SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER
IRRITANTS.
Manner in which radicles bend when they encounter an obstacle in the
soil—Vicia faba, tips of radicles highly sensitive to contact and other
irritants—Effects of too high a temperature—Power of discriminating
between objects attached on opposite sides—Tips of secondary radicles
sensitive—Pisum, tips of radicles sensitive—Effects of such
sensitiveness in overcoming geotropism—Secondary radicles—Phaseolus,
tips of radicles hardly sensitive to contact, but highly sensitive to
caustic and to the removal of a
slice—Tropaeolum—Gossypium—Cucurbita—Raphanus—Æsculus, tip not
sensitive to slight contact, highly sensitive to caustic—Quercus, tip
highly sensitive to contact—Power of discrimination—Zea, tip highly
sensitive, secondary radicles—Sensitiveness of radicles to moist
air—Summary of chapter.
In order to see how the radicles of seedlings would pass over stones,
roots, and other obstacles, which they must incessantly encounter in
the soil, germinating beans (Vicia faba) were so placed that the tips
of the radicles came into contact, almost rectangularly or at a high
angle, with underlying plates of glass. In other cases the beans were
turned about whilst their radicles were growing, so that they descended
nearly vertically on their own smooth, almost flat, broad upper
surfaces. The delicate root-cap, when it first touched any directly
opposing surface, was a little flattened transversely; the flattening
soon became oblique, and in a few hours quite disappeared, the apex now
pointing at right angles, or at nearly right angles, to its former
course. The radicle then seemed to glide in its new direction over the
surface which had opposed
it, pressing on it with very little force. How far such abrupt changes
in its former course are aided by the circumnutation of the tip must be
left doubtful. Thin slips of wood were cemented on more or less steeply
inclined glass-plates, at right angles to the radicles which were
gliding down them. Straight lines had been painted along the growing
terminal part of some of these radicles, before they met the opposing
slip of wood; and the lines became sensibly curved in 2 h. after the
apex had come into contact with the slips. In one case of a radicle,
which was growing rather slowly, the root-cap, after encountering a
rough slip of wood at right angles, was at first slightly flattened
transversely: after an interval of 2 h. 30 m. the flattening became
oblique; and after an additional 3 hours the flattening had wholly
disappeared, and the apex now pointed at right angles to its former
course. It then continued to grow in its new direction alongside the
slip of wood, until it came to the end of it, round which it bent
rectangularly. Soon afterwards when coming to the edge of the plate of
glass, it was again bent at a large angle, and descended
perpendicularly into the damp sand.
When, as in the above cases, radicles encountered an obstacle at right
angles to their course, the terminal growing part became curved for a
length of between .3 and .4 of an inch (8–10 mm.), measured from the
apex. This was well shown by the black lines which had been previously
painted on them. The first and most obvious explanation of the
curvature is, that it results merely from the mechanical resistance to
the growth of the radicle in its original direction. Nevertheless, this
explanation did not seem to us satisfactory. The radicles did not
present the appearance of having been subjected to a sufficient
pressure to account for
their curvature; and Sachs has shown[1] that the growing part is more
rigid than the part immediately above which has ceased to grow, so that
the latter might have been expected to yield and become curved as soon
as the apex encountered an unyielding object; whereas it was the stiff
growing part which became curved. Moreover, an object which yields with
the greatest ease will deflect a radicle: thus, as we have seen, when
the apex of the radicle of the bean encountered the polished surface of
extremely thin tin-foil laid on soft sand, no impression was left on
it, yet the radicle became deflected at right angles. A second
explanation occurred to us, namely, that even the gentlest pressure
might check the growth of the apex, and in this case growth could
continue only on one side, and thus the radicle would assume a
rectangular form; but this view leaves wholly unexplained the curvature
of the upper part, extending for a length of 8–10 mm.
[1] ‘Arbeiten Bot. Inst. Würzburg,’ Heft iii. 1873, p. 398.
We were therefore led to suspect that the apex was sensitive to
contact, and that an effect was transmitted from it to the upper part
of the radicle, which was thus excited to bend away from the touching
object. As a little loop of fine thread hung on a tendril or on the
petiole of a leaf-climbing plant, causes it to bend, we thought that
any small hard object affixed to the tip of a radicle, freely suspended
and growing in damp air, might cause it to bend, if it were sensitive,
and yet would not offer any mechanical resistance to its growth. Full
details will be given of the experiments which were tried, as the
result proved remarkable. The fact of the apex of a radicle being
sensitive to contact has never been observed, though, as we shall
hereafter see, Sachs discovered that the radicle a little above the
apex is sensitive, and bends like a tendril towards the touching
object. But when one side of the apex is pressed by any object, the
growing part bends away from the object; and this seems a beautiful
adaptation for avoiding obstacles in the soil, and, as we shall see,
for following the lines of least resistance. Many organs, when touched,
bend in one fixed direction, such as the stamens of Berberis, the lobes
of Dionaea, etc.; and many organs, such as tendrils, whether modified
leaves or flower-peduncles, and some few stems, bend towards a touching
object; but no case, we believe, is known of an organ bending away from
a touching object.
Sensitiveness of the Apex of the Radicle of Vicia faba.—Common beans,
after being soaked in water for 24 h., were pinned with the hilum
downwards (in the manner followed by Sachs), inside the cork lids of
glass-vessels, which were half filled with water; the sides and the
cork were well moistened, and light was excluded. As soon as the beans
had protruded radicles, some to a length of less than a tenth of an
inch, and others to a length of several tenths, little squares or
oblongs of card were affixed to the short sloping sides of their
conical tips. The squares therefore adhered obliquely with reference to
the longitudinal axis of the radicle; and this is a very necessary
precaution, for if the bits of card accidentally became displaced, or
were drawn by the viscid matter employed so as to adhere parallel to
the side of the radicle, although only a little way above the conical
apex, the radicle did not bend in the peculiar manner which we are here
considering. Squares of about the 1/20th of an inch (i.e. about 1½
mm.), or oblong bits of nearly the same size, were found to
be the most convenient and effective. We employed at first ordinary
thin card, such as visiting cards, or bits of very thin glass, and
various other objects; but afterwards sand-paper was chiefly employed,
for it was almost as stiff as thin card, and the roughened surface
favoured its adhesion. At first we generally used very thick gum-water;
and this of course, under the circumstances, never dried in the least;
on the contrary, it sometimes seemed to absorb vapour, so that the bits
of card became separated by a layer of fluid from the tip. When there
was no such absorption and the card was not displaced, it acted well
and caused the radicle to bend to the opposite side. I should state
that thick gum-water by itself induces no action. In most cases the
bits of card were touched with an extremely small quantity of a
solution of shellac in spirits of wine, which had been left to
evaporate until it was thick; it then set hard in a few seconds, and
fixed the bits of card well. When small drops of the shellac were
placed on the tips without any card, they set into hard little beads,
and these acted like any other hard object, causing the radicles to
bend to the opposite side. Even extremely minute beads of the shellac
occasionally acted in a slight degree, as will hereafter be described.
