SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.
Uses of heliotropism—Insectivorous and climbing plants not
heliotropic—Same organ heliotropic at one age and not at
another—Extraordinary sensitiveness of some plants to light—The effects
of light do not correspond with its intensity—Effects of previous
illumination—Time required for the action of light—After-effects of
light—Apogeotropism acts as soon as light fails—Accuracy with which
plants bend to the light—This dependent on the illumination of one
whole side of the part—Localised sensitiveness to light and its
transmitted effects—Cotyledons of Phalaris, manner of bending—Results
of the exclusion of light from their tips—Effects transmitted beneath
the surface of the ground—Lateral illumination of the tip determines
the direction of the curvature of the base—Cotyledons of Avena,
curvature of basal part due to the illumination of upper part—Similar
results with the hypocotyls of Brassica and Beta—Radicles of Sinapis
apheliotropic, due to the sensitiveness of their tips—Concluding
remarks and summary of chapter—Means by which circumnutation has been
converted into heliotropism or apheliotropism.
No one can look at the plants growing on a bank or on the borders of a
thick wood, and doubt that the young stems and leaves place themselves
so that the leaves may be well illuminated. They are thus enabled to
decompose carbonic acid. But the sheath-like cotyledons of some
Gramineæ, for instance, those of Phalaris, are not green and contain
very little starch; from which fact we may infer that they decompose
little or no carbonic acid. Nevertheless, they are extremely
heliotropic; and this probably serves them in another way, namely, as a
guide from the buried seeds through fissures in the ground or through
overlying masses of vegetation, into the light and air. This view
is strengthened by the fact that with Phalaris and Avena the first true
leaf, which is bright green and no doubt decomposes carbonic acid,
exhibits hardly a trace of heliotropism. The heliotropic movements of
many other seedlings probably aid them in like manner in emerging from
the ground; for apogeotropism by itself would blindly guide them
upwards, against any overlying obstacle.
Heliotropism prevails so extensively among the higher plants, that
there are extremely few, of which some part, either the stem,
flower-peduncle, petiole, or leaf, does not bend towards a lateral
light. Drosera rotundifolia is one of the few plants the leaves of
which exhibit no trace of heliotropism. Nor could we see any in
Dionaea, though the plants were not so carefully observed. Sir J.
Hooker exposed the pitchers of Sarracenia for some time to a lateral
light, but they did not bend towards it.[1] We can understand the
reason why these insectivorous plants should not be heliotropic, as
they do not live chiefly by decomposing carbonic acid; and it is much
more important to them that their leaves should occupy the best
position for capturing insects, than that they should be fully exposed
to the light.
[1] According to F. Kurtz (‘Verhandl. des Bot. Vereins der Provinz
Brandenburg,’ Bd. xx. 1878) the leaves or pitchers of Darlingtonia
Californica are strongly apheliotropic. We failed to detect this
movement in a plant which we possessed for a short time.
Tendrils, which consist of leaves or of other organs modified, and the
stems of twining plants, are, as Mohl long ago remarked, rarely
heliotropic; and here again we can see the reason why, for if they had
moved towards a lateral light they would have been drawn away from
their supports. But some tendrils are apheliotropic, for instance those
of Bignonia capreolata
and of Smilax aspera; and the stems of some plants which climb by
rootlets, as those of the Ivy and Tecoma radicans, are likewise
apheliotropic, and they thus find a support. The leaves, on the other
hand, of most climbing plants are heliotropic; but we could detect no
signs of any such movement in those of Mutisia clematis.
As heliotropism is so widely prevalent, and as twining plants are
distributed throughout the whole vascular series, the apparent absence
of any tendency in their stems to bend towards the light, seemed to us
so remarkable a fact as to deserve further investigation, for it
implies that heliotropism can be readily eliminated. When twining
plants are exposed to a lateral light, their stems go on revolving or
circumnutating about the same spot, without any evident deflection
towards the light; but we thought that we might detect some trace of
heliotropism by comparing the average rate at which the stems moved to
and from the light during their successive revolutions.[2] Three young
plants (about a foot in height) of Ipomœa caerulea and four of I.
purpurea, growing in separate pots, were placed on a bright day before
a north-east window in a room otherwise darkened, with the tips of
their revolving stems fronting the window. When the tip of each plant
pointed directly from the window, and when again towards it, the times
were recorded. This was continued from 6.45 A.M. till a little after 2
P.M. on June 17th. After a few observations we concluded that we could
safely estimate the time
taken by each semicircle, within a limit of error of at most 5 minutes.
Although the rate of movement in different parts of the same revolution
varied greatly, yet 22 semicircles to the light were completed, each on
an average in 73.95 minutes; and 22 semicircles from the light each in
73.5 minutes. It may, therefore, be said that they travelled to and
from the light at exactly the same average rate; though probably the
accuracy of the result was in part accidental. In the evening the stems
were not in the least deflected towards the window. Nevertheless, there
appears to exist a vestige of heliotropism, for with 6 out of the 7
plants, the first semicircle from the light, described in the early
morning after they had been subjected to darkness during the night and
thus probably rendered more sensitive, required rather more time, and
the first semicircle to the light considerably less time, than the
average. Thus with all 7 plants, taken together, the mean time of the
first semicircle in the morning from the light, was 76.8 minutes,
instead of 73.5 minutes, which is the mean of all the semicircles
during the day from the light; and the mean of the first semicircle to
the light was only 63.1, instead of 73.95 minutes, which was the mean
of all the semicircles during the day to the light.
[2] Some erroneous statements are unfortunately given on this subject,
in ‘The Movements and Habits of Climbing Plants,’ 1875, pp. 28, 32,
40, and 53. Conclusions were drawn from an insufficient number of
observations, for we did not then know at how unequal a rate the stems
and tendrils of climbing plants sometimes travel in different parts of
the same revolution.
Similar observations were made on Wistaria Sinensis, and the mean of 9
semicircles from the light was 117 minutes, and of 7 semicircles to the
light 122 minutes, and this difference does not exceed the probable
limit of error. During the three days of exposure, the shoot did not
become at all bent towards the window before which it stood. In this
case the first semicircle from the light in the early morning of each
day, required rather less time for its performance than did the first
semicircle to the light; and this result,
if not accidental, appears to indicate that the shoots retain a trace
of an original apheliotropic tendency. With Lonicera brachypoda the
semicircles from and to the light differed considerably in time; for 5
semicircles from the light required on a mean 202.4 minutes, and 4 to
the light, 229.5 minutes; but the shoot moved very irregularly, and
under these circumstances the observations were much too few.
It is remarkable that the same part on the same plant may be affected
by light in a widely different manner at different ages, and as it
appears at different seasons. The hypocotyledonous stems of Ipomœa
caerulea and purpurea are extremely heliotropic, whilst the stems of
older plants, only about a foot in height, are, as we have just seen,
almost wholly insensible to light. Sachs states (and we have observed
the same fact) that the hypocotyls of the Ivy (Hedera helix) are
slightly heliotropic; whereas the stems of plants grown to a few inches
in height become so strongly apheliotropic, that they bend at right
angles away from the light. Nevertheless, some young plants which had
behaved in this manner early in the summer again became distinctly
heliotropic in the beginning of September; and the zigzag courses of
their stems, as they slowly curved towards a north-east window, were
traced during 10 days. The stems of very young plants of Tropaeolum
majus are highly heliotropic, whilst those of older plants, according
to Sachs, are slightly apheliotropic. In all these cases the
heliotropism of the very young stems serves to expose the cotyledons,
or when the cotyledons are hypogean the first true leaves, fully to the
light; and the loss of this power by the older stems, or their becoming
apheliotropic, is connected with their habit of climbing.
Most seedling plants are strongly heliotropic, and
it is no doubt a great advantage to them in their struggle for life to
expose their cotyledons to the light as quickly and as fully as
possible, for the sake of obtaining carbon. It has been shown in the
first chapter that the greater number of seedlings circumnutate largely
and rapidly; and as heliotropism consists of modified circumnutation,
we are tempted to look at the high development of these two powers in
seedlings as intimately connected. Whether there are any plants which
circumnutate slowly and to a small extent, and yet are highly
heliotropic, we do not know; but there are several, and there is
nothing surprising in this fact, which circumnutate largely and are not
at all, or only slightly, heliotropic. Of such cases Drosera
rotundifolia offers an excellent instance. The stolons of the
strawberry circumnutate almost like the stems of climbing plants, and
they are not at all affected by a moderate light; but when exposed late
in the summer to a somewhat brighter light they were slightly
heliotropic; in sunlight, according to De Vries, they are
apheliotropic. Climbing plants circumnutate much more widely than any
other plants, yet they are not at all heliotropic.
Although the stems of most seedling plants are strongly heliotropic,
some few are but slightly heliotropic, without our being able to assign
any reason. This is the case with the hypocotyl of Cassia tora, and we
were struck with the same fact with some other seedlings, for instance,
those of Reseda odorata. With respect to the degree of sensitiveness of
the more sensitive kinds, it was shown in the last chapter that
seedlings of several species, placed before a north-east window
protected by several blinds, and exposed in the rear to the diffused
light of the room, moved with unerring certainty towards the window,
although
it was impossible to judge, excepting by the shadow cast by an upright
pencil on a white card, on which side most light entered, so that the
excess on one side must have been extremely small.
