LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.
General considerations—Vicia faba, effects of amputating the tips of
the radicles—Regeneration of the tips—Effects of a short exposure of
the tips to geotropic action and their subsequent amputation—Effects of
amputating the tips obliquely—Effects of cauterising the tips—Effects
of grease on the tips—Pisum sativum, tips of radicles cauterised
transversely, and on their upper and lower sides—Phaseolus,
cauterisation and grease on the tips—Gossypium—Cucurbita, tips
cauterised transversely, and on their upper and lower sides—Zea, tips
cauterised—Concluding remarks and summary of chapter—Advantages of the
sensibility to geotropism being localised in the tips of the radicles.
Ciesielski states[1] that when the roots of Pisum, Lens and Vicia were
extended horizontally with their tips cut off, they were not acted on
by geotropism; but some days afterwards, when a new root-cap and
vegetative point had been formed, they bent themselves perpendicularly
downwards. He further states that if the tips are cut off, after the
roots have been left extended horizontally for some little time, but
before they have begun to bend downwards, they may be placed in any
position, and yet will bend as if still acted on by geotropism; and
this shows that some influence had been already transmitted to the
bending part from the tip before it was amputated. Sachs repeated these
experiments; he cut off a length of between .05 and 1 mm. (measured
from the apex of the
vegetative point) of the tips of the radicles of the bean (Vicia faba),
and placed them horizontally or vertically in damp air, earth, and
water, with the result that they became bowed in all sorts of
directions.[2] He therefore disbelieved in Ciesielski’s conclusions.
But as we have seen with several plants that the tip of the radicle is
sensitive to contact and to other irritants, and that it transmits some
influence to the upper growing part causing it to bend, there seemed to
us to be no a priori improbability in Ciesielski’s statements. We
therefore determined to repeat his experiments, and to try others on
several species by different methods.
[1] ‘Abwartskrümmung der Wurzel,’ Inaug. Dissert. Breslau, 1871, p.
29.
[2] ‘Arbeiten des Bot. Instituts in Würzburg,’ Heft. iii. 1873, p.
432.
Vicia faba.—Radicles of this plant were extended horizontally either
over water or with their lower surfaces just touching it. Their tips
had previously been cut off, in a direction as accurately transverse as
could be done, to different lengths, measured from the apex of the
root-cap, and which will be specified in each case. Light was always
excluded. We had previously tried hundreds of unmutilated radicles
under similar circumstances, and found that every one that was healthy
became plainly geotropic in under 12 h. In the case of four radicles
which had their tips cut off for a length of 1.5 mm., new root caps and
new vegetative points were re-formed after an interval of 3 days 20 h.;
and these when placed horizontally were acted on by geotropism. On some
other occasions this regeneration of the tips and reacquired
sensitiveness occurred within a somewhat shorter time. Therefore,
radicles having their tips amputated should be observed in from 12 to
48 h. after the operation.
Four radicles were extended horizontally with their lower surfaces
touching the water, and with their tips cut off for a length of only
0.5 mm.: after 23 h. three of them were still horizontal; after 47 h.
one of the three became fairly geotropic; and after 70 h. the other two
showed a trace of this action. The fourth radicle was vertically
geotropic after 23 h.; but by an
accident the root-cap alone and not the vegetative point was found to
have been amputated; so that this case formed no real exception and
might have been excluded.
Five radicles were extended horizontally like the last, and had their
tips cut off for a length of 1 mm.; after 22–23 h., four of them were
still horizontal, and one was slightly geotropic; after 48 h. the
latter had become vertical; a second was also somewhat geotropic; two
remained approximately horizontal; and the last or fifth had grown in a
disordered manner, for it was inclined upwards at an angle of 65° above
the horizon.
Fourteen radicles were extended horizontally at a little height over
the water with their tips cut off for a length of 1.5 mm.; after 12 h.
all were horizontal, whilst five control or standard specimens in the
same jar were all bent greatly downwards. After 24 h. several of the
amputated radicles remained horizontal, but some showed a trace of
geotropism, and one was plainly geotropic, for it was inclined at 40°
beneath the horizon.
Seven horizontally extended radicles from which the tips had been cut
off for the unusual length of 2 mm. unfortunately were not looked at
until 35 h. had elapsed; three were still horizontal, but to our
surprise, four were more or less plainly geotropic.
The radicles in the foregoing cases were measured before their tips
were amputated, and in the course of 24 h. they had all increased
greatly in length; but the measurements are not worth giving. It is of
more importance that Sachs found that the rate of growth of the
different parts of radicles with amputated tips was the same as with
unmutilated ones. Altogether twenty-nine radicles were operated on in
the manner above described, and of these only a few showed any
geotropic curvature within 24 h.; whereas radicles with unmutilated
tips always became, as already stated, much bent down in less than half
of this time. The part of the radicle which bends most lies at the
distance of from 3 to 6 mm. from the tip, and as the bending part
continues to grow after the operation, there does not seem any reason
why it should not have been acted on by geotropism, unless its
curvature depended on some influence transmitted from the tip. And we
have clear evidence of such transmission in Ciesielski’s experiments,
which we repeated and extended in the following manner.
