Honorary Fellow of Christ's College, Cambridge.
My father's interest in plants was of two kinds, which may be roughly
distinguished as EVOLUTIONARY and PHYSIOLOGICAL. Thus in his purely
evolutionary work, for instance in "The Origin of Species" and in his
book on "Variation under Domestication", plants as well as animals
served as material for his generalisations. He was largely dependent
on the work of others for the facts used in the evolutionary work, and
despised himself for belonging to the "blessed gang" of compilers. And
he correspondingly rejoiced in the employment of his wonderful power of
observation in the physiological problems which occupied so much of his
later life. But inasmuch as he felt evolution to be his life's work, he
regarded himself as something of an idler in observing climbing plants,
insectivorous plants, orchids, etc. In this physiological work he was
to a large extent urged on by his passionate desire to understand the
machinery of all living things. But though it is true that he worked
at physiological problems in the naturalist's spirit of curiosity, yet
there was always present to him the bearing of his facts on the problem
of evolution. His interests, physiological and evolutionary, were indeed
so interwoven that they cannot be sharply separated. Thus his original
interest in the fertilisation of flowers was evolutionary. "I was
led" ("Life and Letters", I. page 90.), he says, "to attend to the
cross-fertilisation of flowers by the aid of insects, from having come
to the conclusion in my speculations on the origin of species, that
crossing played an important part in keeping specific forms constant."
In the same way the value of his experimental work on heterostyled
plants crystalised out in his mind into the conclusion that the product
of illegitimate unions are equivalent to hybrids--a conclusion of the
greatest interest from an evolutionary point of view. And again his work
"Cross and Self Fertilisation" may be condensed to a point of view
of great importance in reference to the meaning and origin of sexual
reproduction. (See Professor Goebel's article in the present volume.)
The whole of his physiological work may be looked at as an illustration
of the potency of his theory as an "instrument for the extension of the
realm of natural knowledge." (Huxley in Darwin's "Life and Letters." II.
page 204.)
His doctrine of natural selection gave, as is well known, an impulse
to the investigation of the use of organs--and thus created the great
school of what is known in Germany as Biology--a department of science
for which no English word exists except the rather vague term Natural
History. This was especially the case in floral biology, and it is
interesting to see with what hesitation he at first expressed the value
of his book on Orchids ("Life and Letters", III. page 254.), "It will
perhaps serve to illustrate how Natural History may be worked under the
belief of the modification of species" (1861). And in 1862 he speaks
(Loc. cit.) more definitely of the relation of his work to natural
selection: "I can show the meaning of some of the apparently meaningless
ridges (and) horns; who will now venture to say that this or that
structure is useless?" It is the fashion now to minimise the value of
this class of work, and we even find it said by a modern writer that to
inquire into the ends subserved by organs is not a scientific problem.
Those who take this view surely forget that the structure of all living
things is, as a whole, adaptive, and that a knowledge of how the
present forms come to be what they are includes a knowledge of why
they survived. They forget that the SUMMATION of variations on which
divergence depends is under the rule of the environment considered as
a selective force. They forget that the scientific study of the
interdependence of organisms is only possible through a knowledge of the
machinery of the units. And that, therefore, the investigation of such
widely interesting subjects as extinction and distribution must include
a knowledge of function. It is only those who follow this line of
work who get to see the importance of minute points of structure and
understand as my father did even in 1842, as shown in his sketch of the
"Origin" (Now being prepared for publication.), that every grain of sand
counts for something in the balance. Much that is confidently stated
about the uselessness of different organs would never have been
written if the naturalist spirit were commoner nowadays. This spirit
is strikingly shown in my father's work on the movements of plants. The
circumstance that botanists had not, as a class, realised the interest
of the subject accounts for the fact that he was able to gather such a
rich harvest of results from such a familiar object as a twining plant.
The subject had been investigated by H. von Mohl, Palm, and Dutrochet,
but they failed not only to master the problem but (which here concerns
us) to give the absorbing interest of Darwin's book to what they
discovered.
His work on climbing plants was his first sustained piece of work on the
physiology of movement, and he remarks in 1864: "This has been new sort
of work for me." ("Life and Letters", III. page 315. He had, however,
made a beginning on the movements of Drosera.) He goes on to remark with
something of surprise, "I have been pleased to find what a capital guide
for observations a full conviction of the change of species is."