A pot with seedlings of Phalaris Canariensis, which had been raised in
darkness, was placed in a completely darkened room, at 12 feet from a
very small lamp. After 3 h. the cotyledons were doubtfully curved
towards the light, and after 7 h. 40 m. from the first exposure, they
were all plainly, though slightly, curved towards the lamp. Now, at
this distance of 12 feet, the light was so obscure that we could not
see the seedlings themselves, nor read the large Roman figures on the
white face of a watch, nor see a pencil line on paper, but could just
distinguish a line made with Indian ink. It is a more surprising fact
that no visible shadow was cast by a pencil held upright on a white
card; the seedlings, therefore, were acted on by a difference in the
illumination of their two sides, which the human eye could not
distinguish. On another occasion even a less degree of light acted, for
some cotyledons of Phalaris became slightly curved towards the same
lamp at a distance of 20 feet; at this distance we could not see a
circular dot 2.29 mm. (.09 inch) in diameter made with Indian ink on
white paper, though we could just see a dot 3.56 mm. (.14 inch) in
diameter; yet a dot of the former size appears large when seen in the
light.[3]
[3] Strasburger says (‘Wirkung des Lichtes auf Schwärmsporen,’ 1878,
p. 52), that the spores of Haematococcus moved to a light which only
just sufficed to allow middle-sized type to be read.
We next tried how small a beam of light would act; as this bears on
light serving as a guide to seedlings whilst they emerge through
fissured or encumbered ground. A pot with seedlings of Phalaris was
covered
by a tin-vessel, having on one side a circular hole 1.23 mm. in
diameter (i.e. a little less than the 1/20th of an inch); and the box
was placed in front of a paraffin lamp and on another occasion in front
of a window; and both times the seedlings were manifestly bent after a
few hours towards the little hole.
A more severe trial was now made; little tubes of very thin glass,
closed at their upper ends and coated with black varnish, were slipped
over the cotyledons of Phalaris (which had germinated in darkness) and
just fitted them. Narrow stripes of the varnish had been previously
scraped off one side, through which alone light could enter; and their
dimensions were afterwards measured under the microscope. As a control
experiment, similar unvarnished and transparent tubes were tried, and
they did not prevent the cotyledons bending towards the light. Two
cotyledons were placed before a south-west window, one of which was
illuminated by a stripe in the varnish, only .004 inch (0.1 mm.) in
breadth and .016 inch (0.4 mm.) in length; and the other by a stripe
.008 inch in breadth and .06 inch in length. The seedlings were
examined after an exposure of 7 h. 40 m., and were found to be
manifestly bowed towards the light. Some other cotyledons were at the
same time treated similarly, excepting that the little stripes were
directed not to the sky, but in such a manner that they received only
the diffused light from the room; and these cotyledons did not become
at all bowed. Seven other cotyledons were illuminated through narrow,
but comparatively long, cleared stripes in the varnish—namely, in
breadth between .01 and .026 inch, and in length between .15 and .3
inch; and these all became bowed to the side, by which light entered
through the stripes, whether these were directed towards the sky or to
one side of
the room. That light passing through a hole only .004 inch in breadth
by .016 in length, should induce curvature, seems to us a surprising
fact.
Before we knew how extremely sensitive the cotyledons of Phalaris were
to light, we endeavoured to trace their circumnutation in darkness by
the aid of a small wax taper, held for a minute or two at each
observation in nearly the same position, a little on the left side in
front of the vertical glass on which the tracing was made. The
seedlings were thus observed seventeen times in the course of the day,
at intervals of from half to three-quarters of an hour; and late in the
evening we were surprised to find that all the 29 cotyledons were
greatly curved and pointed towards the vertical glass, a little to the
left where the taper had been held. The tracings showed that they had
travelled in zigzag lines. Thus, an exposure to a feeble light for a
very short time at the above specified intervals, sufficed to induce
well-marked heliotropism. An analogous case was observed with the
hypocotyls of Solanum lycopersicum. We at first attributed this result
to the after-effects of the light on each occasion; but since reading
Wiesner’s observations,[4] which will be referred to in the last
chapter, we cannot doubt that an intermittent light is more efficacious
than a continuous one, as plants are especially sensitive to any
contrast in its amount.
[4] ‘Sitz. der k. Akad. der Wissensch.’ (Vienna), Jan. 1880, p. 12.
The cotyledons of Phalaris bend much more slowly towards a very obscure
light than towards a bright one. Thus, in the experiments with
seedlings placed in a dark room at 12 feet from a very small lamp, they
were just perceptibly and doubtfully curved towards it after 3 h., and
only slightly, yet certainly, after 4 h.
After 8 h. 40 m. the chords of their arcs were deflected from the
perpendicular by an average angle of only 16°. Had the light been
bright, they would have become much more curved in between 1 and 2 h.
Several trials were made with seedlings placed at various distances
from a small lamp in a dark room; but we will give only one trial. Six
pots were placed at distances of 2, 4, 8, 12, 16, and 20 feet from the
lamp, before which they were left for 4 h. As light decreases in a
geometrical ratio, the seedlings in the 2nd pot received 1/4th, those
in the 3rd pot 1/16th, those in the 4th 1/36th, those in the 5th
1/64th, and those in the 6th 1/100th of the light received by the
seedlings in the first or nearest pot. Therefore it might have been
expected that there would have been an immense difference in the degree
of their heliotropic curvature in the several pots; and there was a
well-marked difference between those which stood nearest and furthest
from the lamp, but the difference in each successive pair of pots was
extremely small. In order to avoid prejudice, we asked three persons,
who knew nothing about the experiment, to arrange the pots in order
according to the degree of curvature of the cotyledons. The first
person arranged them in proper order, but doubted long between the 12
feet and 16 feet pots; yet these two received light in the proportion
of 36 to 64. The second person also arranged them properly, but doubted
between the 8 feet and 12 feet pots, which received light in the
proportion of 16 to 36. The third person arranged them in wrong order,
and doubted about four of the pots. This evidence shows conclusively
how little the curvature of the seedlings differed in the successive
pots, in comparison with the great difference in the amount of light
which they received; and it should be noted that there was no
excess of superfluous light, for the cotyledons became but little and
slowly curved even in the nearest pot. Close to the 6th pot, at the
distance of 20 feet from the lamp, the light allowed us just to
distinguish a dot 3.56 mm. (.14 inch) in diameter, made with Indian ink
on white paper, but not a dot 2.29 mm. (.09 inch) in diameter.
The degree of curvature of the cotyledons of Phalaris within a given
time, depends not merely on the amount of lateral light which they may
then receive, but on that which they have previously received from
above and on all sides. Analogous facts have been given with respect to
the nyctitropic and periodic movements of plants. Of two pots
containing seedlings of Phalaris which had germinated in darkness, one
was still kept in the dark, and the other was exposed (Sept. 26th) to
the light in a greenhouse during a cloudy day and on the following
bright morning. On this morning (27th), at 10.30 A.M., both pots were
placed in a box, blackened within and open in front, before a
north-east window, protected by a linen and muslin blind and by a
towel, so that but little light was admitted, though the sky was
bright. Whenever the pots were looked at, this was done as quickly as
possible, and the cotyledons were then held transversely with respect
to the light, so that their curvature could not have been thus
increased or diminished. After 50 m. the seedlings which had previously
been kept in darkness, were perhaps, and after 70 m. were certainly,
curved, though very slightly, towards the window. After 85 m. some of
the seedlings, which had previously been illuminated, were perhaps a
little affected, and after 100 m. some of the younger ones were
certainly a little curved towards the light. At this time (i.e. after
100 m.) there was a plain difference
in the curvature of the seedlings in the two pots. After 2 h. 12 m. the
chords of the arcs of four of the most strongly curved seedlings in
each pot were measured, and the mean angle from the perpendicular of
those which had previously been kept in darkness was 19°, and of those
which had previously been illuminated only 7°. Nor did this difference
diminish during two additional hours. As a check, the seedlings in both
pots were then placed in complete darkness for two hours, in order that
apogeotropism should act on them; and those in the one pot which were
little curved became in this time almost completely upright, whilst the
more curved ones in the other pot still remained plainly curved.
Two days afterwards the experiment was repeated, with the sole
difference that even less light was admitted through the window, as it
was protected by a linen and muslin blind and by two towels; the sky,
moreover, was somewhat less bright. The result was the same as before,
excepting that everything occurred rather slower. The seedlings which
had been previously kept in darkness were not in the least curved after
54 m., but were so after 70 m. Those which had previously been
illuminated were not at all affected until 130 m. had elapsed, and then
only slightly. After 145 m. some of the seedlings in this latter pot
were certainly curved towards the light; and there was now a plain
difference between the two pots. After 3 h. 45 m. the chords of the
arcs of 3 seedlings in each pot were measured, and the mean angle from
the perpendicular was 16° for those in the pot which had previously
been kept in darkness, and only 5° for those which had previously been
illuminated.