Beans were embedded in friable peat with the hilum downwards, and after
their radicles had grown perpendicularly down for a length of from ½ to
1 inch, sixteen were selected which
were perfectly straight, and these were placed horizontally on the
peat, being covered by a thin layer of it. They were thus left for an
average period of 1 h. 37 m. The tips were then cut off transversely
for a length of 1.5 mm., and immediately afterwards they were embedded
vertically in the peat. In this position geotropism would not tend to
induce any curvature, but if some influence had already been
transmitted from the tip to the part which bends most, we might expect
that this part would become curved in the direction in which geotropism
had previously acted; for it should be noted that these radicles being
now destitute of their sensitive tips, would not be prevented by
geotropism from curving in any direction. The result was that of the
sixteen vertically embedded radicles, four continued for several days
to grow straight downwards, whilst twelve became more or less bowed
laterally. In two of the twelve, a trace of curvature was perceptible
in 3 h. 30 m., counting from the time when they had first been laid
horizontally; and all twelve were plainly bowed in 6 h., and still more
plainly in 9 h. In every one of them the curvature was directed towards
the side which had been downwards whilst the radicles remained
horizontal. The curvature extended for a length of from 5 to, in one
instance, 8 mm., measured from the cut-off end. Of the twelve bowed
radicles five became permanently bent into a right angle; the other
seven were at first much less bent, and their curvature generally
decreased after 24 h., but did not wholly disappear. This decrease of
curvature would naturally follow, if an exposure of only 1 h. 37 m. to
geotropism, served to modify the turgescence of the cells, but not
their subsequent growth to the full extent. The five radicles which
were rectangularly bent became fixed in this position, and they
continued to grow out horizontally in the peat for a length of about 1
inch during from 4 to 6 days. By this time new tips had been formed;
and it should be remarked that this regeneration occurred slower in the
peat than in water, owing perhaps to the radicles being often looked at
and thus disturbed. After the tips had been regenerated, geotropism was
able to act on them, so that they now became bowed vertically
downwards. An accurate drawing (Fig. 195) is given on the opposite page
of one of these five radicles, reduced to half the natural size.
We next tried whether a shorter exposure to geotropism would suffice to
produce an after-effect. Seven radicles were extended horizontally for
an hour, instead of 1 h. 37 m. as in the
former trial; and after their tips (1.5 mm. in length) had been
amputated, they were placed vertically in damp peat. Of these, three
were not in the least affected and continued for days to grow straight
downwards. Four showed after 8 h. 30 m. a mere trace of curvature in
the direction in which they had been acted on by geotropism; and in
this respect they differed much from those which had been exposed for 1
h. 37 m., for many of the latter were plainly curved in 6 h. The
curvature of one of these four radicles almost disappeared after 24 h.
In the second, the curvature increased during two days and then
decreased. the third radicle became permanently bent, so that its
terminal part made an angle of about 45° with its original vertical
direction. The fourth radicle became horizontal. These two, latter
radicles continued during two more days to grow in the peat in the same
directions, that is, at an angle of 45° beneath the horizon and
horizontally. By the fourth morning new tips had been re-formed, and
now geotropism was able to act on them again, and they became bent
perpendicularly downwards, exactly as in the case of the five radicles
described in the last paragraph and as is shown in (Fig. 195) here
given.
Fig. 195. Vicia faba: radicle, rectangularly bent at A, after the
amputation of the tip, due to the previous influence of geotropism. L,
side of bean which lay on the peat, whilst geotropism acted on the
radicle. A, point of chief curvature of the radicle, whilst standing
vertically downwards. B, point of chief curvature after the
regeneration of the tip, when geotropism again acted. C, regenerated
tip.
Lastly, five other radicles were similarly treated, but were exposed to
geotropism during only 45 m. After 8 h. 30 m. only one was doubtfully
affected; after 24 h. two were just perceptibly curved towards the side
which had been acted on by geotropism; after 48 h. the one first
mentioned had a radius of curvature of 60 mm. That this curvature was
due to the action of geotropism during the horizontal position of the
radicle, was shown after 4 days, when a new tip had been re-formed, for
it then grew perpendicularly downwards. We learn from this
case that when the tips are amputated after an exposure to geotropism
of only 45 m., though a slight influence is sometimes transmitted to
the adjoining part of the radicle, yet this seldom suffices, and then
only slowly, to induce even moderately well-pronounced curvature.
In the previously given experiments on 29 horizontally extended
radicles with their tips amputated, only one grew irregularly in any
marked manner, and this became bowed upwards at an angle of 65°. In
Ciesielski’s experiments the radicles could not have grown very
irregularly, for if they had done so, he could not have spoken
confidently of the obliteration of all geotropic action. It is
therefore remarkable that Sachs, who experimented on many radicles with
their tips amputated, found extremely disordered growth to be the usual
result. As horizontally extended radicles with amputated tips are
sometimes acted on slightly by geotropism within a short time, and are
often acted on plainly after one or two days, we thought that this
influence might possibly prevent disordered growth, though it was not
able to induce immediate curvature. Therefore 13 radicles, of which 6
had their tips amputated transversely for a length of 1.5 mm., and the
other 7 for a length of only 0.5 mm., were suspended vertically in damp
air, in which position they would not be affected by geotropism; but
they exhibited no great irregularity of growth, whilst observed during
4 to 6 days. We next thought that if care were not taken in cutting off
the tips transversely, one side of the stump might be irritated more
than the other, either at first or subsequently during the regeneration
of the tip, and that this might cause the radicle to bend to one side.