It was this point of view that enabled him to develop a broad conception
of the power of climbing as an adaptation by means of which plants are
enabled to reach the light. Instead of being compelled to construct a
stem of sufficient strength to stand alone, they succeed in the struggle
by making use of other plants as supports. He showed that the great
class of tendril- and root-climbers which do not depend on twining round
a pole, like a scarlet-runner, but on attaching themselves as they grow
upwards, effect an economy. Thus a Phaseolus has to manufacture a stem
three feet in length to reach a height of two feet above the ground,
whereas a pea "which had ascended to the same height by the aid of its
tendrils, was but little longer than the height reached." ("Climbing
Plants" (2nd edition 1875), page 193.)
Thus he was led on to the belief that TWINING is the more ancient form
of climbing, and that tendril-climbers have been developed from twiners.
In accordance with this view we find LEAF-CLIMBERS, which may be looked
on as incipient tendril-bearers, occurring in the same genera with
simple twiners. (Loc. cit. page 195.) He called attention to the case
of Maurandia semperflorens in which the young flower-stalks revolve
spontaneously and are sensitive to a touch, but neither of these
qualities is of any perceptible value to the species. This forced him to
believe that in other young plants the rudiments of the faculty needed
for twining would be found--a prophecy which he made good in his "Power
of Movement" many years later.
In "Climbing Plants" he did little more than point out the remarkable
fact that the habit of climbing is widely scattered through the
vegetable kingdom. Thus climbers are to be found in 35 out of the 59
Phanerogamic Alliances of Lindley, so that "the conclusion is forced
on our minds that the capacity of revolving (If a twining plant, e.g.
a hop, is observed before it has begun to ascend a pole, it will
be noticed that, owing to the curvature of the stem, the tip is not
vertical but hangs over in a roughly horizontal position. If such a
shoot is watched it will be found that if, for instance, it points
to the north at a given hour, it will be found after a short interval
pointing north-east, then east, and after about two hours it will once
more be looking northward. The curvature of the stem depends on one side
growing quicker than the opposite side, and the revolving movement,
i.e. circumnutation, depends on the region of quickest growth creeping
gradually round the stem from south through west to south again. Other
plants, e.g. Phaseolus, revolve in the opposite direction.), on which
most climbers depend, is inherent, though undeveloped, in almost every
plant in the vegetable kingdom." ("Climbing Plants", page 205.)
In the "Origin" (Edition I. page 427, Edition VI. page 374.) Darwin
speaks of the "apparent paradox, that the very same characters are
analogical when one class or order is compared with another, but give
true affinities when the members of the same class or order are compared
one with another." In this way we might perhaps say that the climbing
of an ivy and a hop are analogical; the resemblance depending on the
adaptive result rather than on community of blood; whereas the relation
between a leaf-climber and a true tendril-bearer reveals descent. This
particular resemblance was one in which my father took especial delight.
He has described an interesting case occurring in the Fumariaceae.
("Climbing Plants", page 195.) "The terminal leaflets of the
leaf-climbing Fumaria officinalis are not smaller than the other
leaflets; those of the leaf-climbing Adlumia cirrhosa are greatly
reduced; those of Corydalis claviculata (a plant which may be
indifferently called a leaf-climber or a tendril-bearer) are either
reduced to microscopical dimensions or have their blades wholly aborted,
so that this plant is actually in a state of transition; and finally in
the Dicentra the tendrils are perfectly characterized."
It is a remarkable fact that the quality which, broadly speaking, forms
the basis of the climbing habit (namely revolving nutation, otherwise
known as circumnutation) subserves two distinct ends. One of these is
the finding of a support, and this is common to twiners and tendrils.
Here the value ends as far as tendril-climbers are concerned, but in
twiners Darwin believed that the act of climbing round a support is a
continuation of the revolving movement (circumnutation). If we imagine a
man swinging a rope round his head and if we suppose the rope to strike
a vertical post, the free end will twine round it. This may serve as
a rough model of twining as explained in the "Movements and Habits
of Climbing Plants". It is on these points--the nature of revolving
nutation and the mechanism of twining--that modern physiologists differ
from Darwin. (See the discussion in Pfeffer's "The Physiology of Plants"
Eng. Tr. (Oxford, 1906), III. page 34, where the literature is given.
Also Jost, "Vorlesungen uber Pflanzenphysiologie", page 562, Jena,
1904.)