The curvature of the cotyledons of Phalaris towards a lateral light is
therefore certainly influenced by the
degree to which they have been previously illuminated. We shall
presently see that the influence of light on their bending continues
for a short time after the light has been extinguished. These facts, as
well as that of the curvature not increasing or decreasing in nearly
the same ratio with that of the amount of light which they receive, as
shown in the trials with the plants before the lamp, all indicate that
light acts on them as a stimulus, in somewhat the same manner as on the
nervous system of animals, and not in a direct manner on the cells or
cell-walls which by their contraction or expansion cause the curvature.
It has already been incidentally shown how slowly the cotyledons of
Phalaris bend towards a very dim light; but when they were placed
before a bright paraffin lamp their tips were all curved rectangularly
towards it in 2 h. 20 m. The hypocotyls of Solanum lycopersicum had
bent in the morning at right angles towards a north-east window. At 1
P.M. (Oct. 21st) the pot was turned round, so that the seedlings now
pointed from the light, but by 5 P.M. they had reversed their curvature
and again pointed to the light. They had thus passed through 180° in 4
h., having in the morning previously passed through about 90°. But the
reversal of the first half of the curvature will have been aided by
apogeotropism. Similar cases were observed with other seedlings, for
instance, with those of Sinapis alba.
We attempted to ascertain in how short a time light acted on the
cotyledons of Phalaris, but this was difficult on account of their
rapid circumnutating movement; moreover, they differ much in
sensibility, according to age; nevertheless, some of our observations
are worth giving. Pots with seedlings were
placed under a microscope provided with an eye-piece micrometer, of
which each division equalled 1/500th of an inch (0.051 mm.); and they
were at first illuminated by light from a paraffin lamp passing through
a solution of bichromate of potassium, which does not induce
heliotropism. Thus the direction in which the cotyledons were
circumnutating could be observed independently of any action from the
light; and they could be made, by turning round the pots, to
circumnutate transversely to the line in which the light would strike
them, as soon as the solution was removed. The fact that the direction
of the circumnutating movement might change at any moment, and thus the
plant might bend either towards or from the lamp independently of the
action of the light, gave an element of uncertainty to the results.
After the solution had been removed, five seedlings which were
circumnutating transversely to the line of light, began to move towards
it, in 6, 4, 7½, 6, and 9 minutes. In one of these cases, the apex of
the cotyledon crossed five of the divisions of the micrometer (i.e.
1/100th of an inch, or 0.254 mm.) towards the light in 3 m. Of two
seedlings which were moving directly from the light at the time when
the solution was removed, one began to move towards it in 13 m., and
the other in 15 m. This latter seedling was observed for more than an
hour and continued to move towards the light; it crossed at one time 5
divisions of the micrometer (0.254 mm.) in 2 m. 30 s. In all these
cases, the movement towards the light was extremely unequal in rate,
and the cotyledons often remained almost stationary for some minutes,
and two of them retrograded a little. Another seedling which was
circumnutating transversely to the line of light, moved towards it in 4
m. after the solution was removed; it then remained
almost stationary for 10 m.; then crossed 5 divisions of the micrometer
in 6 m.; and then 8 divisions in 11m. This unequal rate of movement,
interrupted by pauses, and at first with occasional retrogressions,
accords well with our conclusion that heliotropism consists of modified
circumnutation.
In order to observe how long the after-effects of light lasted, a pot
with seedlings of Phalaris, which had germinated in darkness, was
placed at 10.40 A.M. before a north-east window, being protected on all
other sides from the light; and the movement of a cotyledon was traced
on a horizontal glass. It circumnutated about the same space for the
first 24 m., and during the next 1 h. 33 m. moved rapidly towards the
light. The light was now (i.e. after 1 h. 57 m.) completely excluded,
but the cotyledon continued bending in the same direction as before,
certainly for more than 15 m., probably for about 27 m. The doubt arose
from the necessity of not looking at the seedlings often, and thus
exposing them, though momentarily, to the light. This same seedling was
now kept in the dark, until 2.18 P.M., by which time it had reacquired
through apogeotropism its original upright position, when it was again
exposed to the light from a clouded sky. By 3 P.M. it had moved a very
short distance towards the light, but during the next 45 m. travelled
quickly towards it. After this exposure of 1 h. 27 m. to a rather dull
sky, the light was again completely excluded, but the cotyledon
continued to bend in the same direction as before for 14 m. within a
very small limit of error. It was then placed in the dark, and it now
moved backwards, so that after 1 h. 7 m. it stood close to where it had
started from at 2.18 P.M. These observations show that the cotyledons
of Phalaris, after being exposed to a lateral
light, continue to bend in the same direction for between a quarter and
half an hour.
In the two experiments just given, the cotyledons moved backwards or
from the window shortly after being subjected to darkness; and whilst
tracing the circumnutation of various kinds of seedlings exposed to a
lateral light, we repeatedly observed that late in the evening, as the
light waned, they moved from it. This fact is shown in some of the
diagrams given in the last chapter. We wished therefore to learn
whether this was wholly due to apogeotropism, or whether an organ after
bending towards the light tended from any other cause to bend from it,
as soon as the light failed. Accordingly, two pots of seedling Phalaris
and one pot of seedling Brassica were exposed for 8 h. before a
paraffin lamp, by which time the cotyledons of the former and the
hypocotyls of the latter were bent rectangularly towards the light. The
pots were now quickly laid horizontally, so that the upper parts of the
cotyledons and of the hypocotyls of 9 seedlings projected vertically
upwards, as proved by a plumb-line. In this position they could not be
acted on by apogeotropism, and if they possessed any tendency to
straighten themselves or to bend in opposition to their former
heliotropic curvature, this would be exhibited, for it would be opposed
at first very slightly by apogeotropism. They were kept in the dark for
4 h., during which time they were twice looked at; but no uniform
bending in opposition to their former heliotropic curvature could be
detected. We have said uniform bending, because they circumnutated in
their new position, and after 2 h. were inclined in different
directions (between 4° and 11°) from the perpendicular. Their
directions were also changed after two additional hours, and again on
the following morning. We may
therefore conclude that the bending back of plants from a light, when
this becomes obscure or is extinguished, is wholly due to
apogeotropism.[5]
[5] It appears from a reference in Wiesner (‘Die Undulirende Nutation
der Internodien,’ p. 7), that H. Müller of Thurgau found that a stem
which is bending heliotropically is at the same time striving, through
apogeotropism, to raise itself into a vertical position.
In our various experiments we were often struck with the accuracy with
which seedlings pointed to a light although of small size. To test
this, many seedlings of Phalaris, which had germinated in darkness in a
very narrow box several feet in length, were placed in a darkened room
near to and in front of a lamp having a small cylindrical wick. The
cotyledons at the two ends and in the central part of the box, would
therefore have to bend in widely different directions in order to point
to the light. After they had become rectangularly bent, a long white
thread was stretched by two persons, close over and parallel, first to
one and then to another cotyledon; and the thread was found in almost
every case actually to intersect the small circular wick of the now
extinguished lamp. The deviation from accuracy never exceeded, as far
as we could judge, a degree or two. This extreme accuracy seems at
first surprising, but is not really so, for an upright cylindrical
stem, whatever its position may be with respect to the light, would
have exactly half its circumference illuminated and half in shadow; and
as the difference in illumination of the two sides is the exciting
cause of heliotropism, a cylinder would naturally bend with much
accuracy towards the light. The cotyledons, however, of Phalaris are
not cylindrical, but oval in section; and the longer axis was to the
shorter axis (in the one which was measured) as 100 to 70.
Nevertheless, no difference could be
detected in the accuracy of their bending, whether they stood with
their broad or narrow sides facing the light, or in any intermediate
position; and so it was with the cotyledons of Avena sativa, which are
likewise oval in section. Now, a little reflection will show that in
whatever position the cotyledons may stand, there will be a line of
greatest illumination, exactly fronting the light, and on each side of
this line an equal amount of light will be received; but if the oval
stands obliquely with respect to the light, this will be diffused over
a wider surface on one side of the central line than on the other. We
may therefore infer that the same amount of light, whether diffused
over a wider surface or concentrated on a smaller surface, produces
exactly the same effect; for the cotyledons in the long narrow box
stood in all sorts of positions with reference to the light, yet all
pointed truly towards it.
That the bending of the cotyledons to the light depends on the
illumination of one whole side or on the obscuration of the whole
opposite side, and not on a narrow longitudinal zone in the line of the
light being affected, was shown by the effects of painting
longitudinally with Indian ink one side of five cotyledons of Phalaris.
These were then placed on a table near to a south-west window, and the
painted half was directed either to the right or left. The result was
that instead of bending in a direct line towards the window, they were
deflected from the window and towards the unpainted side, by the
following angles, 35°, 83°, 31°, 43°, and 39°. It should be remarked
that it was hardly possible to paint one-half accurately, or to place
all the seedlings which are oval in section in quite the same position
relatively to the light; and this will account for the differences in
the angles. Five
cotyledons of Avena were also painted in the same manner, but with
greater care; and they were laterally deflected from the line of the
window, towards the unpainted side, by the following angles, 44°, 44°,
55°, 51°, and 57°. This deflection of the cotyledons from the window is
intelligible, for the whole unpainted side must have received some
light, whereas the opposite and painted side received none; but a
narrow zone on the unpainted side directly in front of the window will
have received most light, and all the hinder parts (half an oval in
section) less and less light in varying degrees; and we may conclude
that the angle of deflection is the resultant of the action of the
light over the whole of the unpainted side.