It has also been shown in Chapter III. that if a thin slice be cut off
one side of the tip of the radicle, this causes the radicle to bend
from the sliced side. Accordingly, 30 radicles, with tips amputated for
a length of 1.5 mm., were allowed to grow perpendicularly downwards
into water. Twenty of them were amputated at an angle of 20° with a
line transverse to their longitudinal axes; and such stumps appeared
only moderately oblique. The remaining ten radicles were amputated at
an angle of about 45°. Under these circumstances no less than 19 out of
the 30 became much distorted in the course of 2 or 3 days. Eleven other
radicles were similarly treated, excepting that only 1 mm. (including
in this and all other cases the root-cap) was amputated; and of these
only one grew much, and two others slightly
distorted; so that this amount of oblique amputation was not
sufficient. Out of the above 30 radicles, only one or two showed in the
first 24 h. any distortion, but this became plain in the 19 cases on
the second day, and still more conspicuous at the close of the third
day, by which time new tips had been partially or completely
regenerated. When therefore a new tip is reformed on an oblique stump,
it probably is developed sooner on one side than on the other: and this
in some manner excites the adjoining part to bend to one side. Hence it
seems probable that Sachs unintentionally amputated the radicles on
which he experimented, not strictly in a transverse direction.
This explanation of the occasional irregular growth of radicles with
amputated tips, is supported by the results of cauterising their tips;
for often a greater length on one side than on the other was
unavoidably injured or killed. It should be remarked that in the
following trials the tips were first dried with blotting-paper, and
then slightly rubbed with a dry stick of nitrate of silver or lunar
caustic. A few touches with the caustic suffice to kill the root-cap
and some of the upper layers of cells of the vegetative point.
Twenty-seven radicles, some young and very short, others of moderate
length, were suspended vertically over water, after being thus
cauterised. Of these some entered the water immediately, and others on
the second day. The same number of uncauterised radicles of the same
age were observed as controls. After an interval of three or four days
the contrast in appearance between the cauterised and control specimens
was wonderfully great. The controls had grown straight downwards, with
the exception of the normal curvature, which we have called Sachs’
curvature. Of the 27 cauterised radicles, 15 had become extremely
distorted; 6 of them grew upwards and formed hoops, so that their tips
sometimes came into contact with the bean above; 5 grew out
rectangularly to one side; only a few of the remaining 12 were quite
straight, and some of these towards the close of our observations
became hooked at their extreme lower ends. Radicles, extended
horizontally instead of vertically, with their tips cauterised, also
sometimes grew distorted, but not so commonly, as far as we could
judge, as those suspended vertically; for this occurred with only 5 out
of 19 radicles thus treated.
Instead of cutting off the tips, as in the first set of experiments, we
next tried the effects of touching horizontally extended radicles with
caustic in the manner just described. But
some preliminary remarks must first be made. It may be objected that
the caustic would injure the radicles and prevent them from bending;
but ample evidence was given in Chapter III. that touching the tips of
vertically suspended radicles with caustic on one side, does not stop
their bending; on the contrary, it causes them to bend from the touched
side. We also tried touching both the upper and the lower sides of the
tips of some radicles of the bean, extended horizontally in damp
friable earth. The tips of three were touched with caustic on their
upper sides, and this would aid their geotropic bending; the tips of
three were touched on their lower sides, which would tend to counteract
the bending downwards; and three were left as controls. After 24 h. an
independent observer was asked to pick out of the nine radicles, the
two which were most and the two which were least bent; he selected as
the latter, two of those which had been touched on their lower sides,
and as the most bent, two of those which had been touched on the upper
side. Hereafter analogous and more striking experiments with Pisum
sativum and Cucurbita ovifera will be given. We may therefore safely
conclude that the mere application of caustic to the tip does not
prevent the radicles from bending.
In the following experiments, the tips of young horizontally extended
radicles were just touched with a stick of dry caustic; and this was
held transversely, so that the tip might be cauterised all round as
symmetrically as possible. The radicles were then suspended in a closed
vessel over water, kept rather cool, viz., 55°–59° F. This was done
because we had found that the tips were more sensitive to contact under
a low than under a high temperature; and we thought that the same rule
might apply to geotropism. In one exceptional trial, nine radicles
(which were rather too old, for they had grown to a length of from 3 to
5 cm.), were extended horizontally in damp friable earth, after their
tips had been cauterised and were kept at too high a temperature, viz.,
of 68° F., or 20° C. The result in consequence was not so striking as
in the subsequent cases for although when after 9 h. 40 m. six of them
were examined, these did not exhibit any geotropic bending, yet after
24 h., when all nine were examined, only two remained horizontal, two
exhibited a trace of geotropism, and five were slightly or moderately
geotropic, yet not comparable in degree with the control specimens.
Marks had been made on seven of these cauterised radicles at 10 mm.
from the tips, which includes
the whole growing portion; and after the 24 h. this part had a mean
length of 37 mm., so that it had increased to more than 3½ times its
original length; but it should be remembered that these beans had been
exposed to a rather high temperature.
Nineteen young radicles with cauterised tips were extended at different
times horizontally over water. In every trial an equal number of
control specimens were observed. In the first trial, the tips of three
radicles were lightly touched with the caustic for 6 or 7 seconds,
which was a longer application than usual. After 23 h. 30 m. (temp.