Their criticism originated in observations made on a revolving shoot
which is removed from the action of gravity by keeping the plant slowly
rotating about a horizontal axis by means of the instrument known as a
klinostat. Under these conditions circumnutation becomes irregular or
ceases altogether. When the same experiment is made with a plant which
has twined spirally up a stick, the process of climbing is checked and
the last few turns become loosened or actually untwisted. From this
it has been argued that Darwin was wrong in his description of
circumnutation as an automatic change in the region of quickest growth.
When the free end of a revolving shoot points towards the north there
is no doubt that the south side has been elongating more than the north;
after a time it is plain from the shoot hanging over to the east that
the west side of the plant has grown most, and so on. This rhythmic
change of the position of the region of greatest growth Darwin ascribes
to an unknown internal regulating power. Some modern physiologists,
however, attempt to explain the revolving movement as due to a
particular form of sensitiveness to gravitation which it is not
necessary to discuss in detail in this place. It is sufficient for my
purpose to point out that Darwin's explanation of circumnutation is
not universally accepted. Personally I believe that circumnutation is
automatic--is primarily due to internal stimuli. It is however in some
way connected with gravitational sensitiveness, since the movement
normally occurs round a vertical line. It is not unnatural that,
when the plant has no external stimulus by which the vertical can be
recognised, the revolving movement should be upset.
Very much the same may be said of the act of twining, namely that most
physiologists refuse to accept Darwin's view (above referred to) that
twining is the direct result of circumnutation. Everyone must allow
that the two phenomena are in some way connected, since a plant
which circumnutates clockwise, i.e. with the sun, twines in the same
direction, and vice versa. It must also be granted that geotropism has a
bearing on the problem, since all plants twine upwards, and cannot twine
along a horizontal support. But how these two factors are combined, and
whether any (and if so what) other factors contribute, we cannot say.
If we give up Darwin's explanation, we must at the same time say with
Pfeffer that "the causes of twining are... unknown." ("The Physiology of
Plants", Eng. Tr. (Oxford, 1906), III. page 37.)
Let us leave this difficult question and consider some other points made
out in the progress of the work on climbing plants. One result of what
he called his "niggling" ("Life and Letters", III. page 312.) work on
tendrils was the discovery of the delicacy of their sense of touch, and
the rapidity of their movement. Thus in a passion-flower tendril, a bit
of platinum wire weighing 1.2 mg. produced curvature ("Climbing
Plants", page 171.), as did a loop of cotton weighing 2 mg. Pfeffer
("Untersuchungen a.d. Bot. Inst. z. Tubingen", Bd. I. 1881-85, page
506.), however, subsequently found much greater sensitiveness: thus
the tendril of Sicyos angulatus reacted to 0.00025 mg., but this only
occurred when the delicate rider of cottonwool fibre was disturbed by
the wind. The same author expanded and explained in a most interesting
way the meaning of Darwin's observation that tendrils are not stimulated
to movement by drops of water resting on them. Pfeffer showed that
DIRTY water containing minute particles of clay in suspension acts as a
stimulus. He also showed that gelatine acts like pure water; if a smooth
glass rod is coated with a 10 per cent solution of gelatine and is then
applied to a tendril, no movement occurs in spite of the fact that the
gelatine is solid when cold. Pfeffer ("Physiology", Eng. Tr. III.
page 52. Pfeffer has pointed out the resemblance between the contact
irritability of plants and the human sense of touch. Our skin is not
sensitive to uniform pressure such as is produced when the finger
is dipped into mercury (Tubingen "Untersuchungen", I. page 504.)
generalises the result in the statement that the tendril has a special
form of irritability and only reacts to "differences of pressure or
variations of pressure in contiguous... regions." Darwin was especially
interested in such cases of specialised irritability. For instance in
May, 1864, he wrote to Asa Gray ("Life and Letters", III. page 314.)
describing the tendrils of Bignonia capreolata, which "abhor a simple
stick, do not much relish rough bark, but delight in wool or moss."
He received, from Gray, information as to the natural habitat of the
species, and finally concluded that the tendrils "are specially adapted
to climb trees clothed with lichens, mosses, or other such productions."
("Climbing Plants", page 102.)