It should have been premised that painting with Indian ink does not
injure plants, at least within several hours; and it could injure them
only by stopping respiration. To ascertain whether injury was thus soon
caused, the upper halves of 8 cotyledons of Avena were thickly coated
with transparent matter,—4 with gum, and 4 with gelatine; they were
placed in the morning before a window, and by the evening they were
normally bowed towards the light, although the coatings now consisted
of dry crusts of gum and gelatine. Moreover, if the seedlings which
were painted longitudinally with Indian ink had been injured on the
painted side, the opposite side would have gone on growing, and they
would consequently have become bowed towards the painted side; whereas
the curvature was always, as we have seen, in the opposite direction,
or towards the unpainted side which was exposed to the light. We
witnessed the effects of injuring longitudinally one side of the
cotyledons of Avena and Phalaris; for before we knew that grease was
highly injurious to them, several were painted down one side
with a mixture of oil and lamp-black, and were then exposed before a
window; others similarly treated were afterwards tried in darkness.
These cotyledons soon became plainly bowed towards the blackened side,
evidently owing to the grease on this side having checked their growth,
whilst growth continued on the opposite side. But it deserves notice
that the curvature differed from that caused by light, which ultimately
becomes abrupt near the ground. These seedlings did not afterwards die,
but were much injured and grew badly.
LOCALISED SENSITIVENESS TO LIGHT, AND ITS TRANSMITTED EFFECTS.
Phalaris Canariensis.—Whilst observing the accuracy with which the
cotyledons of this plant became bent towards the light of a small lamp,
we were impressed with the idea that the uppermost part determined the
direction of the curvature of the lower part. When the cotyledons are
exposed to a lateral light, the upper part bends first, and afterwards
the bending gradually extends down to the base, and, as we shall
presently see, even a little beneath the ground. This holds good with
cotyledons from less than .1 inch (one was observed to act in this
manner which was only .03 in height) to about .5 of an inch in height;
but when they have grown to nearly an inch in height, the basal part,
for a length of .15 to .2 of an inch above the ground, ceases to bend.
As with young cotyledons the lower part goes on bending, after the
upper part has become well arched towards a lateral light, the apex
would ultimately point to the ground instead of to the light, did not
the upper part reverse its curvature and straighten itself, as
soon as the upper convex surface of the bowed-down portion received
more light than the lower concave surface. The position ultimately
assumed by young and upright cotyledons, exposed to light entering
obliquely from above through a window, is shown in the accompanying
figure (Fig. 181); and here it may be seen that the whole upper part
has become very nearly straight. When the cotyledons were exposed
before a bright lamp, standing on the same level with them, the upper
part, which was at first greatly arched towards the light, became
straight and strictly parallel with the surface of the soil in the
pots; the basal part being now rectangularly bent. All this great
amount of curvature, together with the subsequent straightening of the
upper part, was often effected in a few hours.
Fig. 181. Phalaris Canariensis: cotyledons after exposure in a box open
on one side in front of a south-west window during 8 h. Curvature
towards the light accurately traced. The short horizontal lines show
the level of the ground.
After the uppermost part has become bowed a little to the light, its
overhanging weight must tend to increase the curvature of the lower
part; but any such effect was shown in several ways to be quite
insignificant. When little caps of tin-foil (hereafter to be described)
were placed on the summits of the cotyledons, though this must have
added considerably to their weight, the rate or amount of bending was
not thus increased. But the best evidence was afforded by placing pots
with seedlings of Phalaris before a lamp in such a position, that the
cotyledons were horizontally extended and projected at right angles to
the line of light. In the course of 5½ h. they were directed towards
the light with their bases bent at right angles; and this abrupt
curvature could not have been aided in the least by the weight of the
upper part, which acted at right angles to the plane of curvature.
It will be shown that when the upper halves of the cotyledons of
Phalaris and Avena were enclosed in little pipes of tin-foil or of
blackened glass, in which case the upper part was mechanically
prevented from bending, the lower and unenclosed part did not bend when
exposed to a lateral light; and it occurred to us that this fact might
be due, not to the exclusion of the light from the upper part, but to
some necessity of the bending gradually travelling down the cotyledons,
so that unless the upper part first became bent, the lower could not
bend, however much it might be stimulated. It was necessary for our
purpose to ascertain whether this notion was true, and it was proved
false; for the lower halves of several cotyledons became bowed to the
light, although their upper halves were enclosed in little glass tubes
(not blackened), which prevented, as far as we could judge, their
bending. Nevertheless, as the part within the tube might possibly bend
a very little, fine rigid rods or flat splinters of thin glass were
cemented with shellac to one side of the upper part of 15 cotyledons;
and in six cases they were in addition tied on with threads. They were
thus forced to remain quite straight. The result was that the lower
halves of all became bowed to the light, but generally not in so great
a degree as the corresponding part of the free seedlings in the same
pots; and this may perhaps be accounted for by some slight degree of
injury having been caused by a considerable surface having been smeared
with shellac. It may be added, that when the cotyledons of Phalaris and
Avena are acted on by apogeotropism, it is the upper part which begins
first to bend; and when this part was rendered rigid in the manner just
described, the upward curvature of the basal part was not thus
prevented.
To test our belief that the upper part of the cotyledons of Phalaris,
when exposed to a lateral light, regulates the bending of the lower
part, many experiments were tried; but most of our first attempts
proved useless from various causes not worth specifying. Seven
cotyledons had their tips cut off for lengths varying between .1 and
.16 of an inch, and these, when left exposed all day to a lateral
light, remained upright. In another set of 7 cotyledons, the tips were
cut off for a length of only about .05 of an inch (1.27 mm.) and these
became bowed towards
a lateral light, but not nearly so much as the many other seedlings in
the same pots. This latter case shows that cutting off the tips does
not by itself injure the plants so seriously as to prevent
heliotropism; but we thought at the time, that such injury might follow
when a greater length was cut off, as in the first set of experiments.
Therefore, no more trials of this kind were made, which we now regret;
as we afterwards found that when the tips of three cotyledons were cut
off for a length of .2 inch, and of four others for lengths of .14,
.12, .1, and .07 inch, and they were extended horizontally, the
amputation did not interfere in the least with their bending vertically
upwards, through the action of apogeotropism, like unmutilated
specimens. It is therefore extremely improbable that the amputation of
the tips for lengths of from .1 to .14 inch, could from the injury thus
caused have prevented the lower part from bending towards the light.
We next tried the effects of covering the upper part of the cotyledons
of Phalaris with little caps which were impermeable to light; the whole
lower part being left fully exposed before a south-west window or a
bright paraffin lamp. Some of the caps were made of extremely thin
tin-foil blackened within; these had the disadvantage of occasionally,
though rarely, being too heavy, especially when twice folded. The basal
edges could be pressed into close contact with the cotyledons; though
this again required care to prevent injuring them. Nevertheless, any
injury thus caused could be detected by removing the caps, and trying
whether the cotyledons were then sensitive to light. Other caps were
made of tubes of the thinnest glass, which when painted black served
well, with the one great disadvantage that the lower ends could not be
closed. But tubes were used which fitted the cotyledons almost closely,
and black paper was placed on the soil round each, to check the upward
reflection of light from the soil. Such tubes were in one respect far
better than caps of tin-foil, as it was possible to cover at the same
time some cotyledons with transparent and others with opaque tubes; and
thus our experiments could be controlled. It should be kept in mind
that young cotyledons were selected for trial, and that these when not
interfered with become bowed down to the ground towards the light.
We will begin with the glass-tubes. The summits of nine cotyledons,
differing somewhat in height, were enclosed for rather less than half
their lengths in uncoloured or transparent
tubes; and these were then exposed before a south-west window on a
bright day for 8 h. All of them became strongly curved towards the
light, in the same degree as the many other free seedlings in the same
pots; so that the glass-tubes certainly did not prevent the cotyledons
from bending towards the light. Nineteen other cotyledons were, at the
same time, similarly enclosed in tubes thickly painted with Indian ink.
On five of them, the paint, to our surprise, contracted after exposure
to the sunlight, and very narrow cracks were formed, through which a
little light entered; and these five cases were rejected. Of the
remaining 14 cotyledons, the lower halves of which had been fully
exposed to the light for the whole time, 7 continued quite straight and
upright; 1 was considerably bowed to the light, and 6 were slightly
bowed, but with the exposed bases of most of them almost or quite
straight. It is possible that some light may have been reflected
upwards from the soil and entered the bases of these 7 tubes, as the
sun shone brightly, though bits of blackened paper had been placed on
the soil round them. Nevertheless, the 7 cotyledons which were slightly
bowed, together with the 7 upright ones, presented a most remarkable
contrast in appearance with the many other seedlings in the same pots
to which nothing had been done. The blackened tubes were then removed
from 10 of these seedlings, and they were now exposed before a lamp for
8 h.; 9 of them became greatly, and 1 moderately, curved towards the
light, proving that the previous absence of any curvature in the basal
part, or the presence of only a slight degree of curvature there, was
due to the exclusion of light from the upper part.