55°–56° F.) these three radicles, A, B, C (Fig. 196), were still
horizontal, whilst the three control specimens had become within 8 h.
slightly geotropic, and strongly so (D, E, F) in 23 h. 30 m. A dot had
been made on all six radicles at 10 mm. from their tips, when first
placed horizontally. After the 23 h. 30 m. this terminal part,
originally 10 mm. in length, had increased in the cauterised specimens
to a mean length of 17.3 mm., and to 15.7 mm. in the control radicles,
as shown in the figures by the unbroken transverse line; the dotted
line being at 10 mm. from the apex. The control or uncauterised
radicles, therefore, had actually grown less
than the cauterised; but this no doubt was accidental, for radicles of
different ages grow at different rates, and the growth of different
individuals is likewise affected by unknown causes. The state of the
tips of these three radicles, which had been cauterised for a rather
longer time than usual, was as follows: the blackened apex, or the part
which had been actually touched by the caustic, was succeeded by a
yellowish zone, due probably to the absorption of some of the caustic;
in A, both zones together were 1.1 mm. in length, and 1.4 mm. in
diameter at the base of the yellowish zone; in B, the length of both
was only 0.7 mm., and the diameter 0.7 mm.; in C, the length was 0.8
mm., and the diameter 1.2 mm.
Fig. 196. Vicia faba: state of radicles which had been extended
horizontally for 23 h. 30 m.; A, B, C, tips touched with caustic; D, E,
F, tips uncauterised. Lengths of radicles reduced to one-half scale,
but by an accident the beans themselves not reduced in the same degree.
Three other radicles, the tips of which had been touched with caustic
curing 2 or 3 seconds, remained (temp. 58°–59° F.) horizontal for 23
h.; the control radicles having, of course, become geotropic within
this time. The terminal growing part, 10 mm. in length, of the
cauterised radicles had increased in this interval to a mean length of
24.5 mm., and of the controls to a mean of 26 mm. A section of one of
the cauterised tips showed that the blackened part was 0.5 mm. in
length, of which 0.2 mm. extended into the vegetative point; and a
faint discoloration could be detected even to 1.6 mm. from the apex of
the root-cap.
In another lot of six radicles (temp. 55°–57° F.) the three control
specimens were plainly geotropic in 8½ h.; and after 24 h. the mean
length of their terminal part had increased from 10 mm. to 21 mm. When
the caustic was applied to the three cauterised specimens, it was held
quite motionless during 5 seconds, and the result was that the black
marks were extremely minute. Therefore, caustic was again applied,
after 8½ h., during which time no geotropic action had occurred. When
the specimens were re-examined after an additional interval of 15½ h.,
one was horizontal and the other two showed, to our surprise, a trace
of geotropism which in one of them soon afterwards became strongly
marked; but in this latter specimen the discoloured tip was only 2/3
mm. in length. The growing part of these three radicles increased in 24
h. from 10 mm. to an average of 16.5 mm.
It would be superfluous to describe in detail the behaviour of the 10
remaining cauterised radicles. The corresponding control specimens all
became geotropic in 8 h. Of the cauterised, 6 were first looked at
after 8 h., and one alone showed a trace
of geotropism; 4 were first looked at after 14 h., and one alone of
these was slightly geotropic. After 23–24h., 5 of the 10 were still
horizontal, 4 slightly, and 1 decidedly, geotropic. After 48 h. some of
them became strongly geotropic. The cauterised radicles increased
greatly in length, but the measurements are not worth giving.
As five of the last-mentioned cauterised radicles had become in 24 h.
somewhat geotropic, these (together with three which were still
horizontal) had their positions reversed, so that their tips were now a
little upturned, and they were again touched with caustic. After 24 h.
they showed no trace of geotropism; whereas the eight corresponding
control specimens, which had likewise been reversed, in which position
the tips of several pointed to the zenith, all became geotropic; some
having passed in the 24 h. through an angle of 180°, others through
about 135°, and others through only 90°. The eight radicles, which had
been twice cauterised, were observed for an additional day (i.e. for 48
h. after being reversed), and they still showed no signs of geotropism.
Nevertheless, they continued to grow rapidly; four were measured 24 h.
after being reversed, and they had in this time increased in length
between 8 and 11 mm.; the other four were measured 48 h. after being
reversed, and these had increased by 20, 18, 23, and 28 mm.
In coming to a conclusion with respect to the effects of cauterising
the tips of these radicles, we should bear in mind, firstly, that
horizontally extended control radicles were always acted on by
geotropism, and became somewhat bowed downwards in 8 or 9 h.; secondly,
that the chief seat of the curvature lies at a distance of from 3 to 6
mm. from the tip; thirdly, that the tip was discoloured by the caustic
rarely for more than 1 mm. in length; fourthly, that the greater number
of the cauterised radicles, although subjected to the full influence of
geotropism during the whole time, remained horizontal for 24 h., and
some for twice as long; and that those which did become bowed were so
only in a slight degree; fifthly, that the cauterised radicles
continued to grow almost, and sometimes quite, as well as the uninjured
ones along the part which bends most. And lastly, that a touch on the
tip with caustic, if on one side, far from preventing curvature,
actually induces it. Bearing all these facts in mind, we must infer
that under normal conditions the geotropic curvature of the root is due
to an influence transmitted from the apex to the adjoining part where
the bending
takes place; and that when the tip of the root is cauterised it is
unable to originate the stimulus necessary to produce geotropic
curvature.
