or just rubbed on one side with dry nitrate of silver. They were rubbed
thrice, because we supposed from the foregoing trials, that the tips
were not highly sensitive. After 24 h. the tips were found greatly
blackened; 6 were blackened equally all round, so that no curvature to
any one side could be expected; 6 were much blackened on one side for a
length of about 1/10th of an inch, and this length became curved at
right angles towards the blackened surface, the curvature afterwards
increasing in several instances until little hooks were formed. It was
manifest that the blackened side was so much injured that it could not
grow, whilst the opposite side continued to grow. One alone out of
these 13 radicles became curved from the blackened side, the
curvature extending for some little distance above the apex.
After the experience thus gained, the tips of six almost dry radicles
were once touched with the dry caustic on one side; and after an
interval of 10 m. were allowed to enter water, which was kept at a
temperature of 65°–67° F. The result was that after an interval of 8 h.
a minute blackish speck could just be distinguished on one side of the
apex of five of these radicles, all of which became curved towards the
opposite side—in two cases at about an angle of 45°—in two other cases
at nearly a rectangle—and in the fifth case at above a rectangle, so
that the apex was a little hooked; in this latter case the black mark
was rather larger than in the others. After 24 h. from the application
of the caustic, the curvature of three of these radicles (including the
hooked one) had diminished; in the fourth it remained the same, and in
the fifth it had increased, the tip being now hooked. It has been said
that after 8 h. black specks could be seen on one side of the apex of
five of the six radicles; on the sixth the speck, which was extremely
minute, was on the actual apex and therefore central; and this radicle
alone did not become curved. It was therefore again touched on one side
with caustic, and after 15 h. 30 m. was found curved from the
perpendicular and from the blackened side at an angle of 34°, which
increased in nine additional hours to 54°.
It is therefore certain that the apex of the radicle of this Phaseolus
is extremely sensitive to caustic, more so than that of the bean,
though the latter is far more sensitive to pressure. In the experiments
just given, the curvature from the slightly cauterised side of the tip,
extended along the radicle for a length of nearly 10 mm.; whereas in
the first set
of experiments, when the tips of several were greatly blackened and
injured on one side, so that their growth was arrested, a length of
less than 3 mm. became curved towards the much blackened side, owing to
the continued growth of the opposite side. This difference in the
results is interesting, for it shows that too strong an irritant does
not induce any transmitted effect, and does not cause the adjoining,
upper and growing part of the radicle to bend. We have analogous cases
with Drosera, for a strong solution of carbonate of ammonia when
absorbed by the glands, or too great heat suddenly applied to them, or
crushing them, does not cause the basal part of the tentacles to bend,
whilst a weak solution of the carbonate, or a moderate heat, or slight
pressure always induced such bending. Similar results were observed
with Dionaea and Pinguicula.
The effect of cutting off with a razor a thin slice from one side of
the conical apex of 14 young and short radicles was next tried. Six of
them after being operated on were suspended in damp air; the tips of
the other eight, similarly suspended, were allowed to enter water at a
temperature of about 65° F. It was recorded in each case which side of
the apex had been sliced off, and when they were afterwards examined
the direction of the curvature was noted, before the record was
consulted. Of the six radicles in damp air, three had their tips curved
after an interval of 10 h. 15 m. directly away from the sliced surface,
whilst the other three were not affected and remained straight;
nevertheless, one of them after 13 additional hours became slightly
curved from the sliced surface. Of the eight radicles with their tips
immersed in water, seven were plainly curved away from the sliced
surfaces after 10 h. 15 m.; and with
respect to the eighth which remained quite straight, too thick a slice
had been accidentally removed, so that it hardly formed a real
exception to the general result. When the seven radicles were looked at
again, after an interval of 23 h. from the time of slicing, two had
become distorted; four were deflected at an angle of about 70° from the
perpendicular and from the cut surface; and one was deflected at nearly
90°, so that it projected almost horizontally, but with the extreme tip
now beginning to bend downwards through the action of geotropism. It is
therefore manifest that a thin slice cut off one side of the conical
apex, causes the upper growing part of the radicle of this Phaseolus to
bend, through the transmitted effects of the irritation, away from the
sliced surface.
Tropaeolum majus: Sensitiveness of the apex of the Radicle to
contact.—Little squares of card were attached with shellac to one side
of the tips of 19 radicles, some of which were subjected to 78° F., and
others to a much lower temperature. Only 3 became plainly curved from
the squares, 5 slightly, 4 doubtfully, and 7 not at all. These seeds
were, as we believed, old, so we procured a fresh lot, and now the
results were widely different. Twenty-three were tried in the same
manner; five of the squares produced no effect, but three of these
cases were no real exceptions, for in two of them the squares had
slipped and were parallel to the apex, and in the third the shellac was
in excess and had spread equally all round the apex. One radicle was
deflected only slightly from the perpendicular and from the card;
whilst seventeen were plainly deflected. The angles in several of these
latter cases varied between 40° and 65° from the perpendicular; and in
two of them it amounted after 15 h. or 16 h. to about 90°. In one
instance a loop
was nearly completed in 16 h. There can, therefore, be no doubt that
the apex is highly sensitive to slight contact, and that the upper part
of the radicle bends away from the touching object.
Gossypium herbaceum: Sensitiveness of the apex of the Radicle.—Radicles
were experimented on in the same manner as before, but they proved
ill-fitted for our purpose, as they soon became unhealthy when
suspended in damp air. Of 38 radicles thus suspended, at temperatures
varying from 66° to 69° F., with squares of card attached to their
tips, 9 were plainly and 7 slightly or even doubtfully deflected from
the squares and from the perpendicular; 22 not being affected. We
thought that perhaps the above temperature was not high enough, so 19
radicles with attached squares, likewise suspended in damp air, were
subjected to a temperature of from 74° to 79° F., but not one of them
was acted on, and they soon became unhealthy. Lastly, 19 radicles were
suspended in water at a temperature from 70° to 75° F., with bits of
glass or squares of the card attached to their tips by means of
Canada-balsam or asphalte, which adhered rather better than shellac
beneath the water. The radicles did not keep healthy for long. The
result was that 6 were plainly and 2 doubtfully deflected from the
attached objects and the perpendicular; 11 not being affected. The
evidence consequently is hardly conclusive, though from the two sets of
cases tried under a moderate temperature, it is probable that the
radicles are sensitive to contact; and would be more so under
favourable conditions.
