given rise to the plant's
poetical name of the sun-dew."
This secretion, when excited by nutritious matter, becomes distinctly
acid, and contains a digestive ferment allied to the pepsin of the human
stomach. So excited, it is found capable of dissolving boiled white of
egg, muscle, fibrin, cartilage, gelatine, curd of milk, and many other
substances. Further, various substances that animal gastric juice is
unable to digest are not acted upon by the secretion of the sun-dew.
These include all horny matter, starch, fat, and oil. It is not however
prejudiced in favour of animal matter. The sun-dew can absorb nutriment
from living seeds of plants, injuring or killing them, of course, in the
process, while pollen and fresh green leaves yield to its influence.
The action of salts of ammonia and other chemicals was even more
wonderful. "It is an astonishing fact that so inconceivably minute a
quantity as the one twenty-millionth of a grain of phosphate of ammonia
should induce some change in a gland of Drosera sufficient to cause a
motor impulse to be sent down the whole length of the tentacle; this
impulse exciting movement often through an angle of above 180°. I know
not whether to be most astonished at this fact, or that the pressure of
a minute bit of hair, weighing only 1/78700 of a grain, and largely
supported by the dense secretion, should quickly cause conspicuous
movement."
These are but specimens of a multitude of profoundly interesting facts
brought out in this exhaustive investigation. If this single research
were his only title to fame Darwin's name must rank high as an
experimenter of rare ingenuity and success. But he concludes his summary
of results by the utterly modest remark, "We see how little has been
made out in comparison with what remains unexplained and unknown."
The facts relating to Venus' fly-trap (_Dionæa muscipula_) and other
members of the order to which the sun-dew belongs were better known, but
Darwin elicited new truths by his ingenious and varied experiments. The
rapidity with which the two lobes of the leaf of dionæa close together
when anything touches the tiny spikes which stand up vertically from the
upper surface of the lobes, is astonishing, and any insect which causes
the closure is almost certain to be caught. Digestion is accomplished in
the case of the dionæa by a separate agency, consisting of a large
number of minute reddish glands covering the surface of the lobes. These
secrete a digestive fluid when stimulated by the contact of any
nitrogenous matter, and of course this takes place when any insect
is caught. In fact, essentially the same process of digestion and
absorption takes place as in the sun-dew. The insect is held firmly for
days, until its juices have been absorbed, and then the leaf slowly
reopens, not being able to close again for many subsequent days.
It is interesting to note the extreme caution with which the great
naturalist speculates upon the mode by which the varied members of the
sun-dew order became modified from an ordinary plant-form to such a
remarkable degree. The details are too special for quotation here. He
suggests, but he does not in the slightest degree dogmatise. For many
years to come Darwin's suggestions and comments must be the pregnant
soil out of which fruitful research will spring, and his caution will
remain the model, to depart from which will but sow hindrances in the
path of scientific progress.
The order to which the butterwort and the bladderworts belong also
afforded valuable results. The leaf of the butterwort bears glandular
hairs, and its margins curve inwards when excited by contact of various
bodies, especially living insects, and, at the same time, these are
caught in the viscid secretion of the glands, and their juices absorbed
by the plant. The bladderworts are even more remarkably constructed, for
they have a portion of their leaves developed into subaqueous bladders,
with a narrow entrance beneath, defended by a complex valve, which
facilitates the entrance of water insects or crustaceans, but prevents
their exit. The whole interior of the bladder is lined with transparent
four-branched protoplasmic hairs, but nevertheless the bladderwort is
unlike the preceding plants in having no power of digesting its prey,
however long it may remain in captivity. Yet there is no doubt that the
imprisoned creatures do decay in their watery cell, and that the hairs
just described absorb the products of their decay.
Such is a brief account of Darwin's work on "Insectivorous Plants." With
his characteristic expressions he acknowledges the valuable aid given
him by Professor Burdon-Sanderson, and by his son, Mr. Francis Darwin.
The former was enabled to give the first brief account of the process of
digestion in these plants, as observed by Darwin, in a lecture before
the Royal Institution, in June, 1874, and Dr. (now Sir Joseph) Hooker
called general public notice to the subject of Carnivorous Plants in his
lecture before the British Association at Belfast in the same year: so
that a thoroughly awakened attention was given to this new work from
Darwin's pen. The public and the scientific world learnt to appreciate
yet more keenly his varied talent, his long patience, his reserve of
power; and thence dated very definitely a general appreciation of the
fundamental unity of the animal and plant kingdoms, seeing that the
salient faculties of digestion, of purposive locomotion, of rapid
communication and consentaneous action were no longer restricted to
animals, but were possessed in a high degree by plants also. Eager
followers soon brought forward further proofs of unity of functions in
the two kingdoms, and of reciprocal combinations between them, and now
no one in the slightest degree acquainted with modern biology doubts
that life is at bottom one phenomenon, shared equally and manifested in
essentially the same modes by the living substance of plant and animal
alike.