As we had observed that grease was highly injurious to some plants, we
determined to try its effects on radicles. When the cotyledons of
Phalaris and Avena were covered with grease along one side, the growth
of this side was quite stopped or greatly checked, and as the opposite
side continued to grow, the cotyledons thus treated became bowed
towards the greased side. This same matter quickly killed the delicate
hypocotyls and young leaves of certain plants. The grease which we
employed was made by mixing lamp-black and olive oil to such a
consistence that it could be laid on in a thick layer. The tips of five
radicles of the bean were coated with it for a length of 3 mm., and to
our surprise this part increased in length in 23 h. to 7.1 mm.; the
thick layer of grease being curiously drawn out. It thus could not have
checked much, if at all, the growth of the terminal part of the
radicle. With respect to geotropism, the tips of seven horizontally
extended radicles were coated for a length of 2 mm., and after 24 h. no
clear difference could be perceived between their downward curvature
and that of an equal number of control specimens. The tips of 33 other
radicles were coated on different occasions for a length of 3 mm.; and
they were compared with the controls after 8 h., 24 h., and 48 h. On
one occasion, after 24 h., there was very little difference in
curvature between the greased and control specimens; but generally the
difference was unmistakable, those with greased tips being considerably
less curved downwards. The whole growing part (the greased tips
included) of six of these radicles was measured and was found to have
increased in 23 h. from 10 mm. to a mean length of 17.7 mm.; whilst the
corresponding part of the controls had increased to 20.8 mm. It appears
therefore, that although the tip itself, when greased, continues to
grow, yet the growth of the whole radicle is somewhat checked, and that
the geotropic curvature of the upper part, which was free from grease,
was in most cases considerably lessened.
Pisum sativum.—Five radicles, extended horizontally over water, had
their tips lightly touched two or three times with dry caustic. These
tips were measured in two cases, and found to be blackened for a length
of only half a millimeter. Five other radicles were left as controls.
The part which is most bowed through geotropism lies at a distance of
several millimeters from
the apex. After 24 h., and again after 32 h. from the commencement,
four of the cauterised radicles were still horizontal, but one was
plainly geotropic, being inclined at 45° beneath the horizon. The five
controls were somewhat geotropic after 7 h. 20 m., and after 24 h. were
all strongly geotropic; being inclined at the following angles beneath
the horizon, viz., 59°, 60°, 65°, 57°, and 43°. The length of the
radicles was not measured in either set, but it was manifest that the
cauterised radicles had grown greatly.
The following case proves that the action of the caustic by itself does
not prevent the curvature of the radicle. Ten radicles were extended
horizontally on and beneath a layer of damp friable peat-earth; and
before being extended their tips were touched with dry caustic on the
upper side. Ten other radicles similarly placed were touched on the
lower side; and this would tend to make them bend from the cauterised
side; and therefore, as now placed, upwards, or in opposition to
geotropism. Lastly, ten uncauterised radicles were extended
horizontally as controls. After 24 h. all the latter were geotropic;
and the ten with their tips cauterised on the upper side were equally
geotropic; and we believe that they became curved downwards before the
controls. The ten which had been cauterised on the lower side presented
a widely different appearance: No. 1, however, was perpendicularly
geotropic, but this was no real exception, for on examination under the
microscope, there was no vestige of a coloured mark on the tip, and it
was evident that by a mistake it had not been touched with the caustic.
No. 2 was plainly geotropic, being inclined at about 45° beneath the
horizon; No. 3 was slightly, and No. 4 only just perceptibly geotropic;
Nos. 5 and 6 were strictly horizontal; and the four remaining ones were
bowed upwards, in opposition to geotropism. In these four cases the
radius of the upward curvatures (according to Sachs’ cyclometer) was 5
mm., 10 mm., 30 mm., and 70 mm. This curvature was distinct long before
the 24 h. had elapsed, namely, after 8 h. 45 m. from the time when the
lower sides of the tips were touched with the caustic.
Phaseolus multiflorus.—Eight radicles, serving as controls, were
extended horizontally, some in damp friable peat and some in damp air.
They all became (temp 20°–21° C.) plainly geotropic in 8 h. 30 m., for
they then stood at an average angle of 63° beneath the horizon. A
rather greater length of the radicle is bowed downwards by geotropism
than in the case of Vicia faba,
that is to say, rather more than 6 mm. as measured from the apex of the
root-cap. Nine other radicles were similarly extended, three in damp
peat and six in damp air, and dry caustic was held transversely to
their tips during 4 or 5 seconds. Three of their tips were afterwards
examined: in (1) a length of 0.68 mm. was discoloured, of which the
basal 0.136 mm. was yellow, the apical part being black; in (2) the
discoloration was 0.65 mm. in length, of which the basal 0.04 mm. was
yellow; in (3) the discoloration was 0.6 mm. in length, of which the
basal 0.13 mm. was yellow. Therefore less than 1 mm. was affected by
the caustic, but this sufficed almost wholly to prevent geotropic
action; for after 24 h. one alone of the nine cauterised radicles
became slightly geotropic, being now inclined at 10° beneath the
horizon; the eight others remained horizontal, though one was curved a
little laterally.