Tendrils were not the only instance discovered by Darwin of delicacy
of touch in plants. In 1860 he had already begun to observe Sundew
(Drosera), and was full of astonishment at its behaviour. He wrote to
Sir Joseph Hooker ("Life and Letters", III. page 319.): "I have been
working like a madman at Drosera. Here is a fact for you which is
certain as you stand where you are, though you won't believe it, that a
bit of hair 1/78000 of one grain in weight placed on gland, will cause
ONE of the gland-bearing hairs of Drosera to curve inwards." Here again
Pfeffer (Pfeffer in "Untersuchungen a. d. Bot. Inst. z. Tubingen",
I. page 491.) has, as in so many cases, added important facts to my
father's observations. He showed that if the leaf of Drosera is entirely
freed from such vibrations as would reach it if observed on an ordinary
table, it does not react to small weights, so that in fact it was the
vibration of the minute fragment of hair on the gland that produced
movement. We may fancifully see an adaptation to the capture of
insects--to the dancing of a gnat's foot on the sensitive surface.
Darwin was fond of telling how when he demonstrated the sensitiveness
of Drosera to Mr Huxley and (I think) to Sir John Burdon Sanderson, he
could perceive (in spite of their courtesy) that they thought the whole
thing a delusion. And the story ended with his triumph when Mr Huxley
cried out, "It IS moving."
Darwin's work on tendrils has led to some interesting investigations on
the mechanisms by which plants perceive stimuli. Thus Pfeffer (Tubingen
"Untersuchungen" I. page 524.) showed that certain epidermic cells
occurring in tendrils are probably organs of touch. In these cells the
protoplasm burrows as it were into cavities in the thickness of the
external cell-walls and thus comes close to the surface, being separated
from an object touching the tendril merely by a very thin layer of
cell-wall substance. Haberlandt ("Physiologische Pflanzenanatomie",
Edition III. Leipzig, 1904. "Sinnesorgane im Pflanzenreich", Leipzig,
1901, and other publications.) has greatly extended our knowledge of
vegetable structure in relation to mechanical stimulation. He defines a
sense-organ as a contrivance by which the DEFORMATION or forcible change
of form in the protoplasm--on which mechanical stimulation depends--is
rendered rapid and considerable in amplitude ("Sinnesorgane", page 10).
He has shown that in certain papillose and bristle-like contrivances,
plants possess such sense-organs; and moreover that these contrivances
show a remarkable similarity to corresponding sense-organs in animals.
Haberlandt and Nemec ("Ber. d. Deutschen bot. Gesellschaft", XVIII.
1900. See F. Darwin, Presidential Address to Section K, British
Association, 1904.) published independently and simultaneously a
theory of the mechanism by which plants are orientated in relation
to gravitation. And here again we find an arrangement identical in
principle with that by which certain animals recognise the vertical,
namely the pressure of free particles on the irritable wall of a cavity.
In the higher plants, Nemec and Haberlandt believe that special loose
and freely movable starch-grains play the part of the otoliths or
statoliths of the crustacea, while the protoplasm lining the cells in
which they are contained corresponds to the sensitive membrane lining
the otocyst of the animal. What is of special interest in our present
connection is that according to this ingenious theory (The original
conception was due to Noll ("Heterogene Induction", Leipzig, 1892), but
his view differed in essential points from those here given.) the
sense of verticality in a plant is a form of contact-irritability. The
vertical position is distinguished from the horizontal by the fact that,
in the latter case, the loose starch-grains rest on the lateral walls
of the cells instead of on the terminal walls as occurs in the normal
upright position. It should be added that the statolith theory is
still sub judice; personally I cannot doubt that it is in the main a
satisfactory explanation of the facts.
With regard to the RAPIDITY of the reaction of tendrils, Darwin records
("Climbing Plants", page 155. Others have observed movement after about
6".) that a Passion-Flower tendril moved distinctly within 25 seconds of
stimulation. It was this fact, more than any other, that made him doubt
the current explanation, viz. that the movement is due to unequal
growth on the two sides of the tendril. The interesting work of Fitting
(Pringsheim's "Jahrb." XXXVIII. 1903, page 545.) has shown, however,
that the primary cause is not (as Darwin supposed) contraction on the
concave, but an astonishingly rapid increase in growth-rate on the
convex side.
On the last page of "Climbing Plants" Darwin wrote: "It has often been
vaguely asserted that plants are distinguished from animals by not
having the power of movement. It should rather be said that plants
acquire and display this power only when it is of some advantage to
them."