Fifteen radicles which had germinated in friable peat were suspended
vertically over water. Seven of them served as controls, and they
remained quite straight during 24 h. The tips of the other eight
radicles
were just touched with dry caustic on one side. After only 5 h. 10 m.
five of them were slightly curved from the perpendicular and from the
side bearing the little blackish marks. After 8 h. 40 m., 4 out of
these 5 were deflected at angles between 15° and 65° from the
perpendicular. On the other hand, one which had been slightly curved
after 5 h. 10 m., now became straight. After 24 h. the curvature in two
cases had considerably increased; also in four other cases, but these
latter radicles had now become so contorted, some being turned upwards,
that it could no longer be ascertained whether they were still curved
from the cauterised side. The control specimens exhibited no such
irregular growth, and the two sets presented a striking contrast. Out
of the 8 radicles which had been touched with caustic, two alone were
not affected, and the marks left on their tips by the caustic were
extremely minute. These marks in all cases were oval or elongated; they
were measured in three instances, and found to be of nearly the same
size, viz. 2/3 of a mm. in length. Bearing this fact in mind, it should
be observed that the length of the curved part of the radicle, which
had become deflected from the cauterised side in the course of 8 h. 40
m. was found to be in three cases 6, 7, and 9 mm.
Cucurbita ovifera: Sensitiveness of the apex of the Radicle.—The tips
proved ill-fitted for the attachment of cards, as they are extremely
fine and flexible. Moreover, owing to the hypocotyls being soon
developed and becoming arched, the whole radicle is quickly displaced
and confusion is thus caused. A large number of trials were made, but
without any definite result, excepting on two occasions, when out of 23
radicles 10 were deflected from the attached squares
of card, and 13 were not acted on. Rather large squares, though
difficult to affix, seemed more efficient than very small ones.
We were much more successful with caustic; but in our first trial, 15
radicles were too much cauterised, and only two became curved from the
blackened side; the others being either killed on one side, or
blackened equally all round. In our next trial the dried tips of 11
radicles were touched momentarily with dry caustic, and after a few
minutes were immersed in water. The elongated marks thus caused were
never black, only brown, and about ½ mm. in length, or even less. In 4
h. 30 m. after the cauterisation, 6 of them were plainly curved from
the side with the brown mark, 4 slightly, and 1 not at all. The latter
proved unhealthy, and never grew; and the marks on 2 of the 4 slightly
curved radicles were excessively minute, one being distinguishable only
with the aid of a lens. Of 10 control specimens tried in the same jars
at the same time, not one was in the least curved. In 8 h. 40 m. after
the cauterisation, 5 of the radicles out of the 10 (the one unhealthy
one being omitted) were deflected at about 90°, and 3 at about 45° from
the perpendicular and from the side bearing the brown mark. After 24 h.
all 10 radicles had increased immensely in length; in 5 of them the
curvature was nearly the same, in 2 it had increased, and in 3 it had
decreased. The contrast presented by the 10 controls, after both the 8
h. 40 m. and the 24 h. intervals, was very great; for they had
continued to grow vertically downwards, excepting two which, from some
unknown cause, had become somewhat tortuous.
In the chapter on Geotropism we shall see that 10 radicles of this
plant were extended horizontally on and beneath damp friable peat,
under which conditions
they grow better and more naturally than in damp air; and their tips
were slightly cauterised on the lower side, brown marks about ½ mm. in
length being thus caused. Uncauterised specimens similarly placed
became much bent downwards through geotropism in the course of 5 or 6
hours. After 8 h. only 3 of the cauterised ones were bowed downwards,
and this in a slight degree; 4 remained horizontal; and 3 were curved
upwards in opposition to geotropism and from the side bearing the brown
mark. Ten other specimens had their tips cauterised at the same time
and in the same degree, on the upper side; and this, if it produced any
effect, would tend to increase the power of geotropism; and all these
radicles were strongly bowed downwards after 8 h. From the several
foregoing facts, there can be no doubt that the cauterisation of the
tip of the radicle of this Cucurbita on one side, if done lightly
enough, causes the whole growing part to bend to the opposite side.
Raphanus sativus: Sensitiveness of the apex of the Radicle.—We here
encountered many difficulties in our trials, both with squares of card
and with caustic; for when seeds were pinned to a cork-lid, many of the
radicles, to which nothing had been done, grew irregularly, often
curving upwards, as if attracted by the damp surface above; and when
they were immersed in water they likewise often grew irregularly. We
did not therefore dare to trust our experiments with attached squares
of card; nevertheless some of them seemed to indicate that the tips
were sensitive to contact. Our trials with caustic generally failed
from the difficulty of not injuring too greatly the extremely fine
tips. Out of 7 radicles thus tried, one became bowed after 22 h. at an
angle of 60°, a second at 40°,
and a third very slightly from the perpendicular and from the
cauterised side.
Æsculus hippocastanum: Sensitiveness of the apex of the Radicle.—Bits
of glass and squares of card were affixed with shellac or gum-water to
the tips of 12 radicles of the horse-chestnut; and when these objects
fell off, they were refixed; but not in a single instance was any
curvature thus caused. These massive radicles, one of which was above 2
inches in length and .3 inch in diameter at its base, seemed insensible
to so slight a stimulus as any small attached object. Nevertheless,
when the apex encountered an obstacle in its downward course, the
growing part became so uniformly and symmetrically curved, that its
appearance indicated not mere mechanical bending, but increased growth
along the whole convex side, due to the irritation of the apex.
That this is the correct view may be inferred from the effects of the
more powerful stimulus of caustic. The bending from the cauterised side
occurred much slower than in the previously described species, and it
will perhaps be worth while to give our trials in detail.
The seeds germinated in sawdust, and one side of the tips of the
radicles were slightly rubbed once with dry nitrate of silver; and
after a few minutes were allowed to dip into water. They were subjected
to a rather varying temperature, generally between 52° and 58° F. A few
cases have not been thought worth recording, in which the whole tip was
blackened, or in which the seedling soon became unhealthy.
(1.) The radicle was slightly deflected from the cauterised side in one
day (i.e. 24 h.); in three days it stood at 60° from the perpendicular;
in four days at 90°; on the fifth day it was curved up about 40° above
the horizon; so that it had passed through an angle of 130° in the five
days, and this was the greatest amount of curvature observed.
(2.) In two days radicle slightly deflected; after seven days
deflected 69° from the perpendicular and from the cauterised side;
after eight days the angle amounted to nearly 90°.
(3.) After one day slight deflection, but the cauterised mark was so
faint that the same side was again touched with caustic. In four days
from the first touch deflection amounted to 78°, which in an additional
day increased to 90°.
(4.) After two days slight deflection, which during the next three days
certainly increased but never became great; the radicle did not grow
well and died on the eighth day.
(5.) After two days very slight deflection; but this on the fourth day
amounted to 56° from the perpendicular and from the cauterised side.
(6.) After three days doubtfully, but after four days certainly
deflected from the cauterised side. On the fifth day deflection
amounted to 45° from the perpendicular, and this on the seventh day
increased to about 90°.
(7.) After two days slightly deflected; on the third day the deflection
amounted to 25° from the perpendicular, and this did not afterwards
increase.
(8.) After one day deflection distinct; on the third day it amounted to
44°, and on the fourth day to 72° from the perpendicular and the
cauterised side.
(9.) After two days deflection slight, yet distinct; on the third day
the tip was again touched on the same side with caustic and thus
killed.
(10.) After one day slight deflection, which after six days increased
to 50° from the perpendicular and the cauterised side.