The terminal part (10 mm. in length) of the six cauterised radicles in
the damp air, had more than doubled in length in the 24 h., for this
part was now on an average 20.7 mm. long. The increase in length within
the same time was greater in the control specimens, for the terminal
part had grown on an average from 10 mm. to 26.6 mm. But as the
cauterised radicles had more than doubled their length in the 24 h., it
is manifest that they had not been seriously injured by the caustic. We
may here add that when experimenting on the effects of touching one
side of the tip with caustic, too much was applied at first, and the
whole tip (but we believe not more than 1 mm. in length) of six
horizontally extended radicles was killed, and these continued for two
or three days to grow out horizontally.
Many trials were made, by coating the tips of horizontally extended
radicles with the before described thick grease. The geotropic
curvature of 12 radicles, which were thus coated for a length of 2 mm.,
was delayed during the first 8 or 9 h., but after 24 h. was nearly as
great as that of the control specimens. The tips of nine radicles were
coated for a length of 3 mm., and after 7 h. 10 m. these stood at an
average angle of 30° beneath the horizon, whilst the controls stood at
an average of 54°. After 24 h. the two lots differed but little in
their degree of curvature. In some other trials, however, there was a
fairly well-marked difference after 24 h. between those with greased
tips and the controls. The terminal part of eight control specimens
increased in 24 h. from 10 mm. to a mean length of
24.3 mm., whilst the mean increase of those with greased tips was 20.7
mm. The grease, therefore, slightly checked the growth of the terminal
part, but this part was not much injured; for several radicles which
had been greased for a length of 2 mm. continued to grow during seven
days, and were then only a little shorter than the controls. The
appearance presented by these radicles after the seven days was very
curious, for the black grease had been drawn out into the finest
longitudinal striae, with dots and reticulations, which covered their
surfaces for a length of from 26 to 44 mm., or of 1 to 1.7 inch. We may
therefore conclude that grease on the tips of the radicles of this
Phaseolus somewhat delays and lessens the geotropic curvature of the
part which ought to bend most.
Gossypium herbaceum.—The radicles of this plant bend, through the
action of geotropism, for a length of about 6 mm. Five radicles, placed
horizontally in damp air, had their tips touched with caustic, and the
discoloration extended for a length of from 2/3 to 1 mm. They showed,
after 7 h. 45 m. and again after 23 h., not a trace of geotropism; yet
the terminal portion, 9 mm. in length, had increased on an average to
15.9 mm. Six control radicles, after 7 h. 45 m., were all plainly
geotropic, two of them being vertically dependent, and after 23 h. all
were vertical, or nearly so.
Cucurbita ovifera.—A large number of trials proved almost useless, from
the three following causes: Firstly, the tips of radicles which have
grown somewhat old are only feebly geotropic if kept in damp air; nor
did we succeed well in our experiments, until the germinating seeds
were placed in peat and kept at a rather high temperature. Secondly,
the hypocotyls of the seeds which were pinned to the lids of the jars
gradually became arched; and, as the cotyledons were fixed, the
movement of the hypocotyl affected the position of the radicle, and
caused confusion. Thirdly, the point of the radicle is so fine that it
is difficult not to cauterise it either too much or too little. But we
managed generally to overcome this latter difficulty, as the following
experiments show, which are given to prove that a touch with caustic on
one side of the tip does not prevent the upper part of the radicle from
bending. Ten radicles were laid horizontally beneath and on damp
friable peat, and their tips were touched with caustic on the upper
side. After 8 h. all were plainly geotropic, three of them
rectangularly; after 19 h.
all were strongly geotropic, most of them pointing perpendicularly
downwards. Ten other radicles, similarly placed, had their tips touched
with caustic on the lower side; after 8 h. three were slightly
geotropic, but not nearly so much so as the least geotropic of the
foregoing specimens; four remained horizontal; and three were curved
upwards in opposition to geotropism. After 19 h. the three which were
slightly geotropic had become strongly so. Of the four horizontal
radicles, one alone showed a trace of geotropism; of the three
up-curved radicles, one retained this curvature, and the other two had
become horizontal.
The radicles of this plant, as already remarked, do not succeed well in
damp air, but the result of one trial may be briefly given. Nine young
radicles between .3 and .5 inch in length, with their tips cauterised
and blackened for a length never exceeding ½ mm., together with eight
control specimens, were extended horizontally in damp air. After an
interval of only 4 h. 10 m. all the controls were slightly geotropic,
whilst not one of the cauterised specimens exhibited a trace of this
action. After 8 h. 35 m., there was the same difference between the two
sets, but rather more strongly marked. By this time both sets had
increased greatly in length. The controls, however, never became much
more curved downwards; and after 24 h. there was no great difference
between the two sets in their degree of curvature.
Eight young radicles of nearly equal length (average .36 inch) were
placed beneath and on peat-earth, and were exposed to a temp. of
75°–76° F. Their tips had been touched transversely with caustic, and
five of them were blackened for a length of about 0.5 mm., whilst the
other three were only just visibly discoloured. In the same box there
were 15 control radicles, mostly about .36 inch in length, but some
rather longer and older, and therefore less sensitive. After 5 h., the
15 control radicles were all more or less geotropic: after 9 h., eight
of them were bent down beneath the horizon at various angles between
45° and 90°, the remaining seven being only slightly geotropic: after
25 h. all were rectangularly geotropic. The state of the eight
cauterised radicles after the same intervals of time was as follows:
after 5 h. one alone was slightly geotropic, and this was one with the
tip only a very little discoloured: after 9 h. the one just mentioned
was rectangularly geotropic, and two others were slightly so, and these
were the three which had been scarcely
affected by the caustic; the other five were still strictly horizontal.