Similar observations were made on 12 younger cotyledons with their
upper halves enclosed within glass-tubes coated with black varnish, and
with their lower halves fully exposed to bright sunshine. In these
younger seedlings the sensitive zone seems to extend rather lower down,
as was observed on some other occasions, for two became almost as much
curved towards the light as the free seedlings; and the remaining ten
were slightly curved, although the basal part of several of them, which
normally becomes more curved than any other part, exhibited hardly a
trace of curvature. These 12 seedlings taken together differed greatly
in their degree of curvature from all the many other seedlings in the
same pots.
Better evidence of the efficiency of the blackened tubes was
incidentally afforded by some experiments hereafter to be given,
in which the upper halves of 14 cotyledons were enclosed in tubes from
which an extremely narrow stripe of the black varnish had been scraped
off. These cleared stripes were not directed towards the window, but
obliquely to one side of the room, so that only a very little light
could act on the upper halves of the cotyledons. These 14 seedlings
remained during eight hours of exposure before a south-west window on a
hazy day quite upright; whereas all the other many free seedlings in
the same pots became greatly bowed towards the light.
We will now turn to the trials with caps made of very thin tin-foil.
These were placed at different times on the summits of 24 cotyledons,
and they extended down for a length of between .15 and .2 of an inch.
The seedlings were exposed to a lateral light for periods varying
between 6 h. 30 m. and 7 h. 45 m., which sufficed to cause all the
other seedlings in the same pots to become almost rectangularly bent
towards the light. They varied in height from only .04 to 1.15 inch,
but the greater number were about .75 inch. Of the 24 cotyledons with
their summits thus protected, 3 became much bent, but not in the
direction of the light, and as they did not straighten themselves
through apogeotropism during the following night, either the caps were
too heavy or the plants themselves were in a weak condition; and these
three cases may be excluded. There are left for consideration 21
cotyledons; of these 17 remained all the time quite upright; the other
4 became slightly inclined to the light, but not in a degree comparable
with that of the many free seedlings in the same pots. As the
glass-tubes, when unpainted, did not prevent the cotyledons from
becoming greatly bowed, it cannot be supposed that the caps of very
thin tin-foil did so, except through the exclusion of the light. To
prove that the plants had not been injured, the caps were removed from
6 of the upright seedlings, and these were exposed before a paraffin
lamp for the same length of time as before, and they now all became
greatly curved towards the light.
As caps between .15 and .2 of an inch in depth were thus proved to be
highly efficient in preventing the cotyledons from bending towards the
light, 8 other cotyledons were protected with caps between only .06 and
.12 in depth. Of these, two remained vertical, one was considerably and
five slightly curved towards the light, but far less so than the free
seedlings in the same pots.
Another trial was made in a different manner, namely, by bandaging with
strips of tin-foil, about .2 in breadth, the upper part, but not the
actual summit, of eight moderately young seedlings a little over half
an inch in height. The summits and the basal parts were thus left fully
exposed to a lateral light during 8 h.; an upper intermediate zone
being protected. With four of these seedlings the summits were exposed
for a length of .05 inch, and in two of them this part became curved
towards the light, but the whole lower part remained quite upright;
whereas the entire length of the other two seedlings became slightly
curved towards the light. The summits of the four other seedlings were
exposed for a length of .04 inch, and of these one remained almost
upright, whilst the other three became considerably curved towards the
light. The many free seedlings in the same pots were all greatly curved
towards the light.
From these several sets of experiments, including those with the
glass-tubes, and those when the tips were cut off, we may infer that
the exclusion of light from the upper part of the cotyledons of
Phalaris prevents the lower part, though fully exposed to a lateral
light, from becoming curved. The summit for a length of .04 or .05 of
an inch, though it is itself sensitive and curves towards the light,
has only a slight power of causing the lower part to bend. Nor has the
exclusion of light from the summit for a length of .1 of an inch a
strong influence on the curvature of the lower part. On the other hand,
an exclusion for a length of between .15 and .2 of an inch, or of the
whole upper half, plainly prevents the lower and fully illuminated part
from becoming curved in the manner (see Fig. 181) which invariably
occurs when a free cotyledon is exposed to a lateral light. With very
young seedlings the sensitive zone seems to extend rather lower down
relatively to their height than in older seedlings. We must therefore
conclude that when seedlings are freely exposed to a lateral light some
influence is transmitted from the upper to the lower part, causing the
latter to bend.
This conclusion is supported by what may be seen to occur on a small
scale, especially with young cotyledons, without any artificial
exclusion of the light; for they bend beneath the earth where no light
can enter. Seeds of Phalaris were covered with a layer one-fourth of an
inch in thickness of very fine sand, consisting of extremely minute
grains of silex coated with
oxide of iron. A layer of this sand, moistened to the same degree as
that over the seeds, was spread over a glass-plate; and when the layer
was .05 of an inch in thickness (carefully measured) no light from a
bright sky could be seen to pass through it, unless it was viewed
through a long blackened tube, and then a trace of light could be
detected, but probably much too little to affect any plant. A layer .1
of an inch in thickness was quite impermeable to light, as judged by
the eye aided by the tube. It may be worth adding that the layer, when
dried, remained equally impermeable to light. This sand yielded to very
slight pressure whilst kept moist, and in this state did not contract
or crack in the least. In a first trial, cotyledons which had grown to
a moderate height were exposed for 8 h. before a paraffin lamp, and
they became greatly bowed. At their bases on the shaded side opposite
to the light, well-defined, crescentic, open furrows were formed, which
(measured under a microscope with a micrometer) were from .02 to .03 of
an inch in breadth, and these had evidently been left by the bending of
the buried bases of the cotyledons towards the light. On the side of
the light the cotyledons were in close contact with the sand, which was
a very little heaped up. By removing with a sharp knife the sand on one
side of the cotyledons in the line of the light, the bent portion and
the open furrows were found to extend down to a depth of about .1 of an
inch, where no light could enter. The chords of the short buried arcs
formed in four cases angles of 11°, 13°, 15°, and 18°, with the
perpendicular. By the following morning these short bowed portions had
straightened themselves through apogeotropism.
In the next trial much younger cotyledons were similarly treated, but
were exposed to a rather obscure lateral light. After some hours, a
bowed cotyledon, .3 inch in height, had an open furrow on the shaded
side .04 inch in breadth; another cotyledon, only .13 inch in height,
had left a furrow .02 inch in breadth. But the most curious case was
that of a cotyledon which had just protruded above the ground and was
only .03 inch in height, and this was found to be bowed in the
direction of the light to a depth of .2 of an inch beneath the surface.
From what we know of the impermeability of this sand to light, the
upper illuminated part in these several cases must have determined the
curvature of the lower buried portions. But an apparent cause of doubt
may be suggested: as the cotyledons are continually circumnutating,
they tend to form a minute
crack or furrow all round their bases, which would admit a little light
on all sides; but this would not happen when they were illuminated
laterally, for we know that they quickly bend towards a lateral light,
and they then press so firmly against the sand on the illuminated side
as to furrow it, and this would effectually exclude light on this side.
Any light admitted on the opposite and shaded side, where an open
furrow is formed, would tend to counteract the curvature towards the
lamp or other source of the light. It may be added, that the use of
fine moist sand, which yields easily to pressure, was indispensable in
the above experiments; for seedlings raised in common soil, not kept
especially damp, and exposed for 9 h. 30 m. to a strong lateral light,
did not form an open furrow at their bases on the shaded side, and were
not bowed beneath the surface.
Perhaps the most striking proof of the action of the upper on the lower
part of the cotyledons of Phalaris, when laterally illuminated, was
afforded by the blackened glass-tubes (before alluded to) with very
narrow stripes of the varnish scraped off on one side, through which a
little light was admitted. The breadth of these stripes or slits varied
between .01 and .02 inch (.25 and .51 mm.). Cotyledons with their upper
halves enclosed in such tubes were placed before a south-west window,
in such a position, that the scraped stripes did not directly face the
window, but obliquely to one side. The seedlings were left exposed for
8 h., before the close of which time the many free seedlings in the
same pots had become greatly bowed towards the window. Under these
circumstances, the whole lower halves of the cotyledons, which had
their summits enclosed in the tubes, were fully exposed to the light of
the sky, whilst their upper halves received exclusively or chiefly
diffused light from the room, and this only through a very narrow slit
on one side. Now, if the curvature of the lower part had been
determined by the illumination of this part, all the cotyledons
assuredly would have become curved towards the window; but this was far
from being the case. Tubes of the kind just described were placed on
several occasions over the upper halves of 27 cotyledons; 14 of them
remained all the time quite vertical; so that sufficient diffused light
did not enter through the narrow slits to produce any effect whatever;
and they behaved in the same manner as if their upper halves had been
enclosed in completely blackened tubes. The lower halves of the 13
other cotyledons became bowed
not directly in the line of the window, but obliquely towards it; one
pointed at an angle of only 18°, but the remaining 12 at angles varying
between 45° and 62° from the line of the window. At the commencement of
the experiment, pins had been laid on the earth in the direction
towards which the slits in the varnish faced; and in this direction
alone a small amount of diffused light entered. At the close of the
experiment, 7 of the bowed cotyledons pointed exactly in the line of
the pins, and 6 of them in a line between that of the pins and that of
the window. This intermediate position is intelligible, for any light
from the sky which entered obliquely through the slits would be much
more efficient than the diffused light which entered directly through
them. After the 8 h. exposure, the contrast in appearance between these
13 cotyledons and the many other seedlings in the same pots, which were
all (excepting the above 14 vertical ones) greatly bowed in straight
and parallel lines towards the window, was extremely remarkable. It is
therefore certain that a little weak light striking the upper halves of
the cotyledons of Phalaris, is far more potent in determining the
direction of the curvature of the lower halves, than the full
illumination of the latter during the whole time of exposure.