But that it was the cards which chiefly acted in our many trials, was
proved by coating one side of the tip with a little bit of goldbeaters’
skin (which by itself hardly acts), and then fixing a bit of card to
the skin with shellac which never came into contact with the radicle:
nevertheless the radicle bent away from the attached card in the
ordinary manner.
Some preliminary trials were made, presently to be described, by which
the proper temperature was determined, and then the following
experiments were made. It should be premised that the beans were
always fixed to the cork-lids, for the convenience of manipulation,
with the edge from which the radicle and plumule protrudes, outwards;
and it must be remembered that owing to what we have called Sachs’
curvature, the radicles, instead of growing perpendicularly downwards,
often bend somewhat, even as much as about 45° inwards, or under the
suspended bean. Therefore when a square of card was fixed to the apex
in front, the bowing induced by it coincided with Sachs’ curvature, and
could be distinguished from it only by being more strongly pronounced
or by occurring more quickly. To avoid this source of doubt, the
squares
were fixed either behind, causing a curvature in direct opposition to
that of Sachs’, or more commonly to the right or left sides. For the
sake of brevity, we will speak of the bits of card, etc., as fixed in
front, or behind, or laterally. As the chief curvature of the radicle
is at a little distance from the apex, and as the extreme terminal and
basal portions are nearly straight, it is possible to estimate in a
rough manner the amount of curvature by an angle; and when it is said
that the radicle became deflected at any angle from the perpendicular,
this implies that the apex was turned upwards by so many degrees from
the downward direction which it would naturally have followed, and to
the side opposite to that to which the card was affixed. That the
reader may have a clear idea of the kind of movement excited by the
bits of attached card, we append here accurate sketches of three
germinating beans thus treated, and selected out of several specimens
to show the gradations in the degrees of curvature. We will now give in
detail a series of experiments, and afterwards a summary of the
results.
Fig. 65. Vicia faba: A, radicle beginning to bend from the attached
little square of card; B, bent at a rectangle; C, bent into a circle or
loop, with the tip beginning to bend downwards through the action of
geotropism.
In the first 12 trials, little squares or oblongs of sanded card, 1.8
mm. in length, and 1.5 or only 0.9 mm. in breadth (i.e. .071 of an inch
in length and .059 or .035 of an inch in breadth) were fixed with
shellac to the tips of the radicles. In the subsequent trials the
little squares were only occasionally measured, but were of about the
same size.
(1.) A young radicle, 4 mm. in length, had a card fixed behind: after 9
h. deflected in the plane in which the bean is flattened, 50° from the
perpendicular and from the card, and in opposition to Sachs’ curvature:
no change next morning, 23 h. from the time of attachment.
(2.) Radicle 5.5 mm. in length, card fixed behind: after 9 h. deflected
in the plane of the bean 20° from the perpendicular and from the card,
and in opposition to Sachs’ curvature: after 23 h. no change.
(3.) Radicle 11 mm. in length, card fixed behind: after 9 h. deflected
in the plane of the bean 40° from the perpendicular and from the card,
and in opposition to Sachs’ curvature. The tip of the radicle more
curved than the upper part, but in the same plane. After 23 h. the
extreme tip was slightly bent towards the card; the general curvature
of the radicle remaining the same.
(4.) Radicle 9 mm. long, card fixed behind and a little laterally:
after 9 h. deflected in the plane of the bean only about 7° or 8° from
the perpendicular and from the card, in opposition to Sachs’ curvature.
There was in addition a slight lateral curvature directed partly from
the card. After 23 h. no change.
(5.) Radicle 8 mm. long, card affixed almost laterally: after 9 h.
deflected 30° from the perpendicular, in the plane of the bean and in
opposition to Sachs’ curvature; also deflected in a plane at right
angles to the above one, 20° from the perpendicular: after 23 h. no
change.
(6.) Radicle 9 mm. long, card affixed in front: after 9 h. deflected in
the plane of the bean about 40° from the vertical, away from the card
and in the direction of Sachs’ curvature. Here therefore we have no
evidence of the card being the cause of the deflection, except that a
radicle never moves spontaneously, as far as we have seen, as much as
40° in the course of 9 h. After 23 h. no change.
(7.) Radicle 7 mm. long, card affixed to the back: after 9 h. the
terminal part of the radicle deflected in the plane of the bean 20°
from the vertical, away from the card and in opposition to Sachs’
curvature. After 22 h. 30 m. this part of the radicle had become
straight.
(8.) Radicle 12 mm. long, card affixed almost laterally: after 9 h.
deflected laterally in a plane at right angles to that of the bean
between 40° and 50° from the vertical and from the card. In the plane
of the bean itself the deflection amounted to 8° or 9° from the
vertical and from the card, in opposition to Sachs’ curvature. After 22
h. 30 m. the extreme tip had become slightly curved towards the card.
(9.) Card fixed laterally: after 11 h. 30 m. no effect, the radicle
being still almost vertical.
(10.) Card fixed almost laterally: after 11 h. 30 m. deflected 90° from
the vertical and from the card, in a plane intermediate between that of
the bean itself and one at right
angles to it. Radicle consequently partially deflected from Sachs’
curvature.
(11.) Tip of radicle protected with goldbeaters’ skin, with a square of
card of the usual dimensions affixed with shellac: after 11 h. greatly
deflected in the plane of the bean, in the direction of Sachs’
curvature, but to a much greater degree and in less time than ever
occurs spontaneously.
(12.) Tip of radicle protected as in last case: after 11 h. no effect,
but after 24 h. 40 m. radicle clearly deflected from the card. This
slow action was probably due to a portion of the goldbeaters’ skin
having curled round and lightly touched the opposite side of the tip
and thus irritated it.
(13.) A radicle of considerable length had a small square of card fixed
with shellac to its apex laterally: after only 7 h. 15 m. a length of
.4 of an inch from the apex, measured along the middle, was
considerably curved from the side bearing the card.
(14.) Case like the last in all respects, except that a length of only
.25 of an inch of the radicle was thus deflected.
(15.) A small square of card fixed with shellac to the apex of a young
radicle; after 9 h. 15 m. deflected through 90° from the perpendicular
and from the card. After 24 h. deflection much decreased, and after an
additional day, reduced to 23° from the perpendicular.
(16.) Square of card fixed with shellac behind the apex of a radicle,
which from its position having been changed during growth had become
very crooked; but the terminal portion was straight, and this became
deflected to about 45° from the perpendicular and from the card, in
opposition to Sachs’ curvature.
(17.) Square of card affixed with shellac: after 8 h. radicle curved at
right angles from the perpendicular and from the card. After 15
additional hours curvature much decreased.