After 24 h. 40 m. the three with only slightly discoloured tips were
bent down rectangularly; the other five were not in the least affected,
but several of them had grown rather tortuously, though still in a
horizontal plane. The eight cauterised radicles which had at first a
mean length of .36 inch, after 9 h. had increased to a mean length of
.79 inch; and after 24 h. 40 m. to the extraordinary mean length of 2
inches. There was no plain difference in length between the five well
cauterised radicles which remained horizontal, and the three with
slightly cauterised tips which had become abruptly bent down. A few of
the control radicles were measured after 25 h., and they were on an
average only a little longer than the cauterised, viz., 2.19 inches. We
thus see that killing the extreme tip of the radicle of this plant for
a length of about 0.5 mm., though it stops the geotropic bending of the
upper part, hardly interferes with the growth of the whole radicle.
In the same box with the 15 control specimens, the rapid geotropic
bending and growth of which have just been described, there were six
radicles, about .6 inch in length, extended horizontally, from which
the tips had been cut off in a transverse direction for a length of
barely 1 mm. These radicles were examined after 9 h. and again after 24
h. 40 m., and they all remained horizontal. They had not become nearly
so tortuous as those above described which had been cauterised. The
radicles with their tips cut off had grown in the 24 h. 40 m. as much,
judging by the eye, as the cauterised specimens.
Zea mays.—The tips of several radicles, extended horizontally in damp
air, were dried with blotting-paper and then touched in the first trial
during 2 or 3 seconds with dry caustic; but this was too long a
contact, for the tips were blackened for a length of rather above 1 mm.
They showed no signs of geotropism after an interval of 9 h., and were
then thrown away. In a second trial the tips of three radicles were
touched for a shorter time, and were blackened for a length of from 0.5
to 0.75 mm.: they all remained horizontal for 4 h., but after 8 h. 30
m. one of them, in which the blackened tip was only 0.5 mm. in length,
was inclined at 21° beneath the horizon. Six control radicles all
became slightly geotropic in 4 h., and strongly so after 8 h. 30 m.,
with the chief seat of curvature generally between 6 or 7 mm. from the
apex. In the cauterised specimens, the terminal growing part, 10 mm. in
length, increased during
the 8 h. 30 m. to a mean length of 13 mm.; and in the controls to 14.3
mm.
In a third trial the tips of five radicles (exposed to a temp. of
70°–71°) were touched with the caustic only once and very slightly;
they were afterwards examined under the microscope, and the part which
was in any way discoloured was on an average .76 mm. in length. After 4
h. 10 m. none were bent; after 5 h. 45 m., and again after 23 h. 30 m.,
they still remained horizontal, excepting one which was now inclined
20° beneath the horizon. The terminal part, 10 mm. in length, had
increased greatly in length during the 23 h. 30 m., viz., to an average
of 26 mm. Four control radicles became slightly geotropic after the 4
h. 10 m., and plainly so after the 5 h. 45 m. Their mean length after
the 23 h. 30 m. had increased from 10 mm. to 31 mm. Therefore a slight
cauterisation of the tip checks slightly the growth of the whole
radicle, and manifestly stops the bending of that part which ought to
bend most under the influence of geotropism, and which still continues
to increase greatly in length.]
Concluding Remarks.—Abundant evidence has now been given, showing that
with various plants the tip of the radicle is alone sensitive to
geotropism; and that when thus excited, it causes the adjoining parts
to bend. The exact length of the sensitive part seems to be somewhat
variable, depending in part on the age of the radicle; but the
destruction of a length of from less than 1 to 1.5 mm. (about 1/20th of
an inch), in the several species observed, generally sufficed to
prevent any part of the radicle from bending within 24 h., or even for
a longer period. The fact of the tip alone being sensitive is so
remarkable a fact, that we will here give a brief summary of the
foregoing experiments. The tips were cut off 29 horizontally extended
radicles of Vicia faba, and with a few exceptions they did not become
geotropic in 22 or 23 h., whilst unmutilated radicles were always bowed
downwards in 8 or 9 h. It should be borne in mind that the mere act of
cutting
off the tip of a horizontally extended radicle does not prevent the
adjoining parts from bending, if the tip has been previously exposed
for an hour or two to the influence of geotropism. The tip after
amputation is sometimes completely regenerated in three days; and it is
possible that it may be able to transmit an impulse to the adjoining
parts before its complete regeneration. The tips of six radicles of
Cucurbita ovifera were amputated like those of Vicia faba; and these
radicles showed no signs of geotropism in 24 h.; whereas the control
specimens were slightly affected in 5 h., and strongly in 9 h.
With plants belonging to six genera, the tips of the radicles were
touched transversely with dry caustic; and the injury thus caused
rarely extended for a greater length than 1 mm., and sometimes to a
less distance, as judged by even the faintest discoloration. We thought
that this would be a better method of destroying the vegetative point
than cutting it off; for we knew, from many previous experiments and
from some given in the present chapter, that a touch with caustic on
one side of the apex, far from preventing the adjoining part from
bending, caused it to bend. In all the following cases, radicles with
uncauterised tips were observed at the same time and under similar
circumstances, and they became, in almost every instance, plainly bowed
downwards in one-half or one-third of the time during which the
cauterised specimens were observed. With Vicia faba 19 radicles were
cauterised; 12 remained horizontal during 23–24 h.; 6 became slightly
and 1 strongly geotropic. Eight of these radicles were afterwards
reversed, and again touched with caustic, and none of them became
geotropic in 24 h., whilst the reversed control specimens became
strongly bowed downwards within this time.