(11.) After one day decided deflection, which after six days increased
to 62° from the perpendicular and from the cauterised side.
(12.) After one day slight deflection, which on the second day amounted
to 35°, on the fourth day to 50°, and the sixth day to 63° from the
perpendicular and the cauterised side.
(13.) Whole tip blackened, but more on one side than the other; on the
fourth day slightly, and on the sixth day greatly deflected from the
more blackened side; the deflection on the ninth day amounted to 90°
from the perpendicular.
(14.) Whole tip blackened in the same manner as in the last case: on
the second day decided deflection from the more blackened side, which
increased on the seventh day to nearly 90°; on the following day the
radicle appeared unhealthy.
(15.) Here we had the anomalous case of a radicle bending
slightly _towards_ the cauterised side on the first day, and continuing
to do so for the next three days, when the deflection amounted to about
90° from the perpendicular. The cause appeared to lie in the
tendril-like sensitiveness of the upper part of the radicle, against
which the point of a large triangular flap of the seed-coats pressed
with considerable force; and this irritation apparently conquered that
from the cauterised apex.
These several cases show beyond doubt that the irritation of one side
of the apex, excites the upper part of the radicle to bend slowly
towards the opposite side. This fact was well exhibited in one lot of
five seeds pinned to the cork-lid of a jar; for when after 6 days the
lid was turned upside down and viewed from directly above, the little
black marks made by the caustic were now all distinctly visible on the
upper sides of the tips of the laterally bowed radicles. A thin slice
was shaved off with a razor from one side of the tips of 22 radicles,
in the manner described under the common bean; but this kind of
irritation did not prove very effective. Only 7 out of the 22 radicles
became moderately deflected in from 3 to 5 days from the sliced
surface, and several of the others grew irregularly. The evidence,
therefore, is far from conclusive.
Quercus robur: Sensitiveness of the apex of the Radicle.—The tips of
the radicles of the common oak are fully as sensitive to slight contact
as are those of any plant examined by us. They remained healthy in damp
air for 10 days, but grew slowly. Squares of the card-like paper were
fixed with shellac to the tips of 15 radicles, and ten of these became
conspicuously bowed from the perpendicular and from the squares; two
slightly, and three not at all. But two of the latter were not real
exceptions, as they were at first very short, and hardly grew
afterwards. Some of the more
remarkable cases are worth describing. The radicles were examined on
each successive morning, at nearly the same hour, that is, after
intervals of 24 h.
No. 1. This radicle suffered from a series of accidents, and acted in
an anomalous manner, for the apex appeared at first insensible and
afterwards sensitive to contact. The first square was attached on Oct
19th; on the 21st the radicle was not at all curved, and the square was
accidentally knocked off; it was refixed on the 22nd, and the radicle
became slightly curved from the square, but the curvature disappeared
on the 23rd, when the square was removed and refixed. No curvature
ensued, and the square was again accidentally knocked off, and refixed.
On the morning of the 27th it was washed off by having reached the
water in the bottom of the jar. The square was refixed, and on the
29th, that is, ten days after the first square had been attached, and
two days after the attachment of the last square, the radicle had grown
to the great length of 3.2 inches, and now the terminal growing part
had become bent away from the square into a hook (see Fig. 68).
Fig. 68. Quercus robur: radicle with square of card attached to one
side of apex, causing it to become hooked. Drawing one-half natural
scale.
No. 2. Square attached on the 19th; on the 20th radicle slightly
deflected from it and from the perpendicular; on the 21st deflected at
nearly right angles; it remained during the next two days in this
position, but on the 25th the upward curvature was lessened through the
action of geotropism, and still more so on the 26th.
No. 3. Square attached on the 19th; on the 21st a trace of curvature
from the square, which amounted on the 22nd to about 40°, and on the
23rd to 53° from the perpendicular.
No. 4. Square attached on the 21st; on the 22nd trace of curvature from
the square; on the 23rd completely hooked with the point turned up to
the zenith. Three days afterwards (i.e. 26th) the curvature had wholly
disappeared and the apex pointed perpendicularly downwards.
No. 5. Square attached on the 21st; on the 22nd decided
though slight curvature from the square; on the 23rd the tip had curved
up above the horizon, and on the 24th was hooked with the apex pointing
almost to the zenith, as in Fig. 68.
No. 6. Square attached on the 21st; on the 22nd slightly curved from
the square; 23rd more curved; 25th considerably curved; 27th all
curvature lost, and the radicle was now directed perpendicularly
downwards.
No. 7. Square attached on the 21st; on the 22nd a trace of curvature
from the square, which increased next day, and on the 24th amounted to
a right angle.
It is, therefore, manifest that the apex of the radicle of the oak is
highly sensitive to contact, and retains its sensitiveness during
several days. The movement thus induced was, however, slower than in
any of the previous cases, with the exception of that of Æsculus. As
with the bean, the terminal growing part, after bending, sometimes
straightened itself through the action of geotropism, although the
object still remained attached to the tip.
The same remarkable experiment was next tried, as in the case of the
bean; namely, little squares of exactly the same size of the card-like
sanded paper and of very thin paper (the thicknesses of which have been
given under Vicia faba) were attached with shellac on opposite sides
(as accurately as could be done) of the tips of 13 radicles, suspended
in damp air, at a temperature of 65°–66° F. The result was striking,
for 9 out of these 13 radicles became plainly, and 1 very slightly,
curved from the thick paper towards the side bearing the thin paper. In
two of these cases the apex became completely hooked after two days; in
four cases the deflection from the perpendicular and from the side
bearing the thick paper, amounted in from two to four days to angles of
90°, 72°, 60°, and 49°, but in two other cases to only 18° and 15°. It
should, however, be stated that in the
case in which the deflection was 49°, the two squares had accidentally
come into contact on one side of the apex, and thus formed a lateral
gable; and the deflection was directed in part from this gable and in
part from the thick paper. In three cases alone the radicles were not
affected by the difference in thickness of the squares of paper
attached to their tips, and consequently did not bend away from the
side bearing the stiffer paper.
Zea mays: Sensitiveness of the apex of the Radicle to contact.—A large
number of trials were made on this plant, as it was the only
monocotyledon on which we experimented. An abstract of the results will
suffice. In the first place, 22 germinating seeds were pinned to
cork-lids without any object being attached to their radicles, some
being exposed to a temperature of 65°–66° F., and others to between 74°
and 79°; and none of them became curved, though some were a little
inclined to one side. A few were selected, which from having germinated
on sand were crooked, but when suspended in damp air the terminal part
grew straight downwards. This fact having been ascertained, little
squares of the card-like paper were affixed with shellac, on several
occasions, to the tips of 68 radicles. Of these the terminal growing
part of 39 became within 24 h. conspicuously curved away from the
attached squares and from the perpendicular; 13 out of the 39 forming
hooks with their points directed towards the zenith, and 8 forming
loops. Moreover, 7 other radicles out of the 68, were slightly and two
doubtfully deflected from the cards. There remain 20 which were not
affected; but 10 of these ought not to be counted; for one was
diseased, two had their tips quite surrounded by shellac, and the
squares on 7 had slipped so as to stand parallel to the apex, instead
of obliquely
on it. There were therefore only 10 out of the 68 which certainly were
not acted on. Some of the radicles which were experimented on were
young and short, most of them of moderate length, and two or three
exceeded three inches in length. The curvature in the above cases
occurred within 24 h., but it was often conspicuous within a much
shorter period. For instance, the terminal growing part of one radicle
was bent upwards into a rectangle in 8 h. 15 m., and of another in 9 h.