In confirmation of the above results, the effect of thickly painting
with Indian ink one side of the upper part of three cotyledons of
Phalaris, for a length of .2 inch from their tips, may be worth giving.
These were placed so that the unpainted surface was directed not
towards the window, but a little to one side; and they all became bent
towards the unpainted side, and from the line of the window by angles
amounting to 31°, 35°, and 83°. The curvature in this direction
extended down to their bases, although the whole lower part was fully
exposed to the light from the window.
Finally, although there can be no doubt that the illumination of the
upper part of the cotyledons of Phalaris greatly affects the power and
manner of bending of the lower part, yet some observations seemed to
render it probable that the simultaneous stimulation of the lower part
by light greatly favours, or is almost necessary, for its well-marked
curvature; but our experiments were not conclusive, owing to the
difficulty of excluding light from the lower halves without
mechanically preventing their curvature.
Avena sativa.—The cotyledons of this plant become quickly bowed towards
a lateral light, exactly like those of Phalaris.
Experiments similar to the foregoing ones were tried, and we will give
the results as briefly as possible. They are somewhat less conclusive
than in the case of Phalaris, and this may possibly be accounted for by
the sensitive zone varying in extension, in a species so long
cultivated and variable as the common Oat. Cotyledons a little under
three-quarters of an inch in height were selected for trial: six had
their summits protected from light by tin-foil caps, .25 inch in depth,
and two others by caps .3 inch in depth. Of these 8 cotyledons, five
remained upright during 8 hours of exposure, although their lower parts
were fully exposed to the light all the time; two were very slightly,
and one considerably, bowed towards it. Caps only .2 or .22 inch in
depth were placed over 4 other cotyledons, and now only one remained
upright, one was slightly, and two considerably bowed to the light. In
this and the following cases all the free seedlings in the same pots
became greatly bowed to the light.
Our next trial was made with short lengths of thin and fairly
transparent quills; for glass-tubes of sufficient diameter to go over
the cotyledons would have been too heavy. Firstly, the summits of 13
cotyledons were enclosed in unpainted quills, and of these 11 became
greatly and 2 slightly bowed to the light; so that the mere act of
enclosure did not prevent the lower part from becoming bowed. Secondly,
the summits of 11 cotyledons were enclosed in quills .3 inch in length,
painted so as to be impermeable to light; of these, 7 did not become at
all inclined towards the light, but 3 of them were slightly bent more
or less transversely with respect to the line of light, and these might
perhaps have been altogether excluded; one alone was slightly bowed
towards the light. Painted quills, .25 inch in length, were placed over
the summits of 4 other cotyledons; of these, one alone remained
upright, a second was slightly bowed, and the two others as much bowed
to the light as the free seedlings in the same pots. These two latter
cases, considering that the caps were .25 in length, are inexplicable.
Lastly, the summits of 8 cotyledons were coated with flexible and
highly transparent gold-beaters’ skin, and all became as much bowed to
the light as the free seedlings. The summits of 9 other cotyledons were
similarly coated with gold-beaters’ skin, which was then painted to a
depth of between .25 and .3 inch, so as to be impermeable to light; of
these 5 remained upright, and 4 were well bowed to the light, almost or
quite as well as
the free seedlings. These latter four cases, as well as the two in the
last paragraph, offer a strong exception to the rule that the
illumination of the upper part determines the curvature of the lower
part. Nevertheless, 5 of these 8 cotyledons remained quite upright,
although their lower halves were fully illuminated all the time; and it
would almost be a prodigy to find five free seedlings standing
vertically after an exposure for several hours to a lateral light.
The cotyledons of Avena, like those of Phalaris, when growing in soft,
damp, fine sand, leave an open crescentric furrow on the shaded side,
after bending to a lateral light; and they become bowed beneath the
surface at a depth to which, as we know, light cannot penetrate. The
arcs of the chords of the buried bowed portions formed in two cases
angles of 20° and 21° with the perpendicular. The open furrows on the
shaded side were, in four cases, .008, .016, .024, and .024 of an inch
in breadth. Brassica oleracea (Common Red).—It will here be shown that
the upper half of the hypocotyl of the cabbage, when illuminated by a
lateral light, determines the curvature of the lower half. It is
necessary to experimentise on young seedlings about half an inch or
rather less in height, for when grown to an inch and upwards the basal
part ceases to bend. We first tried painting the hypocotyls with Indian
ink, or cutting off their summits for various lengths; but these
experiments are not worth giving, though they confirm, as far as they
can be trusted, the results of the following ones. These were made by
folding gold-beaters’ skin once round the upper halves of young
hypocotyls, and painting it thickly with Indian ink or with black
grease. As a control experiment, the same transparent skin, left
unpainted, was folded round the upper halves of 12 hypocotyls; and
these all became greatly curved to the light, excepting one, which was
only moderately curved. Twenty other young hypocotyls had the skin
round their upper halves painted, whilst their lower halves were left
quite uncovered. These seedlings were then exposed, generally for
between 7 and 8 h., in a box blackened within and open in front, either
before a south-west window or a paraffin lamp. This exposure was amply
sufficient, as was shown by the strongly-marked heliotropism of all the
free seedlings in the same pots; nevertheless, some were left exposed
to the light for a much longer time. Of the 20 hypocotyls thus treated,
14 remained quite upright, and 6 became slightly bowed to the light;
but 2 of these latter cases were not really
exceptions, for on removing the skin the paint was found imperfect and
was penetrated by many small transparent spaces on the side which faced
the light. Moreover, in two other cases the painted skin did not extend
quite halfway down the hypocotyl. Although there was a wonderful
contrast in the several pots between these 20 hypocotyls and the other
many free seedlings, which were all greatly bowed down to their bases
in the direction of the light, some being almost prostrate on the
ground.
The most successful trial on any one day (included in the above
results) is worth describing in detail. Six young seedlings were
selected, the hypocotyls of which were nearly .45 inch, excepting one,
which was .6 inch in height, measured from the bases of their petioles
to the ground. Their upper halves, judged as accurately as could be
done by the eye, were folded once round with gold-beaters’ skin, and
this was painted thickly with Indian ink. They were exposed in an
otherwise darkened room before a bright paraffin lamp, which stood on a
level with the two pots containing the seedlings. They were first
looked at after an interval of 5 h. 10 m., and five of the protected
hypocotyls were found quite erect, the sixth being very slightly
inclined to the light; whereas all the many free seedlings in the same
two pots were greatly bowed to the light. They were again examined
after a continuous exposure to the light of 20 h. 35m.; and now the
contrast between the two sets was wonderfully great; for the free
seedlings had their hypocotyls extended almost horizontally in the
direction of the light, and were curved down to the ground; whilst
those with the upper halves protected by the painted skin, but with
their lower halves fully exposed to the light, still remained quite
upright, with the exception of the one which retained the same slight
inclination to the light which it had before. This latter seedling was
found to have been rather badly painted, for on the side facing the
light the red colour of the hypocotyl could be distinguished through
the paint.
We next tried nine older seedlings, the hypocotyls of which varied
between 1 and 1.6 inch in height. the gold-beaters’ skin round their
upper parts was painted with black grease to a depth of only .3 inch,
that is, from less than a third to a fourth or fifth of their total
heights. They were exposed to the light for 7 h. 15 m.; and the result
showed that the whole of the sensitive zone, which determines the
curvature of the lower
part, was not protected from the action of the light; for all 9 became
curved towards it, 4 of them very slightly, 3 moderately, and 2 almost
as much as the unprotected seedlings. Nevertheless, the whole 9 taken
together differed plainly in their degree of curvature from the many
free seedlings, and from some which were wrapped in unpainted skin,
growing in the same two pots.
Seeds were covered with about a quarter of an inch of the fine sand
described under Phalaris; and when the hypocotyls had grown to a height
of between .4 and .55 inch, they were exposed during 9 h. before a
paraffin lamp, their bases being at first closely surrounded by the
damp sand. They all became bowed down to the ground, so that their
upper parts lay near to and almost parallel to the surface of the soil.