(18.) Square of card affixed with shellac: after 8 h. no effect; after
23 h. 3 m. from time of affixing, radicle much curved from the square.
(19.) Square of card affixed with shellac: after 24 h. no effect, but
the radicle had not grown well and seemed sickly.
(20.) Square of card affixed with shellac: after 24 h. no effect.
(21, 22.) Squares of card affixed with shellac: after 24 h. radicles of
both curved at about 45° from the perpendicular and from the cards.
(23.) Square of card fixed with shellac to young radicle: after
9 h. very slightly curved from the card; after 24 h. tip curved towards
card. Refixed new square laterally, after 9 h. distinctly curved from
the card, and after 24 h. curved at right angles from the perpendicular
and from the card.
(24.) A rather large oblong piece of card fixed with shellac to apex:
after 24 h. no effect, but the card was found not to be touching the
apex. A small square was now refixed with shellac; after 16 h. slight
deflection from the perpendicular and from the card. After an
additional day the radicle became almost straight.
(25.) Square of card fixed laterally to apex of young radicle; after 9
h. deflection from the perpendicular considerable; after 24 h.
deflection reduced. Refixed a fresh square with shellac: after 24 h.
deflection about 40° from the perpendicular and from the card.
(26.) A very small square of card fixed with shellac to apex of young
radicle: after 9 h. the deflection from the perpendicular and from the
card amounted to nearly a right angle; after 24 h. deflection much
reduced; after an additional 24 h. radicle almost straight.
(27.) Square of card fixed with shellac to apex of young radicle: after
9 h. deflection from the card and from the perpendicular a right angle;
next morning quite straight. Refixed a square laterally with shellac;
after 9 h. a little deflection, which after 24 h. increased to nearly
20° from the perpendicular and from the card.
(28.) Square of card fixed with shellac; after 9 h. some deflection;
next morning the card dropped off; refixed it with shellac; it again
became loose and was refixed; and now on the third trial the radicle
was deflected after 14 h. at right angles from the card.
(29.) A small square of card was first fixed with thick gum-water to
the apex. It produced a slight effect but soon fell off. A similar
square was now affixed laterally with shellac: after 9 h. the radicle
was deflected nearly 45° from the perpendicular and from the card.
After 36 additional hours angle of deflection reduced to about 30°.
(30.) A very small piece, less than 1/20th of an inch square, of thin
tin-foil fixed with shellac to the apex of a young radicle; after 24 h.
no effect. Tin-foil removed, and a small square of sanded card fixed
with shellac; after 9 h. deflection at nearly right angles from the
perpendicular and from the card. Next
morning deflection reduced to about 40° from the perpendicular.
(31.) A splinter of thin glass gummed to apex, after 9 h. no effect,
but it was then found not to be touching the apex of the radicle. Next
morning a square of card was fixed with shellac to it, and after 9 h.
radicle greatly deflected from the card. After two additional days the
deflection had decreased and was only 35° from the perpendicular.
(32.) Small square of sanded card, attached with thick gum-water
laterally to the apex of a long straight radicle: after 9 h. greatly
deflected from the perpendicular and from the card. Curvature extended
for a length of .22 of an inch from the apex. After 3 additional hours
terminal portion deflected at right angles from the perpendicular. Next
morning the curved portion was .36 in length.
(33.) Square of card gummed to apex: after 15 h. deflected at nearly
90° from the perpendicular and from the card.
(34.) Small oblong of sanded card gummed to apex: after 15 h. deflected
90° from the perpendicular and from the card: in the course of the
three following days the terminal portion became much contorted and
ultimately coiled into a helix.
(35.) Square of card gummed to apex: after 9 h. deflected from card:
after 24 h. from time of attachment greatly deflected obliquely and
partly in opposition to Sachs’ curvature.
(36.) Small piece of card, rather less than 1/20th of an inch square,
gummed to apex: in 9 h. considerably deflected from card and in
opposition to Sachs’ curvature; after 24 h. greatly deflected in the
same direction. After an additional day the extreme tip was curved
towards the card.
(37.) Square of card, gummed to apex in front, caused after 8 h. 30 m.
hardly any effect; refixed fresh square laterally, after 15 h.
deflected almost 90° from the perpendicular and from the card. After 2
additional days deflection much reduced.
(38.) Square of card gummed to apex: after 9 h. much deflection, which
after 24 h. from time of fixing increased to nearly 90°. After an
additional day terminal portion was curled into a loop, and on the
following day into a helix.
(39.) Small oblong piece of card gummed to apex, nearly in front, but a
little to one side; in 9 h. slightly deflected in the direction of
Sachs’ curvature, but rather obliquely, and to side opposite to card.
Next day more curved in the same direction, and after 2 additional days
coiled into a ring.
(40.) Square of card gummed to apex: after 9 h. slightly curved from
card; next morning radicle straight, and apex had grown beyond the
card. Refixed another square laterally with shellac; in 9 h. deflected
laterally, but also in the direction of Sachs’ curvature. After 2
additional days’ curvature considerably increased in the same
direction.
(41.) Little square of tin-foil fixed with gum to one side of apex of a
young and short radicle: after 15 h. no effect, but tin-foil had become
displaced. A little square of card was now gummed to one side of apex,
which after 8 h. 40 m. was slightly deflected; in 24 h. from the time
of attachment deflected at 90° from the perpendicular and from the
card; after 9 additional hours became hooked, with the apex pointing to
the zenith. In 3 days from the time of attachment the terminal portion
of the radicle formed a ring or circle.
(42.) A little square of thick letter-paper gummed to the apex of a
radicle, which after 9 h. was deflected from it. In 24 h. from time
when the paper was affixed the deflection much increased, and after 2
additional days it amounted to 50° from the perpendicular and from the
paper.
(43.) A narrow chip of a quill was fixed with shellac to the apex of a
radicle. After 9 h. no effect; after 24 h. moderate deflection, but now
the quill had ceased to touch the apex. Removed quill and gummed a
little square of card to apex, which after 8 h. caused slight
deflection. On the fourth day from the first attachment of any object,
the extreme tip was curved towards the card.
(44.) A rather long and narrow splinter of extremely thin glass, fixed
with shellac to apex, it caused in 9 h. slight deflection, which
disappeared in 24 h.; the splinter was then found not touching the
apex. It was twice refixed, with nearly similar results, that is, it
caused slight deflection, which soon disappeared. On the fourth day
from the time of first attachment the tip was bent towards the
splinter.