He gradually came to realise the vividness and variety of vegetable
life, and that a plant like an animal has capacities of behaving in
different ways under different circumstances, in a manner that may be
compared to the instinctive movements of animals. This point of view is
expressed in well-known passages in the "Power of Movement". ("The Power
of Movement in Plants", 1880, pages 571-3.) "It is impossible not to be
struck with the resemblance between the... movements of plants and many
of the actions performed unconsciously by the lower animals." And
again, "It is hardly an exaggeration to say that the tip of the
radicle... having the power of directing the movements of the adjoining
parts, acts like the brain of one of the lower animals; the brain being
seated within the anterior end of the body, receiving impressions from
the sense-organs, and directing the several movements."
The conception of a region of perception distinct from a region of
movement is perhaps the most fruitful outcome of his work on the
movements of plants. But many years before its publication, viz. in
1861, he had made out the wonderful fact that in the Orchid Catasetum
("Life and Letters", III. page 268.) the projecting organs or antennae
are sensitive to a touch, and transmit an influence "for more than one
inch INSTANTANEOUSLY," which leads to the explosion or violent ejection
of the pollinia. And as we have already seen a similar transmission of
a stimulus was discovered by him in Sundew in 1860, so that in 1862 he
could write to Hooker ("Life and Letters", III. page 321.): "I cannot
avoid the conclusion, that Drosera possesses matter at least in some
degree analogous in constitution and function to nervous matter." I
propose in what follows to give some account of the observations on
the transmission of stimuli given in the "Power of Movement". It is
impossible within the space at my command to give anything like a
complete account of the matter, and I must necessarily omit all mention
of much interesting work. One well-known experiment consisted in putting
opaque caps on the tips of seedling grasses (e.g. oat and canary-grass)
and then exposing them to light from one side. The difference, in the
amount of curvature towards the light, between the blinded and
unblinded specimens, was so great that it was concluded that the
light-sensitiveness resided exclusively in the tip. The experiment
undoubtedly proves that the sensitiveness is much greater in the tip
than elsewhere, and that there is a transmission of stimulus from the
tip to the region of curvature. But Rothert (Rothert, Cohn's "Beitrage",
VII. 1894.) has conclusively proved that the basal part where the
curvature occurs is also DIRECTLY sensitive to light. He has shown,
however, that in other grasses (Setaria, Panicum) the cotyledon is the
only part which is sensitive, while the hypocotyl, where the movement
occurs, is not directly sensitive.
It was however the question of the localisation of the gravitational
sense in the tip of the seedling root or radicle that aroused most
attention, and it was on this question that a controversy arose which
has continued to the present day.
The experiment on which Darwin's conclusion was based consisted simply
in cutting off the tip, and then comparing the behaviour of roots so
treated with that of normal specimens. An uninjured root when placed
horizontally regains the vertical by means of a sharp downward
curve; not so a decapitated root which continues to grow more or less
horizontally. It was argued that this depends on the loss of an organ
specialised for the perception of gravity, and residing in the tip of
the root; and the experiment (together with certain important variants)
was claimed as evidence of the existence of such an organ.
It was at once objected that the amputation of the tip might check
curvature by interfering with longitudinal growth, on the distribution
of which curvature depends. This objection was met by showing that an
injury, e.g. splitting the root longitudinally (See F. Darwin, "Linnean
Soc. Journal (Bot)." XIX. 1882, page 218.), which does not remove the
tip, but seriously checks growth, does not prevent geotropism. This
was of some interest in another and more general way, in showing that
curvature and longitudinal growth must be placed in different categories
as regards the conditions on which they depend.
Another objection of a much more serious kind was that the amputation
of the tip acts as a shock. It was shown by Rothert (See his excellent
summary of the subject in "Flora" 1894 (Erganzungsband), page 199.) that
the removal of a small part of the cotyledon of Setaria prevents the
plant curving towards the light, and here there is no question of
removing the sense-organ since the greater part of the sensitive
cotyledon is intact. In view of this result it was impossible to rely on
the amputations performed on roots as above described.
At this juncture a new and brilliant method originated in Pfeffer's
laboratory. (See Pfeffer, "Annals of Botany", VIII. 1894, page 317, and
Czapek, Pringsheim's "Jahrb." XXVII. 1895, page 243.) Pfeffer and Czapek
showed that it is possible to bend the root of a lupine so that, for
instance, the supposed sense-organ at the tip is vertical while the
motile region is horizontal. If the motile region is directly sensitive
to gravity the root ought to curve downwards, but this did not occur: on
the contrary it continued to grow horizontally. This is precisely what
should happen if Darwin's theory is the right one: for if the tip is
kept vertical, the sense-organ is in its normal position and receives no
stimulus from gravitation, and therefore can obviously transmit none to
the region of curvature. Unfortunately this method did not convince the
botanical world because some of those who repeated Czapek's experiment
failed to get his results.