On one occasion a hook was formed in 9 h. Six of the radicles in a jar
containing nine seeds, which stood on a sand-bath, raised to a
temperature varying from 76° to 82° F., became hooked, and a seventh
formed a complete loop, when first looked at after 15 hours.
The accompanying figures of four germinating seeds (Fig. 69) show,
firstly, a radicle (A) the apex of which has become so much bent away
from the attached square as to form a hook. Secondly (B), a hook
converted through the continued irritation of the card, aided perhaps
by geotropism, into an almost complete circle or loop. The tip in the
act of forming a loop generally rubs against the upper part of the
radicle, and pushes off the attached square; the loop then contracts or
closes, but never disappears; and the apex afterwards grows vertically
downwards, being no longer irritated by any attached object. This
frequently occurred, and is represented at C. The jar above mentioned
with the six hooked radicles and another jar were kept for two
additional days, for the sake of observing how the hooks would be
modified. Most of them became converted into simple loops, like that
figured at C; but in one case the apex did not rub against the upper
part of the radicle and thus remove the card; and it consequently made,
owing
to the continued irritation from the card, two complete loops, that is,
a helix of two spires; which afterwards became pressed closely
together. Then geotropism prevailed and caused the apex to grow
perpendicularly downwards. In another case, shown at (D), the apex in
making a second turn or spire, passed through the first loop, which was
at first widely open, and in doing so knocked off the card; it then
grew perpendicularly downwards, and thus tied itself into a knot, which
soon became tight!
Fig. 69. Zea mays: radicles excited to bend away from the little
squares of card attached to one side of their tips.
Secondary Radicles of Zea.—A short time after the first radicle has
appeared, others protrude from the
seed, but not laterally from the primary one. Ten of these secondary
radicles, which were directed obliquely downwards, were experimented on
with very small squares of card attached with shellac to the lower
sides of their tips. If therefore the squares acted, the radicles would
bend upwards in opposition to gravity. The jar stood (protected from
light) on a sand-bath, which varied between 76° and 82° F. After only 5
h. one appeared to be a little deflected from the square, and after 20
h. formed a loop. Four others were considerably curved from the squares
after 20 h., and three of them became hooked, with their tips pointing
to the zenith,—one after 29 h. and the two others after 44 h. By this
latter time a sixth radicle had become bent at a right angle from the
side bearing the square. Thus altogether six out of the ten secondary
radicles were acted on, four not being affected. There can, therefore,
be no doubt that the tips of these secondary radicles are sensitive to
slight contact, and that when thus excited they cause the upper part to
bend from the touching object; but generally, as it appears, not in so
short a time as in the case of the first-formed radicle.
SENSITIVENESS OF THE TIP OF THE RADICLE TO MOIST AIR.
Sachs made the interesting discovery, a few years ago, that the
radicles of many seedling plants bend towards an adjoining damp
surface.[9] We shall here endeavour to show that this peculiar form of
sensitiveness resides in their tips. The movement is directly the
reverse of that excited by the irritants hitherto considered, which
cause the growing part of the
radicle to bend away from the source of irritation. In our experiments
we followed Sachs’ plan, and sieves with seeds germinating in damp
sawdust were suspended so that the bottom was generally inclined at 40°
with the horizon. If the radicles had been acted on solely by
geotropism, they would have grown out of the bottom of the sieve
perpendicularly downwards; but as they were attracted by the adjoining
damp surface they bent towards it and were deflected 50° from the
perpendicular. For the sake of ascertaining whether the tip or the
whole growing part of the radicle was sensitive to the moist air, a
length of from 1 to 2 mm. was coated in a certain number of cases with
a mixture of olive-oil and lamp-black. This mixture was made in order
to give consistence to the oil, so that a thick layer could be applied,
which would exclude, at least to a large extent, the moist air, and
would be easily visible. A greater number of experiments than those
which were actually tried would have been necessary, had not it been
clearly established that the tip of the radicle is the part which is
sensitive to various other irritants.
[9] ‘Arbeiten des Bot. Institut., in Würzburg,’ vol. i. 1872, p. 209.
Phaseolus multiflorus.—Twenty-nine radicles, to which nothing had been
done, growing out of a sieve, were observed at the same time with those
which had their tips greased, and for an equal length of time. Of the
29, 24 curved themselves so as to come into close contact with the
bottom of the sieve. The place of chief curvature was generally at a
distance of 5 or 6 mm. from the apex. Eight radicles had their tips
greased for a length of 2 mm., and two others for a length of 1½ mm.;
they were kept at a temperature of 15°–16° C. After intervals of from
19 h. to 24 h. all were still vertically or almost vertically
dependent, for some of them had moved towards the adjoining damp
surface by about 10°. They had therefore not been acted on, or only
slightly acted on, by the damper air on one side, although the whole
upper part was freely exposed. After 48 h. three of these radicles
became
considerably curved towards the sieve; and the absence of curvature in
some of the others might perhaps be accounted for by their not having
grown very well. But it should be observed that during the first 19 h.
to 24 h. all grew well; two of them having increased 2 and 3 mm. in
length in 11 h.; five others increased 5 to 8 mm. in 19 h.; and two,
which had been at first 4 and 6 mm. in length, increased in 24 h. to 15
and 20 mm.
The tips of 10 radicles, which likewise grew well, were coated with the
grease for a length of only 1 mm., and now the result was somewhat
different; for of these 4 curved themselves to the sieve in from 21 h.
to 24h., whilst 6 did not do so. Five of the latter were observed for
an additional day, and now all excepting one became curved to the
sieve.
The tips of 5 radicles were cauterised with nitrate of silver, and
about 1 mm. in length was thus destroyed. They were observed for
periods varying between 11 h. and 24h., and were found to have grown
well. One of them had curved until it came into contact with the sieve;
another was curving towards it; whilst the remaining three were still
vertically dependent. Of 7 not cauterised radicles observed at the same
time, all had come into contact with the sieve.
The tips of 11 radicles were protected by moistened gold-beaters’ skin,
which adheres closely, for a length varying from 1½ to 2½ mm. After 22
h. to 24 h., 6 of these radicles were clearly bent towards or had come
into contact with the sieve; 2 were slightly curved in this direction,
and 3 not at all. All had grown well. Of 14 control specimens observed
at the same time, all excepting one had closely approached the sieve.