From these experiments it is clear that the apex of the radicle of the
bean is sensitive to contact, and that it causes the upper part to bend
away from the touching object. But before giving a summary of the
results, it will be convenient briefly to give a few other
observations. Bits of very thin glass and little squares
of common card were affixed with thick gum-water to the tips of the
radicles of seven beans, as a preliminary trial. Six of these were
plainly acted on, and in two cases the radicles became coiled up into
complete loops. One radicle was curved into a semi-circle in so short a
period as 6 h. 10 m. The seventh radicle which was not affected was
apparently sickly, as it became brown on the following day; so that it
formed no real exception. Some of these trials were made in the early
spring during cold weather in a sitting-room, and others in a
greenhouse, but the temperature was not recorded. These six striking
cases almost convinced us that the apex was sensitive, but of course we
determined to make many more trials. As we had noticed that the
radicles grew much more quickly when subjected to considerable heat,
and as we imagined that heat would increase their sensitiveness,
vessels with germinating beans suspended in damp air were placed on a
chimney-piece, where they were subjected during the greater part of the
day to a temperature of between 69° and 72° F.; some, however, were
placed in the hot-house where the temperature was rather higher. Above
two dozen beans were thus tried; and when a square of glass or card did
not act, it was removed, and a fresh one affixed, this being often done
thrice to the same radicle. Therefore between five and six dozen trials
were altogether made. But there was moderately distinct deflection from
the perpendicular and from the attached object in only one radicle out
of this large number of cases. In five other cases there was very
slight and doubtful deflection. We were astonished at this result, and
concluded that we had made some inexplicable mistake in the first six
experiments. But before finally relinquishing the subject, we resolved
to make one
other trial for it occurred to us that sensitiveness is easily affected
by external conditions, and that radicles growing naturally in the
earth in the early spring would not be subjected to a temperature
nearly so high as 70° F. We therefore allowed the radicles of 12 beans
to grow at a temperature of between 55° and 60° F. The result was that
in every one of these cases (included in the above-described
experiments) the radicle was deflected in the course of a few hours
from the attached object. All the above recorded successful trials, and
some others presently to be given, were made in a sitting-room at the
temperatures just specified. It therefore appears that a temperature of
about, or rather above, 70° F. destroys the sensitiveness of the
radicles, either directly, or indirectly through abnormally accelerated
growth; and this curious fact probably explains why Sachs, who
expressly states that his beans were kept at a high temperature, failed
to detect the sensitiveness of the apex of the radicle.
But other causes interfere with this sensibility. Eighteen radicles
were tried with little squares of sanded card, some affixed with
shellac and some with gum-water, during the few last days of 1878, and
few first days of the next year. They were kept in a room at the proper
temperature during the day, but were probably too cold at night, as
there was a hard frost at the time. The radicles looked healthy but
grew very slowly. The result was that only 6 out of the 18 were
deflected from the attached cards, and this only to a slight degree and
at a very slow rate. These radicles therefore presented a striking
contrast with the 44 above described. On March 6th and 7th, when the
temperature of the room varied between 53° and 59° F., eleven
germinating beans were tried in the
same manner, and now every one of the radicles became curved away from
the cards, though one was only slightly deflected. Some horticulturists
believe that certain kinds of seeds will not germinate properly in the
middle of the winter, although kept at a right temperature. If there
really is any proper period for the germination of the bean, the feeble
degree of sensibility of the above radicles may have resulted from the
trial having been made in the middle of the winter, and not simply from
the nights being too cold. Lastly, the radicles of four beans, which
from some innate cause germinated later than all the others of the same
lot, and which grew slowly though appearing healthy, were similarly
tried, and even after 24 h. they were hardly at all deflected from the
attached cards. We may therefore infer that any cause which renders the
growth of the radicles either slower or more rapid than the normal
rate, lessens or annuls the sensibility of their tips to contact. It
deserves particular attention that when the attached objects failed to
act, there was no bending of any kind, excepting Sachs’ curvature. The
force of our evidence would have been greatly weakened if occasionally,
though rarely, the radicles had become curved in any direction
independently of the attached objects. In the foregoing numbered
paragraphs, however, it may be observed that the extreme tip sometimes
becomes, after a considerable interval of time, abruptly curved towards
the bit of card; but this is a totally distinct phenomenon, as will
presently be explained.
A Summary of the Results of the foregoing Experiments on the Radicles
of Vicia faba.—Altogether little squares (about 1/20th of an inch),
generally of sanded paper as stiff as thin card (between .15 and .20
mm. in thickness), sometimes of ordinary card, or little
fragments of very thin glass etc., were affixed at different times to
one side of the conical tips of 55 radicles. The 11 last-mentioned
cases, but not the preliminary ones, are here included. The squares,
etc., were most commonly affixed with shellac, but in 19 cases with
thick gum-water. When the latter was used, the squares were sometimes
found, as previously stated, to be separated from the apex by a layer
of thick fluid, so that there was no contact, and consequently no
bending of the radicle; and such few cases were not recorded. But in
every instance in which shellac was employed, unless the square fell
off very soon, the result was recorded. In several instances when the
squares became displaced, so as to stand parallel to the radicle, or
were separated by fluid from the apex, or soon fell off, fresh squares
were attached, and these cases (described under the numbered
paragraphs) are here included. Out of 55 radicles experimented on under
the proper temperature, 52 became bent, generally to a considerable
extent from the perpendicular, and away from the side to which the
object was attached. Of the three failures, one can be accounted for,
as the radicle became sickly on the following day; and a second was
observed only during 11 h. 30 m. As in several cases the terminal
growing part of the radicle continued for some time to bend from the
attached object, it formed itself into a hook, with the apex pointing
to the zenith, or even into a ring, and occasionally into a spire or
helix. It is remarkable that these latter cases occurred more
frequently when objects were attached with thick gum-water, which never
became dry, than when shellac was employed. The curvature was often
well-marked in from 7 h. to 11 h.; and in one instance a semicircle was
formed in 6 h. 10 m, from the time
of attachment. But in order to see the phenomenon as well displayed as
in the above described cases, it is indispensable that the bits of
card, etc., should be made to adhere closely to one side of the conical
apex; that healthy radicles should be selected and kept at not too high
or too low a temperature, and apparently that the trials should not be
made in the middle of the winter.
In ten instances, radicles which had curved away from a square of card
or other object attached to their tips, straightened themselves to a
certain extent, or even completely, in the course of from one to two
days from the time of attachment. This was more especially apt to occur
when the curvature was slight. But in one instance (No. 27) a radicle
which in 9 h. had been deflected about 90° from the perpendicular,
became quite straight in 24 h. from the period of attachment. With No.
26, the radicle was almost straight in 48 h. We at first attributed the
straightening process to the radicles becoming accustomed to a slight
stimulus, in the same manner as a tendril or sensitive petiole becomes
accustomed to a very light loop of thread, and unbends itself though
the loop remains still suspended; but Sachs states[2] that radicles of
the bean placed horizontally in damp air after curving downwards
through geotropism, straighten themselves a little by growth along
their lower or concave sides. Why this should occur is not clear: but
perhaps it likewise occurred in the above ten cases. There is another
occasional movement which must not be passed over: the tip of the
radicle, for a length of from 2 to 3 mm., was found in six instances,
after an interval of about 24 or more hours, bent towards the bit of
still attached card,—that is, in a direction exactly opposite to the
previously induced curvature of the whole growing part for a length of
from 7 to 8 mm. This occurred chiefly when the first curvature was
small, and when an object had been affixed more than once to the apex
of the same radicle. The attachment of a bit of card by shellac to one
side of the tender apex may sometimes mechanically prevent its growth;
or the application of thick gum-water more than once to the same side
may injure it; and then checked growth on this side with continued
growth on the opposite and unaffected side would account for the
reversed curvature of the apex.
