DARWIN'S WORK ON THE MOVEMENTS OF PLANTS.

By FRANCIS DARWIN,
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.