It appears from these cases that a cap of goldbeaters’ skin checks,
though only to a slight degree, the bending of the radicles to an
adjoining damp surface. Whether an extremely thin sheet of this
substance when moistened allows moisture from the air to pass through
it, we do not know. One case indicated that the caps were sometimes
more efficient than appears from the above results; for a radicle,
which after 23 h. had only slightly approached the sieve, had its cap
(1½ mm. in length) removed, and during the next 15½ h. it curved itself
abruptly towards the source of moisture, the chief seat of curvature
being at a distance of 2 to 3 mm. from the apex.
Vicia faba.—The tips of 13 radicles were coated with the grease for a
length of 2 mm.; and it should be remembered that with these radicles
the seat of chief curvature is about
4 or 5 mm. from the apex. Four of them were examined after 22h., three
after 26 h., and six after 36 h., and none had been attracted towards
the damp lower surface of the sieve. In another trial 7 radicles were
similarly treated, and 5 of them still pointed perpendicularly
downwards after 11 h., whilst 2 were a little curved towards the sieve;
by an accident they were not subsequently observed. In both these
trials the radicles grew well; 7 of them, which were at first from 4 to
11 mm. in length, were after 11 h. between 7 and 16 mm.; 3 which were
at first from 6 to 8 mm. after 26 h. were 11.5 to 18 mm. in length; and
lastly, 4 radicles which were at first 5 to 8 mm. after 46 h. were 18
to 23 mm. in length. The control or ungreased radicles were not
invariably attracted towards the bottom of the sieve. But on one
occasion 12 out of 13, which were observed for periods between 22 h.
and 36 h., were thus attracted. On two other occasions taken together,
38 out of 40 were similarly attracted. On another occasion only 7 out
of 14 behaved in this manner, but after two more days the proportion of
the curved increased to 17 out of 23. On a last occasion only 11 out of
20 were thus attracted. If we add up these numbers, we find that 78 out
of 96 of the control specimens curved themselves towards the bottom of
the sieve. Of the specimens with greased tips, 2 alone out of the 20
(but 7 of these were not observed for a sufficiently long time) thus
curved themselves. We can, therefore, hardly doubt that the tip for a
length of 2 mm. is the part which is sensitive to a moist atmosphere,
and causes the upper part to bend towards its source.
The tips of 15 radicles were cauterised with nitrate of silver, and
they grew as well as those above described with greased tips. After an
interval of 24 h., 9 of them were not at all curved towards the bottom
of the sieve; 2 were curved towards it at angles of 20° and 12° from
their former vertical position, and 4 had come into close contact with
it. Thus the destruction of the tip for a length of about 1 mm.
prevented the curvature of the greater number of these radicles to the
adjoining damp surface. Of 24 control specimens, 23 were bent to the
sieve, and on a second occasion 15 out of 16 were similarly curved in a
greater or less degree. These control trials are included in those
given in the foregoing paragraph.
Avena sativa.—The tips of 13 radicles, which projected between 2 and 4
mm. from the bottom of the sieve, many of
them not quite perpendicularly downwards, were coated with the black
grease for a length of from 1 to 1½ mm. The sieves were inclined at 30°
with the horizon. The greater number of these radicles were examined
after 22 h., and a few after 25 h., and within these intervals they had
grown so quickly as to have nearly doubled their lengths. With the
ungreased radicles the chief seat of curvature is at a distance of not
less than between 3.5 and 5.5 mm., and not more than between 7 and 10
mm. from the apex. Out of the 13 radicles with greased tips, 4 had not
moved at all towards the sieve; 6 were deflected towards it and from
the perpendicular by angles varying between 10° and 35°; and 3 had come
into close contact with it. It appears, therefore, at first sight that
greasing the tips of these radicles had checked but little their
bending to the adjoining damp surface. But the inspection of the sieves
on two occasions produced a widely different impression on the mind;
for it was impossible to behold the radicles with the black greased
tips projecting from the bottom, and all those with ungreased tips, at
least 40 to 50 in number, clinging closely to it, and feel any doubt
that the greasing had produced a great effect. On close examination
only a single ungreased radicle could be found which had not become
curved towards the sieve. It is probable that if the tips had been
protected by grease for a length of 2 mm. instead of from 1 to 1½ mm.,
they would not have been affected by the moist air and none would have
become curved.
Triticum vulgare.—Analogous trials were made on 8 radicles of the
common wheat; and greasing their tips produced much less effect than in
the case of the oats. After 22 h., 5 of them had come into contact with
the bottom of the sieve; 2 had moved towards it 10° and 15°, and one
alone remained perpendicular. Not one of the very numerous ungreased
radicles failed to come into close contact with the sieve. These trials
were made on Nov. 28th, when the temperature was only 4.8° C. at 10
A.M. We should hardly have thought this case worth notice, had it not
been for the following circumstance. In the beginning of October, when
the temperature was considerably higher, viz., 12° to 13° C., we found
that only a few of the ungreased radicles became bent towards the
sieve; and this indicates that sensitiveness to moisture in the air is
increased by a low temperature, as we have seen with the radicles of
Vicia faba relatively to objects attached to their tips. But in the
present instance it is possible that a difference in the dryness
of the air may have caused the difference in the results at the two
periods.
Finally, the facts just given with respect to Phaseolus multiflorus,
Vicia faba, and Avena sativa show, as it seems to us, that a layer of
grease spread for a length of 1½ to 2 mm. over the tip of the radicle,
or the destruction of the tip by caustic, greatly lessens or quite
annuls in the upper and exposed part the power of bending towards a
neighbouring source of moisture. We should bear in mind that the part
which bends most, lies at some little distance above the greased or
cauterised tip; and that the rapid growth of this part, proves that it
has not been injured by the tips having been thus treated. In those
cases in which the radicles with greased tips became curved, it is
possible that the layer of grease was not sufficiently thick wholly to
exclude moisture, or that a sufficient length was not thus protected,
or, in the case of the caustic, not destroyed. When radicles with
greased tips are left to grow for several days in damp air, the grease
is drawn out into the finest reticulated threads and dots, with narrow
portions of the surface left clean. Such portions would, it is
probable, be able to absorb moisture, and thus we can account for
several of the radicles with greased tips having become curved towards
the sieve after an interval of one or two days. On the whole, we may
infer that sensitiveness to a difference in the amount of moisture in
the air on the two sides of a radicle resides in the tip, which
transmits some influence to the upper part, causing it to bend towards
the source of moisture. Consequently, the movement is the reverse of
that caused by objects attached to one side of the tip, or by a thin
slice being cut off, or by being slightly cauterised. In a future
chapter it will be shown that sensitiveness to the attraction of
gravity likewise resides in the tip; so that it is the tip which
excites the adjoining parts of a horizontally extended radicle to bend
towards the centre of the earth.
SECONDARY RADICLES BECOMING VERTICALLY GEOTROPIC BY THE DESTRUCTION OR
INJURY OF THE TERMINAL PART OF THE PRIMARY RADICLE.