[2] ‘Arbeiten Bot. Instit., Würzburg,’ Heft iii. p. 456.
Various trials were made for ascertaining, as far as we could, the
nature and degree of irritation to which the apex must be subjected, in
order that the terminal growing part should bend away, as if to avoid
the cause of irritation. We have seen in the numbered experiments, that
a little square of rather thick letter-paper gummed to the apex
induced, though slowly, considerable deflection. Judging from several
cases in which various objects had been affixed with gum, and had soon
become separated from the apex by a layer of fluid, as well as from
some trials in which drops of thick gum-water alone had been applied,
this fluid never causes bending. We have also seen in the numbered
experiments that narrow splinters of quill and of very thin glass,
affixed with shellac, caused only a slight degree of deflection, and
this may perhaps have been due to the shellac itself. Little squares of
goldbeaters’ skin, which is excessively thin, were damped, and thus
made to adhere to one side of the tips of two radicles; one of these,
after 24 h., produced no effect; nor did the
other in 8 h., within which time squares of card usually act; but after
24 h. there was slight deflection.
An oval bead, or rather cake, of dried shellac, 1.01 mm. in length and
0.63 in breadth, caused a radicle to become deflected at nearly right
angles in the course of only 6 h.; but after 23 h. it had nearly
straightened itself. A very small quantity of dissolved shellac was
spread over a bit of card, and the tips of 9 radicles were touched
laterally with it; only two of them became slightly deflected to the
side opposite to that bearing the speck of dried shellac, and they
afterwards straightened themselves. These specks were removed, and both
together weighed less than 1/100th of a grain; so that a weight of
rather less than 1/200th of a grain (0.32 mg.) sufficed to excite
movement in two out of the nine radicles. Here then we have apparently
reached nearly the minimum weight which will act.
A moderately thick bristle (which on measurement was found rather
flattened, being 0.33 mm. in one diameter, and 0.20 mm. in the other)
was cut into lengths of about 1/20th of an inch. These after being
touched with thick gum-water, were placed on the tips of eleven
radicles. Three of them were affected; one being deflected in 8 h. 15
m. to an angle of about 90° from the perpendicular; a second to the
same amount when looked at after 9 h.; but after 24 h. from the time of
first attachment the deflection had decreased to only 19°; the third
was only slightly deflected after 9 h., and the bit of bristle was then
found not touching the apex; it was replaced, and after 15 additional
hours the deflection amounted to 26° from the perpendicular. The
remaining eight radicles were not at all acted on by the bits of
bristle, so that we here appear to have nearly reached the minimum
of size of an object which will act on the radicle of the bean. But it
is remarkable that when the bits of bristle did act, that they should
have acted so quickly and efficiently.
As the apex of a radicle in penetrating the ground must be pressed on
all sides, we wished to learn whether it could distinguish between
harder or more resisting, and softer substances. A square of the sanded
paper, almost as stiff as card, and a square of extremely thin paper
(too thin for writing on), of exactly the same size (about 1/20th of an
inch), were fixed with shellac on opposite sides of the apices of 12
suspended radicles. The sanded card was between 0.15 and 0.20 mm. (or
between 0.0059 and 0.0079 of an inch), and the thin paper only 0.045
mm. (or 0.00176 of an inch) in thickness. In 8 out of the 12 cases
there could be no doubt that the radicle was deflected from the side to
which the card-like paper was attached, and towards the opposite side,
bearing the very thin paper. This occurred in some instances in 9 h.,
but in others not until 24 h. had elapsed. Moreover, some of the four
failures can hardly be considered as really failures: thus, in one of
them, in which the radicle remained quite straight, the square of thin
paper was found, when both were removed from the apex, to have been so
thickly coated with shellac that it was almost as stiff as the card: in
the second case, the radicle was bent upwards into a semicircle, but
the deflection was not directly from the side bearing the card, and
this was explained by the two squares having become cemented laterally
together, forming a sort of stiff gable, from which the radicle was
deflected: in the third case, the square of card had been fixed by
mistake in front, and though there was deflection from it, this might
have been due to Sachs’ curvature:
in the fourth case alone no reason could be assigned why the radicle
had not been at all deflected. These experiments suffice to prove that
the apex of the radicle possesses the extraordinary power of
discriminating between thin card and very thin paper, and is deflected
from the side pressed by the more resisting or harder substance.
Some trials were next made by irritating the tips without any object
being left in contact with them. Nine radicles, suspended over water,
had their tips rubbed, each six times with a needle, with sufficient
force to shake the whole bean; the temperature was favourable, viz.
about 63° F. In 7 out of these cases no effect whatever was produced;
in the eighth case the radicle became slightly deflected from, and in
the ninth case slightly deflected towards, the rubbed side; but these
two latter opposed curvatures were probably accidental, as radicles do
not always grow perfectly straight downwards. The tips of two other
radicles were rubbed in the same manner for 15 seconds with a little
round twig, two others for 30 seconds, and two others for 1 minute, but
without any effect being produced. We may therefore conclude from these
15 trials that the radicles are not sensitive to temporary contact, but
are acted on only by prolonged, though very slight, pressure.
We then tried the effects of cutting off a very thin slice parallel to
one of the sloping sides of the apex, as we thought that the wound
would cause prolonged irritation, which might induce bending towards
the opposite side, as in the case of an attached object. Two
preliminary trials were made: firstly, slices were cut from the
radicles of 6 beans suspended in damp air, with a pair of scissors,
which, though sharp, probably caused considerable crushing, and no
curvature followed. Secondly, thin slices were cut with a razor
obliquely off the tips of three radicles similarly suspended; and after
44 h. two were found plainly bent from the sliced surface; and the
third, the whole apex of which had been cut off obliquely by accident,
was curled upwards over the bean, but it was not clearly ascertained
whether the curvature had been at first directed from the cut surface.
These results led us to pursue the experiment, and 18 radicles, which
had grown vertically downwards in damp air, had one side of their
conical tips sliced off with a razor. The tips were allowed just to
enter the water in the jars, and they were exposed to a temperature
14°–16° C. (57°–61° F.). The observations were made at different times.