Czapek ("Berichte d. Deutsch. bot. Ges." XV. 1897, page 516, and
numerous subsequent papers. English readers should consult Czapek in the
"Annals of Botany", XIX. 1905, page 75.) has devised another interesting
method which throws light on the problem. He shows that roots, which
have been placed in a horizontal position and have therefore been
geotropically stimulated, can be distinguished by a chemical test from
vertical, i.e. unstimulated roots. The chemical change in the root can
be detected before any curvature has occurred and must therefore be a
symptom of stimulation, not of movement. It is particularly interesting
to find that the change in the root, on which Czapek's test depends,
takes place in the tip, i.e. in the region which Darwin held to be the
centre for gravitational sensitiveness.
In 1899 I devised a method (F. Darwin, "Annals of Botany", XIII. 1899,
page 567.) by which I sought to prove that the cotyledon of Setaria is
not only the organ for light-perception, but also for gravitation. If
a seedling is supported horizontally by pushing the apical part
(cotyledon) into a horizontal tube, the cotyledon will, according to
my supposition, be stimulated gravitationally and a stimulus will be
transmitted to the basal part of the stem (hypocotyl) causing it to
bend. But this curvature merely raises the basal end of the seedling,
the sensitive cotyledon remains horizontal, imprisoned in its tube; it
will therefore be continually stimulated and will continue to transmit
influences to the bending region, which should therefore curl up into a
helix or corkscrew-like form,--and this is precisely what occurred.
I have referred to this work principally because the same method was
applied to roots by Massart (Massart, "Mem. Couronnes Acad. R. Belg."
LXII. 1902.) and myself (F. Darwin, "Linnean Soc. Journ." XXXV. 1902,
page 266.) with a similar though less striking result. Although these
researches confirmed Darwin's work on roots, much stress cannot be laid
on them as there are several objections to them, and they are not easily
repeated.
The method which--as far as we can judge at present--seems likely
to solve the problem of the root-tip is most ingenious and is due to
Piccard. (Pringsheim's "Jahrb." XL. 1904, page 94.)
Andrew Knight's celebrated experiment showed that roots react to
centrifugal force precisely as they do to gravity. So that if a bean
root is fixed to a wheel revolving rapidly on a horizontal axis, it
tends to curve away from the centre in the line of a radius of the
wheel. In ordinary demonstrations of Knight's experiment the seed is
generally fixed so that the root is at right angles to a radius, and as
far as convenient from the centre of rotation. Piccard's experiment is
arranged differently. (A seed is depicted below a horizontal dotted
line AA, projecting a root upwards.) The root is oblique to the axis
of rotation, and the extreme tip projects beyond that axis. Line AA
represents the axis of rotation, T is the tip of the root just above the
line AA, and B is the region just below line AA in which curvature takes
place. If the motile region B is directly sensitive to gravitation (and
is the only part which is sensitive) the root will curve (down and away
from the vertical) away from the axis of rotation, just as in Knight's
experiment. But if the tip T is alone sensitive to gravitation the
result will be exactly reversed, the stimulus originating in T and
conveyed to B will produce curvature (up towards the vertical). We may
think of the line AA as a plane dividing two worlds. In the lower one
gravity is of the earthly type and is shown by bodies falling and roots
curving downwards: in the upper world bodies fall upwards and roots
curve in the same direction. The seedling is in the lower world, but its
tip containing the supposed sense-organ is in the strange world where
roots curve upwards. By observing whether the root bends up or down we
can decide whether the impulse to bend originates in the tip or in the
motile region.
Piccard's results showed that both curvatures occurred and he
concluded that the sensitive region is not confined to the tip. (Czapek
(Pringsheim's "Jahrb." XXXV. 1900, page 362) had previously given
reasons for believing that, in the root, there is no sharp line of
separation between the regions of perception and movement.)