Sachs has shown that the lateral or secondary radicles of the bean, and
probably of other plants, are acted on by geotropism in so peculiar a
manner, that they grow out horizontally or a little inclined downwards;
and he has further shown[10] the interesting fact, that if the end of
the primary radicle be cut off, one of the nearest secondary radicles
changes its nature and grows perpendicularly downwards, thus replacing
the primary radicle. We repeated this experiment, and planted beans
with amputated radicles in friable peat, and saw the result described
by Sachs; but generally two or three of the secondary radicles grew
perpendicularly downwards. We also modified the experiment, by pinching
young radicles a little way above their tips, between the arms of a
U-shaped piece of thick leaden wire. The part pinched was thus
flattened, and was afterwards prevented from growing thicker. Five
radicles had their ends cut off, and served as controls or standards.
Eight were pinched; of these 2 were pinched too severely and their ends
died and dropped off; 2 were not pinched enough and were not sensibly
affected; the remaining 4 were pinched sufficiently to check the growth
of the terminal part, but did not appear otherwise injured. When the
U-shaped wires were removed, after an
interval of 15 days, the part beneath the wire was found to be very
thin and easily broken, whilst the part above was thickened. Now in
these four cases, one or more of the secondary radicles, arising from
the thickened part just above the wire, had grown perpendicularly
downwards. In the best case the primary radicle (the part below the
wire being 1½ inch in length) was somewhat distorted, and was not half
as long as three adjoining secondary radicles, which had grown
vertically, or almost vertically, downwards. Some of these secondary
radicles adhered together or had become confluent. We learn from these
four cases that it is not necessary, in order that a secondary radicle
should assume the nature of a primary one, that the latter should be
actually amputated; it is sufficient that the flow of sap into it
should be checked, and consequently should be directed into the
adjoining secondary radicles; for this seems to be the most obvious
result of the primary radicle being pinched between the arms of a
U-shaped wire.
[10] ‘Arbeiten Bot. Institut., Würzburg,’ Heft iv. 1874, p. 622.
This change in the nature of secondary radicles is clearly analogous,
as Sachs has remarked, to that which occurs with the shoots of trees,
when the leading one is destroyed and is afterwards replaced by one or
more of the lateral shoots; for these now grow upright instead of
sub-horizontally. But in this latter case the lateral shoots are
rendered apogeotropic, whereas with radicles the lateral ones are
rendered geotropic. We are naturally led to suspect that the same cause
acts with shoots as with roots, namely, an increased flow of sap into
the lateral ones. We made some trials with Abies communis and
pectinata, by pinching with wire the leading and all the lateral shoots
excepting one. But we believe that they were too old when experimented
on; and some were pinched too severely, and
some not enough. Only one case succeeded, namely, with the spruce-fir.
The leading shoot was not killed, but its growth was checked; at its
base there were three lateral shoots in a whorl, two of which were
pinched, one being thus killed; the third was left untouched. These
lateral shoots, when operated on (July 14th) stood at an angle of 8°
above the horizon; by Sept. 8th the unpinched one had risen 35°; by
Oct. 4th it had risen 46°, and by Jan. 26th 48°, and it had now become
a little curved inwards. Part of this rise of 48° may be attributed to
ordinary growth, for the pinched shoot rose 12° within the same period.
It thus follows that the unpinched shoot stood, on Jan. 26th, 56° above
the horizon, or 34° from the vertical; and it was thus obviously almost
ready to replace the slowly growing, pinched, leading shoot.
Nevertheless, we feel some doubt about this experiment, for we have
since observed with spruce-firs growing rather unhealthily, that the
lateral shoots near the summit sometimes become highly inclined, whilst
the leading shoot remains apparently sound.
A widely different agency not rarely causes shoots which naturally
would have brown out horizontally to grow up vertically. The lateral
branches of the Silver Fir (A. pectinata) are often affected by a
fungus, Æcidium elatinum, which causes the branch to enlarge into an
oval knob formed of hard wood, in one of which we counted 24 rings of
growth. According to De Bary[11], when the mycelium penetrates a bud
beginning to elongate, the shoot developed from it grows vertically
upwards. Such upright shoots
afterwards produce lateral and horizontal branches; and they then
present a curious appearance, as if a young fir-tree had grown out of a
ball of clay surrounding the branch. These upright shoots have
manifestly changed their nature and become apogeotropic; for if they
had not been affected by the Æcidium, they would have grown out
horizontally like all the other twigs on the same branches. This change
can hardly be due to an increased flow of sap into the part; but the
presence of the mycelium will have greatly disturbed its natural
constitution.
[11] See his valuable article in ‘Bot. Zeitung,’ 1867, p. 257, on
these monstrous growths, which are called in German “Hexenbesen,” or
“witch-brooms.”
According to Mr. Meehan,[12] the stems of three species of Euphorbia
and of Portulaca oleracea are “normally prostrate or procumbent;” but
when they are attacked by an Æcidium, they “assume an erect habit.” Dr.
Stahl informs us that he knows of several analogous cases; and these
seem to be closely related to that of the Abies. The rhizomes of
Sparganium ramosum grow out horizontally in the soil to a considerable
length, or are diageotropic; but F. Elfving found that when they were
cultivated in water their tips turned upwards, and they became
apogeotropic. The same result followed when the stem of the plant was
bent until it cracked or was merely much bowed.[13]
[12] ‘Proc. Acad. Nat. Sc. Philadelphia,’ June 16th, 1874, and July
23rd, 1875.
[13] See F. Elfving’s interesting paper in ‘Arbeiten Bot. Institut.,
in Würzburg,’ vol. ii. 1880, p. 489. Carl Kraus (Triesdorf) had
previously observed (‘Flora,’ 1878, p. 324) that the underground
shoots of Triticum repens bend vertically up when the parts above
ground are removed, and when the rhizomes are kept partly immersed in
water.
No explanation has hitherto been attempted of such cases as the
foregoing,—namely, of secondary radicles growing vertically downwards,
and of lateral shoots growing vertically upwards, after the amputation
of
the primary radicle or of the leading shoot. The following
considerations give us, as we believe, the clue. Firstly, any cause
which disturbs the constitution[14] is apt to induce reversion; such as
the crossing of two distinct races, or a change of conditions, as when
domestic animals become feral. But the case which most concerns us, is
the frequent appearance of peloric flowers on the summit of a stem, or
in the centre of the inflorescence,—parts which, it is believed,
receive the most sap; for when an irregular flower becomes perfectly
regular or peloric, this may be attributed, at least partly, to
reversion to a primitive and normal type. Even the position of a seed
at the end of the capsule sometimes gives to the seedling developed
from it a tendency to revert. Secondly, reversions often occur by means
of buds, independently of reproduction by seed; so that a bud may
revert to the character of a former state many bud-generations ago. In
the case of animals, reversions may occur in the individual with
advancing age. Thirdly and lastly, radicles when they first protrude
from the seed are always geotropic, and plumules or shoots almost
always apogeotropic. If then any cause, such as an increased flow of
sap or the presence of mycelium, disturbs the constitution of a lateral
shoot or of a secondary radicle, it is apt to revert to its primordial
state; and it becomes either apogeotropic or geotropic, as the case may
be, and consequently grows either vertically upwards or downwards. It
is indeed
possible, or even probable, that this tendency to reversion may have
been increased, as it is manifestly of service to the plant.