Three were examined 12 h. after being sliced, and were all slightly
curved from the cut surface; and the curvature increased considerably
after an additional 12 h. Eight were examined after 19 h.; four after
22 h. 30 m.; and three after 25 h. The final result was that out of the
18 radicles thus tried, 13 were plainly bent from the cut surface after
the above intervals of time; and one other became so after an
additional interval of 13 h. 30 m. So that only 4 out of the 18
radicles were not acted on. To these 18 cases the 3 previously
mentioned ones should be added. It may, therefore, be concluded that a
thin slice removed by a razor from one side of the conical apex of the
radicle causes irritation, like that from an attached object, and
induces curvature from the injured surface.
Lastly, dry caustic (nitrate of silver) was employed to irritate one
side of the apex. If one side of the apex or of the whole terminal
growing part of a radicle, is by any means killed or badly injured, the
other side continues to grow; and this causes the part
to bend over towards the injured side.[3] But in the following
experiments we endeavoured, generally with success, to irritate the
tips on one side, without badly injuring them. This was effected by
first drying the tip as far as possible with blotting-paper, though it
still remained somewhat damp, and then touching it once with quite dry
caustic. Seventeen radicles were thus treated, and were suspended in
moist air over water at a temperature of 58° F. They were examined
after an interval of 21 h. or 24h. The tips of two were found blackened
equally all round, so that they could tell nothing and were rejected,
15 being left. Of these, 10 were curved from the side which had been
touched, where there was a minute brown or blackish mark. Five of these
radicles, three of which were already slightly deflected, were allowed
to enter the water in the jar, and were re-examined after an additional
interval of 27 h. (i.e. in 48 h. after the application of the caustic),
and now four of them had become hooked, being bent from the discoloured
side, with their points directed to the zenith; the fifth remained
unaffected and straight. Thus 11 radicles out of the 15 were acted on.
But the curvature of the four just described was so plain, that they
alone would have sufficed to show that the radicles of the bean bend
away from that side of the apex which has been slightly irritated by
caustic.
[3] Ciesielski found this to be the case (‘Untersuchungen über die
Abwartskrümmung der Wurzel,’ 1871, p. 28) after burning with heated
platinum one side of a radicle. So did we when we painted
longitudinally half of the whole length of 7 radicles, suspended over
water, with a thick layer of grease, which is very injurious or even
fatal to growing parts; for after 48 hours five of these radicles were
curved towards the greased side, two remaining straight.
_The Power of an Irritant on the apex of the Radicle_
_of the Bean, compared with that of Geotropism_.—We know that when a
little square of card or other object is fixed to one side of the tip
of a vertically dependent radicle, the growing part bends from it often
into a semicircle, in opposition to geotropism, which force is
conquered by the effect of the irritation from the attached object.
Radicles were therefore extended horizontally in damp air, kept at the
proper low temperature for full sensitiveness, and squares of card were
affixed with shellac on the lower sides of their tips, so that if the
squares acted, the terminal growing part would curve upwards. Firstly,
eight beans were so placed that their short, young, horizontally
extended radicles would be simultaneously acted on both by geotropism
and by Sachs’ curvature, if the latter came into play; and they all
eight became bowed downwards to the centre of the earth in 20 h.,
excepting one which was only slightly acted on. Two of them were a
little bowed downwards in only 5 h.! Therefore the cards, affixed to
the lower sides of their tips, seemed to produce no effect; and
geotropism easily conquered the effects of the irritation thus caused.
Secondly, 5 oldish radicles, 1½ inch in length, and therefore less
sensitive than the above-mentioned young ones, were similarly placed
and similarly treated. From what has been seen on many other occasions,
it may be safely inferred that if they had been suspended vertically
they would have bent away from the cards; and if they had been extended
horizontally, without cards attached to them, they would have quickly
bent vertically downwards through geotropism; but the result was that
two of these radicles were still horizontal after 23 h.; two were
curved only slightly, and the fifth as much as 40° beneath the horizon.
Thirdly, 5 beans were fastened
with their flat surfaces parallel to the cork-lid, so that Sachs’
curvature would not tend to make the horizontally extended radicles
turn either upwards or downwards, and little squares of card were
affixed as before, to the lower sides of their tips. The result was
that all five radicles were bent down, or towards the centre of the
earth, after only 8 h. 20 m. At the same time and within the same jars,
3 radicles of the same age, with squares affixed to one side, were
suspended vertically; and after 8 h. 20 m. they were considerably
deflected from the cards, and therefore curved upwards in opposition to
geotropism. In these latter cases the irritation from the squares had
over-powered geotropism; whilst in the former cases, in which the
radicles were extended horizontally, geotropism had overpowered the
irritation. Thus within the same jars, some of the radicles were
curving upwards and others downwards at the same time—these opposite
movements depending on whether the radicles, when the squares were
first attached to them, projected vertically down, or were extended
horizontally. This difference in their behaviour seems at first
inexplicable, but can, we believe, be simply explained by the
difference between the initial power of the two forces under the above
circumstances, combined with the well-known principle of the
after-effects of a stimulus. When a young and sensitive radicle is
extended horizontally, with a square attached to the lower side of the
tip, geotropism acts on it at right angles, and, as we have seen, is
then evidently more efficient than the irritation from the square; and
the power of geotropism will be strengthened at each successive period
by its previous action—that is, by its after-effects. On the other
hand, when a square is affixed to a vertically dependent radicle, and
the apex begins to
curve upwards, this movement will be opposed by geotropism acting only
at a very oblique angle, and the irritation from the card will be
strengthened by its previous action. We may therefore conclude that the
initial power of an irritant on the apex of the radicle of the bean, is
less than that of geotropism when acting at right angles, but greater
than that of geotropism when acting obliquely on it.
Sensitiveness of the tips of the Secondary Radicles of the Bean to
contact.—All the previous observations relate to the main or primary
radicle. Some beans suspended to cork-lids, with their radicles dipping
into water, had developed secondary or lateral radicles, which were
afterwards kept in very damp air, at the proper low temperature for
full sensitiveness. They projected, as usual, almost horizontally, with
only a slight downward curvature, and retained this position during
several days. Sachs has shown[4] that these secondary roots are acted
on in a peculiar manner by geotropism, so that if displaced they
reassume their former sub-horizontal position, and do not bend
vertically downwards like the primary radicle. Minute squares of the
stiff sanded paper were affixed by means of shellac (but in some
instances with thick gum-water) to the tips of 39 secondary radicles of
different ages, generally the uppermost ones. Most of the squares were
fixed to the lower sides of the apex, so that if they acted the radicle
would bend upwards; but some were fixed laterally, and a few on the
upper side. Owing to the extreme tenuity of these radicles, it was very
difficult to attach the square to the actual apex. Whether owing to
this or some other circumstance, only nine of the squares induced any
curvature. The curvature amounted in some cases to about 45° above the
horizon, in others to 90°, and then the tip pointed to the zenith. In
one instance a distinct upward curvature was observed in 8 h. 15 m.,
but usually not until 24 h. had elapsed. Although only 9 out of 39
radicles were affected, yet the curvature was so distinct in several of
them, that there could be no doubt that the tip is sensitive to slight
contact, and that the growing part bends away from the touching object.