On the side of the light their bases were in close contact with the
sand, which was here a very little heaped up; on the opposite or shaded
side there were open, crescentic cracks or furrows, rather above .01 of
an inch in width; but they were not so sharp and regular as those made
by Phalaris and Avena, and therefore could not be so easily measured
under the microscope. The hypocotyls were found, when the sand was
removed on one side, to be curved to a depth beneath the surface in
three cases of at least .1 inch, in a fourth case of .11, and in a
fifth of .15 inch. The chords of the arcs of the short, buried, bowed
portions formed angles of between 11° and 15° with the perpendicular.
From what we have seen of the impermeability of this sand to light, the
curvature of the hypocotyls certainly extended down to a depth where no
light could enter; and the curvature must have been caused by an
influence transmitted from the upper illuminated part.
The lower halves of five young hypocotyls were surrounded by unpainted
gold-beaters’ skin, and these, after an exposure of 8 h. before a
paraffin lamp, all became as much bowed to the light as the free
seedlings. The lower halves of 10 other young hypocotyls, similarly
surrounded with the skin, were thickly painted with Indian ink; their
upper and unprotected halves became well curved to the light, but their
lower and protected halves remained vertical in all the cases excepting
one, and on this the layer of paint was imperfect. This result seems to
prove that the influence transmitted from the upper part is not
sufficient to cause the lower part to bend, unless it be at the same
time illuminated; but there remains the doubt, as in
the case of Phalaris, whether the skin covered with a rather thick
crust of dry Indian ink did not mechanically prevent their curvature.
Beta vulgaris.—A few analogous experiments were tried on this plant,
which is not very well adapted for the purpose, as the basal part of
the hypocotyl, after it has grown to above half an inch in height, does
not bend much on exposure to a lateral light. Four hypocotyls were
surrounded close beneath their petioles with strips of thin tin-foil,
.2 inch in breadth, and they remained upright all day before a paraffin
lamp; two others were surrounded with strips .15 inch in breadth, and
one of these remained upright, the other becoming bowed; the bandages
in two other cases were only .1 inch in breadth, and both of these
hypocotyls became bowed, though one only slightly, towards the light.
The free seedlings in the same pots were all fairly well curved towards
the light; and during the following night became nearly upright. The
pots were now turned round and placed before a window, so that the
opposite sides of the seedlings were exposed to the light, towards
which all the unprotected hypocotyls became bent in the course of 7 h.
Seven out of the 8 seedlings with bandages of tin-foil remained
upright, but one which had a bandage only .1 inch in breadth, became
curved to the light. On another occasion, the upper halves of 7
hypocotyls were surrounded with painted gold-beaters’ skin; of these 4
remained upright, and 3 became a little curved to the light: at the
same time 4 other seedlings surrounded with unpainted skin, as well as
the free ones in the same pots, all became bowed towards the lamp,
before which they had been exposed during 22 hours.
Radicles of Sinapis alba.—The radicles of some plants are indifferent,
as far as curvature is concerned, to the action of light; whilst others
bend towards and others from it.[6] Whether these movements are of any
service to the plant is very doubtful, at least in the case of
subterranean roots; they probably result from the radicles being
sensitive to contact, moisture, and gravitation, and as a consequence
to other irritants which are never naturally encountered. The radicles
of Sinapis alba, when immersed in water and exposed to a lateral light,
bend from it, or are apheliotropic. They become bent for a length of
about 4 mm. from their tips. To ascertain whether this movement
generally occurred, 41 radicles, which had germinated in damp sawdust,
were immersed in water and exposed to a lateral light; and they all,
with two doubtful exceptions, became curved from the light. At the same
time the tips of 54 other radicles, similarly exposed, were just
touched with nitrate of silver. They were blackened for a length of
from .05 to .07 mm., and probably killed; but it should be observed
that this did not check materially, if at all, the growth of the upper
part; for several, which were measured, increased in the course of only
8–9 h. by 5 to 7 mm. in length. Of the 54 cauterised radicles one case
was doubtful, 25 curved themselves from the light in the normal manner,
and 28, or more than half, were not in the least apheliotropic. There
was a considerable difference, which we cannot account for, in the
results of the experiments tried towards the end of April and in the
middle of September. Fifteen radicles (part of the above 54) were
cauterised at the former period and were exposed to sunshine, of which
12 failed to be apheliotropic, 2 were still apheliotropic, and 1 was
doubtful. In September, 39 cauterised radicles were exposed to a
northern light, being kept at a proper temperature; and now 23
continued to be apheliotropic in the normal manner, and only 16 failed
to bend from the light. Looking at the aggregate results at both
periods, there can be no doubt that the destruction of the tip for less
than a millimeter in length destroyed in more than half the cases their
power of moving from the light. It is probable that if the tips had
been cauterised for the length of a whole millimeter, all signs of
apheliotropism would have disappeared. It may be suggested that
although the application of caustic does not stop growth, yet enough
may be absorbed to destroy the power of movement in the upper part; but
this suggestion must be rejected, for we have seen and shall again see,
that cauterising one side of the tip of various kinds of radicles
actually excites movement. The conclusion seems inevitable that
sensitiveness to light resides in the tip of the radicle of Sinapis
alba; and that the tip when thus stimulated transmits some influence to
the upper part, causing it to bend. The case in this respect is
parallel with that of the radicles of several plants, the tips of which
are sensitive to contact and to other irritants, and, as will be shown
in the eleventh chapter, to gravitation.
[6] Sachs, ‘Physiologie Végétale,’ 1868, p. 44.
CONCLUDING REMARKS AND SUMMARY OF CHAPTER.
We do not know whether it is a general rule with seedling plants that
the illumination of the upper part determines the curvature of the
lower part. But as this occurred in the four species examined by us,
belonging to such distinct families as the Gramineæ, Cruciferae, and
Chenopodeae, it is probably of common occurrence. It can hardly fail to
be of service to seedlings, by aiding them to find the shortest path
from the buried seed to the light, on nearly the same principle that
the eyes of most of the lower crawling animals are seated at the
anterior ends of their bodies. It is extremely doubtful whether with
fully developed plants the illumination of one part ever affects the
curvature of another part. The summits of 5 young plants of Asparagus
officinalis (varying in height between 1.1 and 2.7 inches, and
consisting of several short internodes) were covered with caps of
tin-foil from 0.3 to 0.35 inch in depth; and the lower uncovered parts
became as much curved towards a lateral light, as were the free
seedlings in the same pots. Other seedlings of the same plant had their
summits painted with Indian ink with the same negative result. Pieces
of blackened paper were gummed to the edges and over the blades of some
leaves on young plants of Tropaeolum majus and Ranunculus ficaria;
these were then placed in a box before a window, and the petioles of
the protected leaves became curved towards the light, as much as those
of the unprotected leaves.
The foregoing cases with respect to seedling plants have been fully
described, not only because the transmission of any effect from light
is a new physiological fact, but because we think it tends to modify
somewhat the current views on heliotropic movements. Until
lately such movements were believed to result simply from increased
growth on the shaded side. At present it is commonly admitted[7] that
diminished light increases the turgescence of the cells, or the
extensibility of the cell-walls, or of both together, on the shaded
side, and that this is followed by increased growth. But Pfeffer has
shown that a difference in the turgescence on the two sides of a
pulvinus,—that is, an aggregate of small cells which have ceased to
grow at an early age,—is excited by a difference in the amount of light
received by the two sides; and that movement is thus caused without
being followed by increased growth on the more turgescent side.[8] All
observers apparently believe that light acts directly on the part which
bends, but we have seen with the above described seedlings that this is
not the case. Their lower halves were brightly illuminated for hours,
and yet did not bend in the least towards the light, though this is the
part which under ordinary circumstances bends the most. It is a still
more striking fact, that the faint illumination of a narrow stripe on
one side of the upper part of the cotyledons of Phalaris determined the
direction of the curvature of the lower part; so that this latter part
did not bend towards the bright light by which it had been fully
illuminated,
but obliquely towards one side where only a little light entered. These
results seem to imply the presence of some matter in the upper part
which is acted on by light, and which transmits its effects to the
lower part. It has been shown that this transmission is independent of
the bending of the upper sensitive part. We have an analogous case of
transmission in Drosera, for when a gland is irritated, the basal and
not the upper or intermediate part of the tentacle bends. The flexible
and sensitive filament of Dionaea likewise transmits a stimulus,
without itself bending; as does the stem of Mimosa.
[7] Emil Godlewski has given (‘Bot. Zeitung,’ 1879, Nos. 6–9) an
excellent account (p. 120) of the present state of the question. See
also Vines in ‘Arbeiten des Bot. Inst. in Würzburg,’ 1878, B. ii. pp.
114–147. Hugo de Vries has recently published a still more important
article on this subject: ‘Bot Zeitung,’ Dec. 19th and 26th, 1879.
[8] ‘Die Periodischen Bewegungen der Blattorgane,’ 1875, pp. 7, 63,
123, etc. Frank has also insisted (‘Die Naturliche wägerechte Richtung
von Pflanzentheilen,’ 1870, p. 53) on the important part which the
pulvini of the leaflets of compound leaves play in placing the
leaflets in a proper position with respect to the light. This holds
good, especially with the leaves of climbing plants, which are carried
into all sorts of positions, ill-adapted for the action of the light.