[14] The facts on which the following conclusions are founded are
given in ‘The Variation of Animals and Plants under Domestication,’
2nd edit. 1875. On the causes leading to reversion see chap. xii. vol.
ii. and p. 59, chap. xiv. On peloric flowers, chap. xiii. p. 32; and
see p. 337 on their position on the plant. With respect to seeds, p.
340. On reversion by means of buds, p. 438, chap. xi. vol. i.
A SUMMARY OF CHAPTER.
A part or organ may be called sensitive, when its irritation excites
movement in an adjoining part. Now it has been shown in this chapter,
that the tip of the radicle of the bean is in this sense sensitive to
the contact of any small object attached to one side by shellac or
gum-water; also to a slight touch with dry caustic, and to a thin slice
cut off one side. The radicles of the pea were tried with attached
objects and caustic, both of which acted. With Phaseolus multiflorus
the tip was hardly sensitive to small squares of attached card, but was
sensitive to caustic and to slicing. The radicles of Tropaeolum were
highly sensitive to contact; and so, as far as we could judge, were
those of Gossypium herbaceum, and they were certainly sensitive to
caustic. The tips of the radicles of Cucurbita ovifera were likewise
highly sensitive to caustic, though only moderately so to contact.
Raphanus sativus offered a somewhat doubtful case. With Æsculus the
tips were quite indifferent to bodies attached to them, though
sensitive to caustic. Those of Quercus robur and Zea mays were highly
sensitive to contact, as were the radicles of the latter to caustic. In
several of these cases the difference in sensitiveness of the tip to
contact and to caustic was, as we believe, merely apparent; for with
Gossypium, Raphanus, and Cucurbita, the tip was so fine and flexible
that it was very difficult to attach any object to one of its sides.
With the radicles of Æsculus, the tips were not at all sensitive to
small bodies attached to them; but it does not follow from this
fact that they would not have been sensitive to somewhat greater
continued pressure, if this could have been applied.
The peculiar form of sensitiveness which we are here considering, is
confined to the tip of the radicle for a length of from 1 mm. to 1.5
mm. When this part is irritated by contact with any object, by caustic,
or by a thin slice being cut off, the upper adjoining part of the
radicle, for a length of from 6 or 7 to even 12 mm., is excited to bend
away from the side which has been irritated. Some influence must
therefore be transmitted from the tip along the radicle for this
length. The curvature thus caused is generally symmetrical. The part
which bends most apparently coincides with that of the most rapid
growth. The tip and the basal part grow very slowly and they bend very
little.
Considering the widely separated position in the vegetable series of
the several above-named genera, we may conclude that the tips of the
radicles of all, or almost all, plants are similarly sensitive, and
transmit an influence causing the upper part to bend. With respect to
the tips of the secondary radicles, those of Vicia faba, Pisum sativum,
and Zea mays were alone observed, and they were found similarly
sensitive.
In order that these movements should be properly displayed, it appears
necessary that the radicles should grow at their normal rate. If
subjected to a high temperature and made to grow rapidly, the tips seem
either to lose their sensitiveness, or the upper part to lose the power
of bending. So it appears to be if they grow very slowly from not being
vigorous, or from being kept at too low a temperature; also when they
are forced to germinate in the middle of the winter.
The curvature of the radicle sometimes occurs within from 6 to 8 hours
after the tip has been irritated, and almost always within 24 h.,
excepting in the case of the massive radicles of Æsculus. The curvature
often amounts to a rectangle,—that is, the terminal part bends upwards
until the tip, which is but little curved, projects almost
horizontally. Occasionally the tip, from the continued irritation of
the attached object, continues to bend up until it forms a hook with
the point directed towards the zenith, or a loop, or even a spire.
After a time the radicle apparently becomes accustomed to the
irritation, as occurs in the case of tendrils, for it again grows
downwards, although the bit of card or other object may remain attached
to the tip. It is evident that a small object attached to the free
point of a vertically suspended radicle can offer no mechanical
resistance to its growth as a whole, for the object is carried
downwards as the radicle elongates, or upwards as the radicle curves
upwards. Nor can the growth of the tip itself be mechanically checked
by an object attached to it by gum-water, which remains all the time
perfectly soft. The weight of the object, though quite insignificant,
is opposed to the upward curvature. We may therefore conclude that it
is the irritation due to contact which excites the movement. The
contact, however, must be prolonged, for the tips of 15 radicles were
rubbed for a short time, and this did not cause them to bend. Here then
we have a case of specialised sensibility, like that of the glands of
Drosera; for these are exquisitely sensitive to the slightest pressure
if prolonged, but not to two or three rough touches.
When the tip of a radicle is lightly touched on one side with dry
nitrate of silver, the injury caused is
very slight, and the adjoining upper part bends away from the
cauterised point, with more certainty in most cases than from an object
attached on one side. Here it obviously is not the mere touch, but the
effect produced by the caustic, which induces the tip to transmit some
influence to the adjoining part, causing it to bend away. If one side
of the tip is badly injured or killed by the caustic, it ceases to
grow, whilst the opposite side continues growing; and the result is
that the tip itself bends towards the injured side and often becomes
completely hooked; and it is remarkable that in this case the adjoining
upper part does not bend. The stimulus is too powerful or the shock too
great for the proper influence to be transmitted from the tip. We have
strictly analogous cases with Drosera, Dionaea and Pinguicula, with
which plants a too powerful stimulus does not excite the tentacles to
become incurved, or the lobes to close, or the margin to be folded
inwards.
With respect to the degree of sensitiveness of the apex to contact
under favourable conditions, we have seen that with Vicia faba a little
square of writing-paper affixed with shellac sufficed to cause
movement; as did on one occasion a square of merely damped goldbeaters’
skin, but it acted very slowly. Short bits of moderately thick bristle
(of which measurements have been given) affixed with gum-water acted in
only three out of eleven trials, and beads of dried shellac under
1/200th of a grain in weight acted only twice in nine cases; so that
here we have nearly reached the minimum of necessary irritation. The
apex, therefore, is much less sensitive to pressure than the glands of
Drosera, for these are affected by far thinner objects than bits of
bristle, and by a very much less weight than 1/200th of a grain.
But the most interesting evidence of the delicate sensitiveness of the
tip of the radicle, was afforded by its power of discriminating between
equal-sized squares of card-like and very thin paper, when these were
attached on opposite sides, as was observed with the radicles of the
bean and oak.