It is possible that some secondary radicles are more sensitive than
others; for Sachs has proved[5] the interesting fact that each
individual secondary radicle possesses its own peculiar constitution.
[4] ‘Arbeiten Bot. Inst., Würzburg,’ Heft iv. 1874, p. 605–617.
[5] ‘Arbeiten Bot. Instit., Würzburg,’ Heft, iv. 1874, p. 620.
Sensitiveness to contact of the Primary Radicle, a little above the
apex, in the Bean (Vicia faba) and Pea (Pisum sativum).—The
sensitiveness of the apex of the radicle in the previously described
cases, and the consequent curvature of the upper part from the touching
object or other source of irritation, is the more remarkable, because
Sachs[6] has shown that pressure at the distance of a few millimeters
above the apex causes the radicle to bend, like a tendril, towards the
touching object. By fixing pins so that they pressed against the
radicles of beans suspended vertically in damp air, we saw this kind of
curvature; but rubbing the part with a twig or needle for a few minutes
produced no effect. Haberlandt remarks,[7] that these radicles in
breaking through the seed-coats often rub and press against the
ruptured edges, and consequently bend round them. As little squares of
the card-like paper affixed with shellac to the tips were highly
efficient in causing the radicles to bend away from them, similar
pieces (of about 1/20th
inch square, or rather less) were attached in the same manner to one
side of the radicle at a distance of 3 or 4 mm. above the apex. In our
first trial on 15 radicles no effect was produced. In a second trial on
the same number, three became abruptly curved (but only one strongly)
towards the card within 24 h. From these cases we may infer that the
pressure from a bit of card affixed with shellac to one side above the
apex, is hardly a sufficient irritant; but that it occasionally causes
the radicle to bend like a tendril towards this side.
[6] Ibid. Heft iii. 1873, p. 437.
[7] ‘Die Schutzeinrichtungen der Keimpflanze,’ 1877, p. 25.
We next tried the effect of rubbing several radicles at a distance of 4
mm. from the apex for a few seconds with lunar caustic (nitrate of
silver); and although the radicles had been wiped dry and the stick of
caustic was dry, yet the part rubbed was much injured and a slight
permanent depression was left. In such cases the opposite side
continues to grow, and the radicle necessarily becomes bent towards the
injured side. But when a point 4 mm. from the apex was momentarily
touched with dry caustic, it was only faintly discoloured, and no
permanent injury was caused. This was shown by several radicles thus
treated straightening themselves after one or two days; yet at first
they became curved towards the touched side, as if they had been there
subjected to slight continued pressure. These cases deserve notice,
because when one side of the apex was just touched with caustic, the
radicle, as we have seen, curved itself in an opposite direction, that
is, away from the touched side.
The radicle of the common pea at a point a little above the apex is
rather more sensitive to continued pressure than that of the bean, and
bends towards the pressed side.[8] We experimented on a variety
(_Yorkshire Hero_) which has a much wrinkled tough skin, too large for
the included cotyledons; so that out of 30 peas which had been soaked
for 24 h. and allowed to germinate on damp sand, the radicles of three
were unable to escape, and were crumpled up in a strange manner within
the skin; four other radicles were abruptly bent round the edges of the
ruptured skin against which they had pressed. Such abnormalities would
probably never, or very rarely, occur with forms developed in a state
of nature and subjected to natural selection. One of the four radicles
just mentioned in doubling backwards came into contact with the pin by
which the pea was fixed to the cork-lid; and now it bent at right
angles round the pin, in a direction quite different from that of the
first curvature due to contact with the ruptured skin; and it thus
afforded a good illustration of the tendril-like sensitiveness of the
radicle a little above the apex.
[8] Sachs, ‘Arbeiten Bot. Institut., Würzburg,’ Heft iii. p. 438.
Little squares of the card-like paper were next affixed to radicles of
the pea at 4 mm. above the apex, in the same manner as with the bean.
Twenty-eight radicles suspended vertically over water were thus treated
on different occasions, and 13 of them became curved towards the cards.
The greatest degree of curvature amounted to 62° from the
perpendicular; but so large an angle was only once formed. On one
occasion a slight curvature was perceptible after 5 h. 45 m., and it
was generally well-marked after 14 h. There can therefore be no doubt
that with the pea, irritation from a bit of card attached to one side
of the radicle above the apex suffices to induce curvature.
Squares of card were attached to one side of the tips of 11 radicles
within the same jars in which the above trials were made, and five of
them became plainly, and one slightly, curved away from this side.
Other
analogous cases will be immediately described. The fact is here
mentioned because it was a striking spectacle, showing the difference
in the sensitiveness of the radicle in different parts, to behold in
the same jar one set of radicles curved away from the squares on their
tips, and another set curved towards the squares attached a little
higher up. Moreover, the kind of curvature in the two cases is
different. The squares attached above the apex cause the radicle to
bend abruptly, the part above and beneath remaining nearly straight; so
that here there is little or no transmitted effect. On the other hand,
the squares attached to the apex affect the radicle for a length of
from about 4 to even 8 mm., inducing in most cases a symmetrical
curvature; so that here some influence is transmitted from the apex for
this distance along the radicle.
Pisum sativum (var. Yorkshire Hero): Sensitiveness of the apex of the
Radicle.—Little squares of the same card-like paper were affixed (April
24th) with shellac to one side of the apex of 10 vertically suspended
radicles: the temperature of the water in the bottom of the jars was
60°–61° F. Most of these radicles were acted on in 8 h. 30 m.; and
eight of them became in the course of 24 h. conspicuously, and the
remaining two slightly, deflected from the perpendicular and from the
side bearing the attached squares. Thus all were acted on; but it will
suffice to describe two conspicuous cases. In one the terminal portion
of the radicle was bent at right angles (A, Fig. 66) after 24h.; and in
the other (B) it had by this time become hooked, with the apex pointing
to the zenith. The two bits of card here used were .07 inch in length
and .04 inch in breadth. Two other radicles, which after 8 h. 30 m.
were moderately deflected, became straight again after 24h. Another
trial was made in the same manner with 15 radicles; but from
circumstances, not worth explaining, they were only once and briefly
examined after the short interval of 5 h. 30 m.; and we merely record
in our notes “almost all bent slightly from the perpendicular, and away
from the squares; the deflection amounting in one or two instances to
nearly a rectangle.” These two sets of cases, especially the first one,
prove that the apex of the radicle is sensitive to slight contact and
that the upper part bends from the touching object. Nevertheless, on
June 1st and 4th, 8 other radicles were tried in the same manner at a
temperature of 58°–60° F., and after 24 h. only 1 was decidedly bent
from the card, 4 slightly, 2 doubtfully, and 1 not in the least. The
amount of curvature was unaccountably small; but all the radicles which
were at all bent, were bent away from the cards.