Light exerts a powerful influence on most vegetable tissues, and there
can be no doubt that it generally tends to check their growth. But when
the two sides of a plant are illuminated in a slightly different
degree, it does not necessarily follow that the bending towards the
illuminated side is caused by changes in the tissues of the same nature
as those which lead to increased growth in darkness. We know at least
that a part may bend from the light, and yet its growth may not be
favoured by light. This is the case with the radicles of Sinapis alba,
which are plainly apheliotropic; nevertheless, they grow quicker in
darkness than in light.[9] So it is with many aërial roots, according
to Wiesner;[10] but there are other opposed cases. It appears,
therefore, that light does not determine the growth of apheliotropic
parts in any uniform manner.
[9] Francis Darwin, ‘Über das Wachsthum negativ heliotropischer
Wurzeln’: ‘Arbeiten des Bot. Inst. in Würzburg,’ B. ii., Heft iii.,
1880, p. 521.
[10] ‘Sitzb. der k. Akad. der Wissensch’ (Vienna), 1880, p. 12.
We should bear in mind that the power of bending to the light is highly
beneficial to most plants. There
is therefore no improbability in this power having been specially
acquired. In several respects light seems to act on plants in nearly
the same manner as it does on animals by means of the nervous
system.[11] With seedlings the effect, as we have just seen, is
transmitted from one part to another. An animal may be excited to move
by a very small amount of light; and it has been shown that a
difference in the illumination of the two sides of the cotyledons of
Phalaris, which could not be distinguished by the human eye, sufficed
to cause them to bend. It has also been shown that there is no close
parallelism between the amount of light which acts on a plant and its
degree of curvature; it was indeed hardly possible to perceive any
difference in the curvature of some seedlings of Phalaris exposed to a
light, which, though dim, was very much brighter than that to which
others had been exposed. The retina, after being stimulated by a bright
light, feels the effect for some time; and Phalaris continued to bend
for nearly half an hour towards the side which had been illuminated.
The retina cannot perceive a dim light after it has been exposed to a
bright one; and plants which had been kept in the daylight during the
previous day and morning, did not move so soon towards an obscure
lateral light as did others which had been kept in complete darkness.
[11] Sachs has made some striking remarks to the same effect with
respect to the various stimuli which excite movement in plants. See
his paper ‘Ueber orthotrope und plagiotrope Pflanzentheile,’ ‘Arb. des
Bot. Inst. in Würzburg,’ 1879, B. ii. p. 282.
Even if light does act in such a manner on the growing parts of plants
as always to excite in them a tendency to bend towards the more
illuminated side—a supposition contradicted by the foregoing
experiments on seedlings and by all apheliotropic
organs—yet the tendency differs greatly in different species, and is
variable in degree in the individuals of the same species, as may be
seen in almost any pot of seedlings of a long cultivated plant.[12]
There is therefore a basis for the modification of this tendency to
almost any beneficial extent. That it has been modified, we see in many
cases: thus, it is of more importance for insectivorous plants to place
their leaves in the best position for catching insects than to turn
their leaves to the light, and they have no such power. If the stems of
twining plants were to bend towards the light, they would often be
drawn away from their supports; and as we have seen they do not thus
bend. As the stems of most other plants are heliotropic, we may feel
almost sure that twining plants, which are distributed throughout the
whole vascular series, have lost a power that their non-climbing
progenitors possessed. Moreover, with Ipomœa, and probably all other
twiners, the stem of the young plant, before it begins to twine, is
highly heliotropic, evidently in order to expose the cotyledons or the
first true leaves fully to the light. With the Ivy the stems of
seedlings are moderately heliotropic, whilst those of the same plants
when grown a little older
are apheliotropic. Some tendrils which consist of modified
leaves—organs in all ordinary cases strongly diaheliotropic—have been
rendered apheliotropic, and their tips crawl into any dark crevice.
[12] Strasburger has shown in his interesting work (‘Wirkung des
Lichtes...auf Schwärmsporen,’ 1878), that the movement of the
swarm-spores of various lowly organised plants to a lateral light is
influenced by their stage of development, by the temperature to which
they are subjected, by the degree of illumination under which they
have been raised, and by other unknown causes; so that the
swarm-spores of the same species may move across the field of the
microscope either to or from the light. Some individuals, moreover,
appear to be indifferent to the light; and those of different species
behave very differently. The brighter the light, the straighter is
their course. They exhibit also for a short time the after-effects of
light. In all these respects they resemble the higher plants. See,
also, Stahl, ‘Ueber den einfluss der Lichts auf die
Bewegungs-erscheinungen der Schwärmsporen’ Verh. d. phys.-med.
Geselsshalft in Würzburg, B. xii. 1878.
Even in the case of ordinary heliotropic movements, it is hardly
credible that they result directly from the action of the light,
without any special adaptation. We may illustrate what we mean by the
hygroscopic movements of plants: if the tissues on one side of an organ
permit of rapid evaporation, they will dry quickly and contract,
causing the part to bend to this side. Now the wonderfully complex
movements of the pollinia of Orchis pyramidalis, by which they clasp
the proboscis of a moth and afterwards change their position for the
sake of depositing the pollen-masses on the double stigma—or again the
twisting movements, by which certain seeds bury themselves in the
ground[13]—follow from the manner of drying of the parts in question;
yet no one will suppose that these results have been gained without
special adaptation. Similarly, we are led to believe in adaptation when
we see the hypocotyl of a seedling, which contains chlorophyll, bending
to the light; for although it thus receives less light, being now
shaded by its own cotyledons, it places them—the more important
organs—in the best position to be fully illuminated. The hypocotyl may
therefore be said to sacrifice itself for the good of the cotyledons,
or rather of the whole plant. But if it be prevented from bending, as
must sometimes occur with seedlings springing up in an entangled mass
of vegetation, the cotyledons themselves bend so as to face the light;
the one farthest off rising
up, and that nearest to the light sinking down, or both twisting
laterally.[14] We may, also, suspect that the extreme sensitiveness to
light of the upper part of the sheath-like cotyledons of the Gramineæ,
and their power of transmitting its effects to the lower part, are
specialised arrangements for finding the shortest path to the light.
With plants growing on a bank, or thrown prostrate by the wind, the
manner in which the leaves move, even rotating on their own axes, so
that their upper surfaces may be again directed to the light, is a
striking phenomenon. Such facts are rendered more striking when we
remember that too intense a light injures the chlorophyll, and that the
leaflets of several Leguminosae when thus exposed bend upwards and
present their edges to the sun, thus escaping injury. On the other
hand, the leaflets of Averrhoa and Oxalis, when similarly exposed, bend
downwards.
[13] Francis Darwin, ‘On the Hygroscopic Mechanism,’ etc.,
‘Transactions Linn. Soc.,’ series ii. vol. i. p. 149, 1876.
[14] Wiesner has made remarks to nearly the same effect with respect
to leaves: ‘Die undulirende Nutation der Internodien,’ p. 6, extracted
from B. lxxvii. (1878). Sitb. der k. Akad. der Wissensch. Wien.
It was shown in the last chapter that heliotropism is a modified form
of circumnutation; and as every growing part of every plant
circumnutates more or less, we can understand how it is that the power
of bending to the light has been acquired by such a multitude of plants
throughout the vegetable kingdom. The manner in which a circumnutating
movement—that is, one consisting of a succession of irregular ellipses
or loops—is gradually converted into a rectilinear course towards the
light, has been already explained. First, we have a succession of
ellipses with their longer axes directed towards the light, each of
which
is described nearer and nearer to its source; then the loops are drawn
out into a strongly pronounced zigzag line, with here and there a small
loop still formed. At the same time that the movement towards the light
is increased in extent and accelerated, that in the opposite direction
is lessened and retarded, and at last stopped. The zigzag movement to
either side is likewise gradually lessened, so that finally the course
becomes rectilinear. Thus under the stimulus of a fairly bright light
there is no useless expenditure of force.
As with plants every character is more or less variable, there seems to
be no great difficulty in believing that their circumnutating movements
may have been increased or modified in any beneficial manner by the
preservation of varying individuals. The inheritance of habitual
movements is a necessary contingent for this process of selection, or
the survival of the fittest; and we have seen good reason to believe
that habitual movements are inherited by plants. In the case of twining
species the circumnutating movements have been increased in amplitude
and rendered more circular; the stimulus being here an internal or
innate one. With sleeping plants the movements have been increased in
amplitude and often changed in direction; and here the stimulus is the
alternation of light and darkness, aided, however, by inheritance. In
the case of heliotropism, the stimulus is the unequal illumination of
the two sides of the plant, and this determines, as in the foregoing
cases, the modification of the circumnutating movement in such a manner
that the organ bends to the light. A plant which has been rendered
heliotropic by the above means, might readily lose this tendency,
judging from the cases already given, as soon as it became useless or
injurious. A species which has ceased to be heliotropic might also be
rendered apheliotropic by the preservation of the individuals which
tended to circumnutate (though the cause of this and most other
variations is unknown) in a direction more or less opposed to that
whence the light proceeded. In like manner a plant might be rendered
diaheliotropic.