With Pisum sativum, five radicles had their tips touched with caustic,
and after 32 h. four were still horizontal. The control specimens were
slightly geotropic in 7 h. 20 m., and strongly so in 24 h. The tips of
9 other radicles of this plant were touched only on the lower side, and
6 of them remained horizontal for 24 h., or were upturned in opposition
to geotropism; 2 were slightly, and 1 plainly geotropic. With Phaseolus
multiflorus, 15 radicles were cauterised, and 8 remained horizontal for
24 h.; whereas all the controls were plainly geotropic in 8 h. 30 m. Of
5 cauterised radicles of Gossypium herbaceum, 4 remained horizontal for
23 h. and 1 became slightly geotropic; 6 control radicles were
distinctly geotropic in 7 h. 45 m. Five radicles of Cucurbita ovifera
remained horizontal in peat-earth during 25 h., and 9 remained so in
damp air during 8½ h.; whilst the controls became slightly geotropic in
4 h. 10 m. The tips of 10 radicals of this plant were touched on their
lower sides, and 6 of them remained horizontal or were upturned after
19 h., 1 being slightly and 3 strongly geotropic.
Lastly, the tips of several radicles of Vicia faba and Phaseolus
multiflorus were thickly coated with grease for a length of 3 mm. This
matter, which is highly injurious to most plants, did not kill or stop
the growth of the tips, and only slightly lessened the rate of growth
of the whole radicle; but it generally delayed a little the geotropic
bending of the upper part.
The several foregoing cases would tell us nothing, if the tip itself
was the part which became most bent; but we know that it is a part
distant from the tip by some millimeters which grows quickest, and
which, under the influence of geotropism, bends most. We have no reason
to suppose that this part is injured by the death or injury of the tip;
and it is certain
that after the tip has been destroyed this part goes on growing at such
a rate, that its length was often doubled in a day. We have also seen
that the destruction of the tip does not prevent the adjoining part
from bending, if this part has already received some influence from the
tip. As with horizontally extended radicles, of which the tip has been
cut off or destroyed, the part which ought to bend most remains
motionless for many hours or days, although exposed at right angles to
the full influence of geotropism, we must conclude that the tip alone
is sensitive to this power, and transmits some influence or stimulus to
the adjoining parts, causing them to bend. We have direct evidence of
such transmission; for when a radicle was left extended horizontally
for an hour or an hour and a half, by which time the supposed influence
will have travelled a little distance from the tip, and the tip was
then cut off, the radicle afterwards became bent, although placed
perpendicularly. The terminal portions of several radicles thus treated
continued for some time to grow in the direction of their
newly-acquired curvature; for as they were destitute of tips, they were
no longer acted on by geotropism. But after three or four days when new
vegetative points were formed, the radicles were again acted on by
geotropism, and now they curved themselves perpendicularly downwards.
To see anything of the above kind in the animal kingdom, we should have
to suppose than an animal whilst lying down determined to rise up in
some particular direction; and that after its head had been cut off, an
impulse continued to travel very slowly along the nerves to the proper
muscles; so that after several hours the headless animal rose up in the
predetermined direction.
As the tip of the radicle has been found to be the
part which is sensitive to geotropism in the members of such distinct
families as the Leguminosae, Malvaceae, Cucurbitaceæ and Gramineæ, we
may infer that this character is common to the roots of most seedling
plants. Whilst a root is penetrating the ground, the tip must travel
first; and we can see the advantage of its being sensitive to
geotropism, as it has to determine the course of the whole root.
Whenever the tip is deflected by any subterranean obstacle, it will
also be an advantage that a considerable length of the root should be
able to bend, more especially as the tip itself grows slowly and bends
but little, so that the proper downward course may be soon recovered.
But it appears at first sight immaterial whether this were effected by
the whole growing part being sensitive to geotropism, or by an
influence transmitted exclusively from the tip. We should, however,
remember that it is the tip which is sensitive to the contact of hard
objects, causing the radicle to bend away from them, thus guiding it
along the lines of least resistance in the soil. It is again the tip
which is alone sensitive, at least in some cases, to moisture, causing
the radicle to bend towards its source. These two kinds of
sensitiveness conquer for a time the sensitiveness to geotropism,
which, however, ultimately prevails. Therefore, the three kinds of
sensitiveness must often come into antagonism; first one prevailing,
and then another; and it would be an advantage, perhaps a necessity,
for the interweighing and reconciling of these three kinds of
sensitiveness, that they should be all localised in the same group of
cells which have to transmit the command to the adjoining parts of the
radicle, causing it to bend to or from the source of irritation.
Finally, the fact of the tip alone being sensitive to
the attraction of gravity has an important bearing on the theory of
geotropism. Authors seem generally to look at the bending of a radicle
towards the centre of the earth, as the direct result of gravitation,
which is believed to modify the growth of the upper or lower surfaces,
in such a manner as to induce curvature in the proper direction. But we
now know that it is the tip alone which is acted on, and that this part
transmits some influence to the adjoining parts, causing them to curve
downwards. Gravity does not appear to act in a more direct manner on a
radicle, than it does on any lowly organised animal, which moves away
when it feels some weight or pressure.