Fig. 66. Pisum sativum: deflection produced within 24 hours in the
growth of vertically dependent radicles, by little squares of card
affixed with shellac to one side of apex: A, bent at right angles; B,
hooked.
We now tried the effects of widely different temperatures on the
sensitiveness of these radicles with squares
of card attached to their tips. Firstly, 13 peas, most of them having
very short and young radicles, were placed in an ice-box, in which the
temperature rose during three days from 44° to 47° F. They grew slowly,
but 10 out of the 13 became in the course of the three days very
slightly curved from the squares; the other 3 were not affected; so
that this temperature was too low for any high degree of sensitiveness
or for much movement. Jars with 13 other radicles were next placed on a
chimney-piece, where they were subjected to a temperature of between
68° and 72° F., and after 24 h., 4 were conspicuously curved from the
cards, 2 slightly, and 7 not at all; so that this temperature was
rather too high. Lastly 12 radicles were subjected to a temperature
varying between 72° and 85° F., and none of them were in the least
affected by the squares. The above several trials, especially the first
recorded one, indicate that the most favourable temperature for the
sensitiveness of the radicle of the pea is about 60° F.
The tips of 6 vertically dependent radicles were touched once with dry
caustic, in the manner described under Vicia faba. After 24 h. four of
them were bent from the side bearing a minute black mark; and the
curvature increased in one case after 38 h., and in another case after
48 h., until the terminal part projected almost horizontally. The two
remaining radicles were not affected.
With radicles of the bean, when extended horizontally in damp air,
geotropism always conquered the effects of the irritation caused by
squares of card attached to the lower sides of their tips. A similar
experiment was tried on 13 radicles of the pea; the squares being
attached with shellac, and the temperature between 58°–60° F. The
result was somewhat different; for
these radicles are either less strongly acted on by geotropism, or,
what is more probable, are more sensitive to contact. After a time
geotropism always prevailed, but its action was often delayed; and in
three instances there was a most curious struggle between geotropism
and the irritation caused by the cards. Four of the 13 radicles were a
little curved downwards within 6 or 8 h., always reckoning from the
time when the squares were first attached, and after 23 h. three of
them pointed vertically downwards, and the fourth at an angle of 45°
beneath the horizon. These four radicles therefore did not seem to have
been at all affected by the attached squares. Four others were not
acted on by geotropism within the first 6 or 8 h., but after 23 h. were
much bowed down. Two others remained almost horizontal for 23 h., but
afterwards were acted on. So that in these latter six cases the action
of geotropism was much delayed. The eleventh radicle was slightly
curved down after 8 h., but when looked at again after 23 h. the
terminal portion was curved upwards; if it had
been longer observed, the tip no doubt would have been found again
curved down, and it would have formed a loop as in the following case.
The twelfth radicle after 6 h. was slightly curved downwards; but when
looked at again after 21 h., this curvature had disappeared and the
apex pointed upwards; after 30 h. the radicle formed a hook, as shown
at A (Fig. 67); which hook after 45 h. was converted into a loop (B).
The thirteenth radicle after 6 h. was slightly curved downwards, but
within 21 h. had curved considerably up, and then down again at an
angle of 45° beneath the horizon, afterwards becoming perpendicular. In
these three last cases geotropism and the irritation caused by the
attached squares alternately prevailed in a highly remarkable manner;
geotropism being ultimately victorious.
Fig. 67. Pisum sativum: a radicle extended horizontally in damp air
with a little square of card affixed to the lower side of its tip,
causing it to bend upwards in opposition to geotropism. The deflection
of the radicle after 21 hours is shown at A, and of the same radicle
after 45 hours at B, now forming a loop.
Similar experiments were not always quite so successful as in the above
cases. Thus 6 radicles, horizontally extended with attached squares,
were tried on June 8th at a proper temperature, and after 7 h. 30 m.
none were in the least curved upwards and none were distinctly
geotropic; whereas of 6 radicles without any attached squares, which
served as standards of comparison or controls, 3 became slightly and 3
almost rectangularly geotropic within the 7 h. 30 m.; but after 23 h.
the two lots were equally geotropic. On July 10th another trial was
made with 6 horizontally extended radicles, with squares attached in
the same manner beneath their tips; and after 7 h. 30 m., 4 were
slightly geotropic, 1 remained horizontal, and 1 was curved upwards in
opposition to gravity or geotropism. This latter radicle after 48 h.
formed a loop, like that at B (Fig. 67).
An analogous trial was now made, but instead of attaching squares of
card to the lower sides of the
tips, these were touched with dry caustic. The details of the
experiment will be given in the chapter on Geotropism, and it will
suffice here to say that 10 peas, with radicles extended horizontally
and not cauterised, were laid on and under damp friable peat; these,
which served as standards or controls, as well as 10 others which had
been touched on the upper side with the caustic, all became strongly
geotropic in 24 h. Nine radicles, similarly placed, had their tips
touched on the lower side with the caustic; and after 24 h., 3 were
slightly geotropic, 2 remained horizontal, and 4 were bowed upwards in
opposition to gravity and to geotropism. This upward curvature was
distinctly visible in 8 h. 45m. after the lower sides of the tips had
been cauterised.
Little squares of card were affixed with shellac on two occasions to
the tips of 22 young and short secondary radicles, which had been
emitted from the primary radicle whilst growing in water, but were now
suspended in damp air. Besides the difficulty of attaching the squares
to such finely pointed objects as were these radicles, the temperature
was too high,—varying on the first occasion from 72° to 77° F., and on
the second being almost steadily 78° F.; and this probably lessened the
sensitiveness of the tips. The result was that after an interval of 8
h. 30 m., 6 of the 22 radicles were bowed upwards (one of them greatly)
in opposition to gravity, and 2 laterally; the remaining 14 were not
affected. Considering the unfavourable circumstances, and bearing in
mind the case of the bean, the evidence appears sufficient to show that
the tips of the secondary radicles of the pea are sensitive to slight
contact.
Phaseolus multiflorus: Sensitiveness of the apex of the
Radicle.—Fifty-nine radicles were tried with squares
of various sizes of the same card-like paper, also with bits of thin
glass and rough cinders, affixed with shellac to one side of the apex.
Rather large drops of the dissolved shellac were also placed on them
and allowed to set into hard beads. The specimens were subjected to
various temperatures between 60° and 72° F., more commonly at about the
latter. But out of this considerable number of trials only 5 radicles
were plainly bent, and 8 others slightly or even doubtfully, from the
attached objects; the remaining 46 not being at all affected. It is
therefore clear that the tips of the radicles of this Phaseolus are
much less sensitive to contact than are those of the bean or pea. We
thought that they might be sensitive to harder pressure, but after
several trials we could not devise any method for pressing harder on
one side of the apex than on the other, without at the same time
offering mechanical resistance to its growth. We therefore tried other
irritants.