Following "Insectivorous Plants" came "The Effects of Cross and
Self-Fertilisation in the Vegetable Kingdom," in 1876. Darwin had led
the way in the study of this subject by his book on Orchids, and his
lead had been excellently followed by Hildebrand, Hermann Müller, Sir
John Lubbock, and others. The path having been indicated, it had
appeared comparatively easy for botanists to follow it up. But there yet
remained a region of experimental inquiry which it required Darwin's
patience and ingenuity to master and to expound conclusively. Although
it might be practically granted that natural selection developed
a process because advantage was gained by it, was it possible to
demonstrate that flowers cross-fertilised bear more and larger seeds,
which produce healthier offspring than those fertilised from their own
pollen? This Darwin set himself exhaustively to do. For more than a
dozen years after his book on orchids appeared, unwearied experiments
on plants were progressing, and nature was being questioned acutely,
untiringly. Competitive germination was carried on. The two classes of
seeds were placed on damp sand in a warm room. As often as a pair
germinated at the same time, they were planted on opposite sides of the
same pot, with a partition between. Besides these pairs of competitors,
others were planted in beds, so that the descendants of the crossed
and self-fertilised flowers might compete. The resulting seeds were
carefully compared, and their produce again compared. Species were
selected from widely distinct families, inhabiting various countries.
From a large number of plants, when insects were quite excluded by a
thin net covering the plant, few or no seeds were produced. The extent
of transport of pollen by insects was unveiled, and the relation between
the structure, odour, and conspicuousness of flowers, the visits of
insects, and the advantages of cross-fertilisation was shown. "We
certainly," says Darwin, "owe the beauty and odour of our flowers, and
the storage of a large supply of honey, to the existence of insects."
The multitude of facts gathered about insects could only have been
discovered and rightly appreciated by one who was a true entomologist
as well as a botanist.
In the last chapter of the book the author discusses with remarkable
power the causes of the phenomena he has discovered. He believes that
the favourable effects of crossing are due to the parents having been
subjected to diverse conditions; but what the precise benefit is, or how
it can operate so as to render the offspring more healthy and vigorous,
he cannot discern. "And so it is," he observes, "with many other facts,
which are so obscure that we stand in awe before the mystery of life."
So it is. The man who probably understood nature better than any man who
has ever lived, who had not only asked her multitudinous questions, but
to whom very many answers had been undoubtedly vouchsafed in response to
his persevering, humble, diligent, acute questioning, acknowledges that
he knows little; that much remains a mystery. But from all we know of
him, from his books, his letters, his friends, his was the joy of a
soul in sympathy with the master power of the universe. He marched
continually on the confines of the unknown, and to him was granted the
felicity of largely extending the boundaries of the known.
Again, in 1877, a new work proceeded from Darwin's pen, "The Different
Forms of Flowers in Plants of the same Species," dedicated to Professor
Asa Gray. It gathered up the contents of numerous papers read before the
Linnean Society, with later additions, and showed conclusively how many
plants possess distinctive forms of flowers in the same species, adapted
to, and in some cases absolutely necessitating, reciprocal fertilisation
through the visits of insects. It gave evidence of all the well-known
Darwinian characteristics of long-continued labour, thought, and
experiment.
In 1880 "The Power of Movement in Plants" was exemplified in a fresh
volume, in which the veteran was materially assisted by his son, Mr.
Francis Darwin. Its object was to describe and connect together several
large classes of movements, common to almost all plants. The surprising
fact was established, that all the parts or organs of plants, whilst
they continue to grow, are continually revolving, or circumnutating as
Darwin called it. This movement commences even before the young seedling
has broken through the ground. The combination of this with the effects
of gravity and light explains countless phenomena in the life of plants.
The tip of the rootlet is thus enabled to penetrate the ground, and it
is proved to be more sensitive than the most delicate tendril. Movement
goes on through all stages of life. Every growing shoot of a great tree
is continually describing small ellipses; the tip of every rootlet
endeavours to do the same. The changes of position of leaves and of
climbing plants, and the sleep of leaves are all brought under this
great principle of circumnutation. It is impossible in reading the book
not to be struck with the great resemblance between the movements of
plants and many of the actions performed unconsciously by the lower
animals. "With plants an astonishingly small stimulus suffices, and,
even with allied plants, one may be highly sensitive to the slightest
continued pressure, and another highly sensitive to a slight momentary
touch. The habit of moving at certain periods is inherited both by
plants and animals, and several other points of similitude have been
specified. But the most striking resemblance is the localisation of
their sensitiveness, and the transmission of an influence from the
excited part to another which consequently moves. Yet plants do not
of course possess nerves or a central nervous system; and we may
infer that with animals such structures serve only for the more
perfect transmission of impressions, and for the more complete
intercommunication of the several parts."
Here we see how much light may be thrown on animal structures and
functions by vegetable physiology. We learn to limit our ideas of the
superiority of animals by discovering how much of what we consider
peculiar to them is found in plants. We appreciate the unity of biology,
indivisible without injury to our knowledge of its parts. No structure
in plants appears more wonderful, as Darwin describes it, than the
tip of the rootlet of a seedling. It is impressed by and transmits
influences of pressure, injury, moisture, light, and gravity to other
parts, and determines the course pursued by the rootlet in penetrating
the ground. "It is hardly an exaggeration to say that the tip of the
radicle thus endowed, and having the power of directing the movements of
the adjoining parts, acts like the brain of one of the lower animals;"
and the brain of Charles Darwin, in working out this acquisition of
knowledge for mankind, has added a new department to vegetable
physiology and to biology.