Haberlandt (Pringsheim's "Jahrb." XLV. 1908, page 575.) has recently
repeated the experiment with the advantage of better apparatus and more
experience in dealing with plants, and has found as Piccard did that
both the tip and the curving region are sensitive to gravity, but with
the important addition that the sensitiveness of the tip is much greater
than that of the motile region. The case is in fact similar to that of
the oat and canary-grass. In both instances my father and I were wrong
in assuming that the sensitiveness is confined to the tip, yet there
is a concentration of irritability in that region and transmission of
stimulus is as true for geotropism as it is for heliotropism. Thus after
nearly thirty years the controversy of the root-tip has apparently ended
somewhat after the fashion of the quarrels at the "Rainbow" in
"Silas Marner"--"you're both right and you're both wrong." But the
"brain-function" of the root-tip at which eminent people laughed in
early days turns out to be an important part of the truth. (By using
Piccard's method I have succeeded in showing that the gravitational
sensitiveness of the cotyledon of Sorghum is certainly much greater than
the sensitiveness of the hypocotyl--if indeed any such sensitiveness
exists. See Wiesner's "Festschrift", Vienna, 1908.)
Another observation of Darwin's has given rise to much controversy.
("Power of Movement", page 133.) If a minute piece of card is fixed
obliquely to the tip of a root some influence is transmitted to the
region of curvature and the root bends away from the side to which
the card was attached. It was thought at the time that this proved the
root-tip to be sensitive to contact, but this is not necessarily the
case. It seems possible that the curvature is a reaction to the injury
caused by the alcoholic solution of shellac with which the cards were
cemented to the tip. This agrees with the fact given in the "Power of
Movement" that injuring the root-tip on one side, by cutting or burning
it, induced a similar curvature. On the other hand it was shown that
curvature could be produced in roots by cementing cards, not to the
naked surface of the root-tip, but to pieces of gold-beaters skin
applied to the root; gold-beaters skin being by itself almost without
effect. But it must be allowed that, as regards touch, it is not clear
how the addition of shellac and card can increase the degree of contact.
There is however some evidence that very close contact from a solid
body, such as a curved fragment of glass, produces curvature: and this
may conceivably be the explanation of the effect of gold-beaters skin
covered with shellac. But on the whole it is perhaps safer to classify
the shellac experiments with the results of undoubted injury rather than
with those of contact.
Another subject on which a good deal of labour was expended is the sleep
of leaves, or as Darwin called it their NYCTITROPIC movement. He showed
for the first time how widely spread this phenomenon is, and attempted
to give an explanation of the use to the plant of the power of sleeping.
His theory was that by becoming more or less vertical at night the
leaves escape the chilling effect of radiation. Our method of testing
this view was to fix some of the leaves of a sleeping plant so that they
remained horizontal at night and therefore fully exposed to radiation,
while their fellows were partly protected by assuming the nocturnal
position. The experiments showed clearly that the horizontal leaves were
more injured than the sleeping, i.e. more or less vertical, ones. It may
be objected that the danger from cold is very slight in warm countries
where sleeping plants abound. But it is quite possible that a lowering
of the temperature which produces no visible injury may nevertheless
be hurtful by checking the nutritive processes (e.g. translocation of
carbohydrates), which go on at night. Stahl ("Bot. Zeitung", 1897, page
81.) however has ingeniously suggested that the exposure of the leaves
to radiation is not DIRECTLY hurtful because it lowers the temperature
of the leaf, but INDIRECTLY because it leads to the deposition of dew on
the leaf-surface. He gives reasons for believing that dew-covered leaves
are unable to transpire efficiently, and that the absorption of mineral
food-material is correspondingly checked. Stahl's theory is in no way
destructive of Darwin's, and it is possible that nyctitropic leaves are
adapted to avoid the indirect as well as the direct results of cooling
by radiation.
In what has been said I have attempted to give an idea of some of the
discoveries brought before the world in the "Power of Movement" (In 1881
Professor Wiesner published his "Das Bewegungsvermogen der Pflanzen",
a book devoted to the criticism of "The Power of Movement in Plants". A
letter to Wiesner, published in "Life and Letters", III. page 336, shows
Darwin's warm appreciation of his critic's work, and of the spirit in
which it is written.) and of the subsequent history of the problems.
We must now pass on to a consideration of the central thesis of the
book,--the relation of circumnutation to the adaptive curvatures of
plants.
Darwin's view is plainly stated on pages 3-4 of the "Power of Movement".
Speaking of circumnutation he says, "In this universally present
movement we have the basis or groundwork for the acquirement, according
to the requirements of the plant, of the most diversified movements."