When radicles of the bean are extended horizontally with squares of
card attached to the lower sides of their tips, the irritation thus
caused was always conquered by geotropism, which then acts under the
most favourable conditions at right angles to the radicle. But when
objects were attached to the radicles of any of the above-named genera,
suspended vertically, the irritation conquered geotropism, which latter
power at first acted obliquely on the radicle; so that the immediate
irritation from the attached object, aided by its after-effects,
prevailed and caused the radicle to bend upwards, until sometimes the
point was directed to the zenith. We must, however, assume that the
after-effects of the irritation of the tip by an attached object come
into play, only after movement has been excited. The tips of the
radicles of the pea seem to be more sensitive to contact than those of
the bean, for when they were extended horizontally with squares of card
adhering to their lower sides, a most curious struggle occasionally
arose, sometimes one and sometimes the other force prevailing, but
ultimately geotropism was always victorious; nevertheless, in two
instances the terminal part became so much curved upwards that loops
were subsequently formed. With the pea, therefore, the irritation from
an attached object, and from geotropism when acting at right angles to
the radicle, are nearly balanced forces. Closely similar results were
observed with the horizontally extended radicles of _Cucurbita
ovifera_,
when their tips were slightly cauterised on the lower side.
Finally, the several co-ordinated movements by which radicles are
enabled to perform their proper functions are admirably perfect. In
whatever direction the primary radicle first protrudes from the seed,
geotropism guides it perpendicularly downwards; and the capacity to be
acted on by the attraction of gravity resides in the tip. But Sachs has
proved[15] that the secondary radicles, or those emitted by the primary
one, are acted on by geotropism in such a manner that they tend to bend
only obliquely downwards. If they had been acted on like the primary
radicle, all the radicles would have penetrated the ground in a close
bundle. We have seen that if the end of the primary radicle is cut off
or injured, the adjoining secondary radicles become geotropic and grow
vertically downwards. This power must often be of great service to the
plant, when the primary radicle has been destroyed by the larvae of
insects, burrowing animals, or any other accident. The tertiary
radicles, or those emitted by the secondary ones, are not influenced,
at least in the case of the bean, by geotropism; so they grow out
freely in all directions. From this manner of growth of the various
kinds of radicles, they are distributed, together with their absorbent
hairs, throughout the surrounding soil, as Sachs has remarked, in the
most advantageous manner; for the whole soil is thus closely searched.
[15] ‘Arbeiten Bot. Institut, Würzburg,’ Heft iv. 1874, pp. 605–631.
Geotropism, as was shown in the last chapter, excites the primary
radicle to bend downwards with very little force, quite insufficient to
penetrate the ground. Such penetration is effected by the pointed
apex (protected by the root-cap) being pressed down by the longitudinal
expansion or growth of the terminal rigid portion, aided by its
transverse expansion, both of which forces act powerfully. It is,
however, indispensable that the seeds should be at first held down in
some manner. When they lie on the bare surface they are held down by
the attachment of the root-hairs to any adjoining objects; and this
apparently is effected by the conversion of their outer surfaces into a
cement. But many seeds get covered up by various accidents, or they
fall into crevices or holes. With some seeds their own weight suffices.
The circumnutating movement of the terminal growing part both of the
primary and secondary radicles is so feeble that it can aid them very
little in penetrating the ground, excepting when the superficial layer
is very soft and damp. But it must aid them materially when they happen
to break obliquely into cracks, or into burrows made by earth-worms or
larvae. This movement, moreover, combined with the sensitiveness of the
tip to contact, can hardly fail to be of the highest importance; for as
the tip is always endeavouring to bend to all sides it will press on
all sides, and will thus be able to discriminate between the harder and
softer adjoining surfaces, in the same manner as it discriminated
between the attached squares of card-like and thin paper. Consequently
it will tend to bend from the harder soil, and will thus follow the
lines of least resistance. So it will be if it meets with a stone or
the root of another plant in the soil, as must incessantly occur. If
the tip were not sensitive, and if it did not excite the upper part of
the root to bend away, whenever it encountered at right angles some
obstacle in the ground, it would be liable
to be doubled up into a contorted mass. But we have seen with radicles
growing down inclined plates of glass, that as soon as the tip merely
touched a slip of wood cemented across the plate, the whole terminal
growing part curved away, so that the tip soon stood at right angles to
its former direction; and thus it would be with an obstacle encountered
in the ground, as far as the pressure of the surrounding soil would
permit. We can also understand why thick and strong radicles, like
those of Æsculus, should be endowed with less sensitiveness than more
delicate ones; for the former would be able by the force of their
growth to overcome any slight obstacle.
After a radicle, which has been deflected by some stone or root from
its natural downward course, reaches the edge of the obstacle,
geotropism will direct it to grow again straight downward; but we know
that geotropism acts with very little force, and here another excellent
adaptation, as Sachs has remarked,[16] comes into play. For the upper
part of the radicle, a little above the apex, is, as we have seen,
likewise sensitive; and this sensitiveness causes the radicle to bend
like a tendril towards the touching object, so that as it rubs over the
edge of an obstacle, it will bend downwards; and the curvature thus
induced is abrupt, in which respect it differs from that caused by the
irritation of one side of the tip. This downward bending coincides with
that due to geotropism, and both will cause the root to resume its
original course.
[16] ‘Arbeiten Bot. Inst., Würzburg,’ Heft iii. p. 456.
As radicles perceive an excess of moisture in the air on one side and
bend towards this side, we may infer that they will act in the same
manner with respect to moisture in the earth. The sensitiveness to
moisture
resides in the tip, which determines the bending of the upper part.
This capacity perhaps partly accounts for the extent to which
drain-pipes often become choked with roots.
Considering the several facts given in this chapter, we see that the
course followed by a root through the soil is governed by
extraordinarily complex and diversified agencies,—by geotropism acting
in a different manner on the primary, secondary, and tertiary
radicles,—by sensitiveness to contact, different in kind in the apex
and in the part immediately above the apex, and apparently by
sensitiveness to the varying dampness of different parts of the soil.
These several stimuli to movement are all more powerful than
geotropism, when this acts obliquely on a radicle, which has been
deflected from its perpendicular downward course. The roots, moreover,
of most plants are excited by light to bend either to or from it; but
as roots are not naturally exposed to the light it is doubtful whether
this sensitiveness, which is perhaps only the indirect result of the
radicles being highly sensitive to other stimuli, is of any service to
the plant. The direction which the apex takes at each successive period
of the growth of a root, ultimately determines its whole course; it is
therefore highly important that the apex should pursue from the first
the most advantageous direction; and we can thus understand why
sensitiveness to geotropism, to contact and to moisture, all reside in
the tip, and why the tip determines the upper growing part to bend
either from or to the exciting cause. A radicle may be compared with a
burrowing animal such as a mole, which wishes to penetrate
perpendicularly down into the ground. By continually moving his head
from side to side, or circumnutating, he will feel any stone
or other obstacle, as well as any difference in the hardness of the
soil, and he will turn from that side; if the earth is damper on one
than on the other side he will turn thitherward as a better
hunting-ground. Nevertheless, after each interruption, guided by the
sense of gravity, he will be able to recover his downward course and to
burrow to a greater depth.