He then points out that curvatures such as those towards the light or
towards the centre of the earth can be shown to be exaggerations of
circumnutation in the given directions. He finally points out that
the difficulty of conceiving how the capacities of bending in definite
directions were acquired is diminished by his conception. "We know that
there is always movement in progress, and its amplitude, or direction,
or both, have only to be modified for the good of the plant in relation
with internal or external stimuli."
It may at once be allowed that the view here given has not been accepted
by physiologists. The bare fact that circumnutation is a general
property of plants (other than climbing species) is not generally
rejected. But the botanical world is no nearer to believing in the
theory of reaction built on it.
If we compare the movements of plants with those of the lower animals we
find a certain resemblance between the two. According to Jennings (H.S.
Jennings, "The Behavior of the Lower Animals". Columbia U. Press, N.Y.
1906.) a Paramoecium constantly tends to swerve towards the aboral side
of its body owing to certain peculiarities in the set and power of
its cilia. But the tendency to swim in a circle, thus produced, is
neutralised by the rotation of the creature about its longitudinal
axis. Thus the direction of the swerves IN RELATION TO THE PATH of the
organism is always changing, with the result that the creature moves in
what approximates to a straight line, being however actually a spiral
about the general line of progress. This method of motion is strikingly
like the circumnutation of a plant, the apex of which also describes a
spiral about the general line of growth. A rooted plant obviously cannot
rotate on its axis, but the regular series of curvatures of which its
growth consists correspond to the aberrations of Paramoecium distributed
regularly about its course by means of rotation. (In my address to the
Biological Section of the British Association at Cardiff (1891) I
have attempted to show the connection between circumnutation and
RECTIPETALITY, i.e. the innate capacity of growing in a straight line.)
Just as a plant changes its direction of growth by an exaggeration of
one of the curvature-elements of which circumnutation consists, so
does a Paramoecium change its course by the accentuation of one of
the deviations of which its path is built. Jennings has shown that the
infusoria, etc., react to stimuli by what is known as the "method of
trial." If an organism swims into a region where the temperature is too
high or where an injurious substance is present, it changes its course.
It then moves forward again, and if it is fortunate enough to escape the
influence, it continues to swim in the given direction. If however its
change of direction leads it further into the heated or poisonous region
it repeats the movement until it emerges from its difficulties. Jennings
finds in the movements of the lower organisms an analogue with what
is known as pain in conscious organisms. There is certainly this much
resemblance that a number of quite different sub-injurious agencies
produce in the lower organisms a form of reaction by the help of which
they, in a partly fortuitous way, escape from the threatening element
in their environment. The higher animals are stimulated in a parallel
manner to vague and originally purposeless movements, one of which
removes the discomfort under which they suffer, and the organism finally
learns to perform the appropriate movement without going through the
tentative series of actions.
I am tempted to recognise in circumnutation a similar groundwork of
tentative movements out of which the adaptive ones were originally
selected by a process rudely representative of learning by experience.
It is, however, simpler to confine ourselves to the assumption that
those plants have survived which have acquired through unknown causes
the power of reacting in appropriate ways to the external stimuli of
light, gravity, etc. It is quite possible to conceive this occurring in
plants which have no power of circumnutating--and, as already pointed
out, physiologists do as a fact neglect circumnutation as a factor in
the evolution of movements. Whatever may be the fate of Darwin's theory
of circumnutation there is no doubt that the research he carried out
in support of, and by the light of, this hypothesis has had a powerful
influence in guiding the modern theories of the behaviour of plants.
Pfeffer ("The Physiology of Plants", Eng. Tr. III. page 11.), who more
than any one man has impressed on the world a rational view of the
reactions of plants, has acknowledged in generous words the great value
of Darwin's work in the same direction. The older view was that, for
instance, curvature towards the light is the direct mechanical result of
the difference of illumination on the lighted and shaded surfaces of the
plant. This has been proved to be an incorrect explanation of the
fact, and Darwin by his work on the transmission of stimuli has greatly
contributed to the current belief that stimuli act indirectly. Thus we
now believe that in a root and a stem the mechanism for the perception
of gravitation is identical, but the resulting movements are different
because the motor-irritabilities are dissimilar in the two cases. We
must come back, in fact, to Darwin's comparison of plants to animals.
In both there is perceptive machinery by which they are made delicately
alive to their environment, in both the existing survivors are those
whose internal constitution has enabled them to respond in a beneficial
way to the disturbance originating in their sense-organs.