GROWTH AND ITS RESPONSIVE VARIATIONS.
X.--THE HIGH MAGNIFICATION CRESCOGRAPH FOR RESEARCHES ON GROWTH[S]
_By_
SIR J. C. BOSE,
_Assisted by_
GURUPRASANNA DAS, L.M.S.
In discussing the difficulties connected with investigations relating
to longitudinal growth and its variations, special stress must be
laid on the importance of maintaining external conditions absolutely
constant. This constancy can only be maintained in practice for a
short time. Lengthy periods of observation, moreover, introduce the
uncertainty of complication arising from spontaneous variation of
growth. The possibility of accurate investigation, therefore lies in
reducing the period of the experiment to a few minutes during which we
have to determine the normal rate of growth and its variation under
a given changed condition. This would necessitate the devising of a
method of very high magnification for record of the rate of growth.[S]
[S] A short account of my researches with the High Magnification
Crescograph has been published in the _Proceedings_ of the Royal
Society. I shall in the following Papers give a detailed account of my
investigations on growth and on allied phenomena.
With auxanometers now in use, which give a magnification of about
twenty times, it takes nearly four hours to determine the influence of
changed condition in inducing variation of growth. It will be seen
that if we succeeded in enhancing magnification from twenty to ten
thousand times, the necessary period for experiment would be reduced
from four hours to thirty seconds. The importance of securing a
magnification of this order is sufficiently obvious.
The problem of high magnification was first solved by my Optical
Lever.[T] The tip of the growing organ was attached to the short arm of
a lever, the axis of which carried a small mirror; in this way it was
possible to obtain a magnification of a thousand times. The magnified
movement of growth was followed with a pen on a revolving drum. The
record laboured under the disadvantage of not being automatic. This
defect was overcome by the use of the photographic method which however
entailed the inconvenience and discomfort of a dark room.
[T] BOSE--“Plant Response,” p 412.
I have, for the past six years, been working with a different method,
which has now been brought to a great state of perfection. The problem
to be solved was the devising of a direct method of high magnification
and the automatic record of the magnified rate of growth.
METHOD OF HIGH MAGNIFICATION.
The magnification in my Crescograph is obtained by a compound system of
two levers. The growing plant is attached to the short arm of a lever,
the long arm of which is attached to the short arm of the second lever.
If the magnification by the first lever be _m_, and that by the second,
_n_, the resulting magnification would be _mn_.
The practical difficulties met with in carrying out this idea are
very numerous. It will be understood that just as the imperceptible
movement is highly magnified by the compound system of levers, the
various errors and difficulties are likely to be magnified in the
same proportion. The principal difficulties met with were due: (1) to
the weight of the compound lever which exerted a great tension on the
growing plant, (2) to the yielding of flexible connections by which
the plant was attached to the first lever, and the first lever to the
second, and (3) to the friction at the fulcrums.
_Weight of the Lever._--As the first lever is to exert a pull on
the second, it has to be made rigid. The second lever serves as an
index, and can therefore be made of fine glass fibre. The securing of
rigidity of the first lever entails large cross section and consequent
weight, which exerts considerable tension on the plant. Excessive
tension greatly modifies growth; even the weight of the index used
in self-recording auxanometers is found to modify the normal rate of
growth. The weight of the levers introduces an additional difficulty in
the increased friction at the fulcrums, on account of which there is an
obstruction of the free movement of the recording arm of the lever. The
conditions essential for overcoming these difficulties therefore are:
(1) construction of a very light lever possessing sufficient rigidity,
and (2) arranging the levers in such a way that the tension on the
plant may be reduced to any extent, or even eliminated.
I found in _navaldum_, an alloy of aluminium, a light material
possessing sufficient rigidity. The first lever is constructed out of a
thin narrow sheet 25 cm. in length; it has, as explained before, to be
fairly rigid in order to exert a pull on the second without undergoing
any bending; this rigidity is secured by giving the thin narrow plate
of the lever a T-shape. The first lever balances, to a certain extent,
the second. Finer adjustments are made by means of an adjustable
counterpoise B, at the end of the levers. By this means the tension on
the plant can be greatly reduced; or a constant tension may be exerted
by means of a weight T (Fig. 56). In my later type of the apparatus
the plant connection is made to the right, instead of the left side of
the first fulcrum. This gives certain practical advantages. The second
lever is then made practically to balance the first, only a very slight
weight being necessary for exact counterpoise. The reduction of total
weight thus secured reduces materially the friction at the fulcrum with
great enhancement of efficiency of the apparatus.
[Illustration: FIG. 56.--Compound lever. P, plant attached to short arm
of lever L; T, weight exerting tension; C, connecting link; L′, second
lever with bent tip for record; B, B, balancing counterpoise. Fork F,
carries at its side two conical agate cups, on which lever rests by two
pin-points. (From a photograph.)]
The second or the recording lever has a normal excursion through 8 cm.
on the recording surface, which is a very thin sheet of glass 8×8 cm.
coated with a layer of smoke. As the recording lever is about 40 cm.
in length, the curvature in the record is slight, and practically
negligible in the middle portion of 4 cm. The dimensions given allow
a magnification of ten thousand times. A far more compact apparatus
is made with 15 cm. length of levers. This gives a magnification of a
thousand times.
AUTOMATIC RECORD OF THE RATE OF GROWTH.
Another great difficulty in obtaining an accurate record of the curve
of growth arises from the friction of contact of the bent tip of
the writing lever against the recording surface. This I was able to
overcome by an oscillating device by which the contact, instead of
being continuous, was made intermittent. The smoked glass plate, G,
is made to oscillate, to and fro, at regular intervals of time, say
one second. The bent tip of the recording lever comes periodically in
contact with the glass plate during its extreme forward oscillation.
The record would thus consist of a series of dots, the distance between
successive dots representing magnified growth during a second.
The drawback in connection with the obtaining of record on the
oscillating plate lies in the fact that if the plate approaches the
recording point with anything like suddenness, then the stroke on
the flexible lever causes an after-oscillation; the multiple dots,
thus produced, spoil the record. In order to overcome this, a special
contrivance is necessary, by which the speed of approach of the plate
should be gradually reduced to zero at contact with the recording
point. The rate of recession should, on the other hand, continuously
increase from zero to maximum. The recording point will in this manner
be gently pressed against the glass plate, marking the dot, and then
gradually set free. It was only after strict observance of these
conditions that the disturbing effect of after-vibration of the lever
could be obviated.
[Illustration: FIG. 57.--Eccentric for oscillation of plate K, crank;
S, slide; P, holder for glass plate G. A, adjusting screws; L,
recording lever. Clock releases string C for lateral movement of the
plate. (From a photograph.)]
This particular contrivance consists of an eccentric rod actuated by a
rotating wheel. A cylindrical rod is supported eccentrically, so that
semi-rotation of the eccentric causing a pull on the crank K (Fig. 57)
pushes the plate carrier gradually forward. On the return movement of
the eccentric, a light antagonistic spring makes the plate recede.
The rate of the movement of the crank itself is further regulated by
the device of the revolving wheel. This is released periodically by
clockwork at intervals of one, two, five, ten, or fifteen seconds
respectively, according to the requirements of the experiment. The
complete apparatus is shown in figure 58.
[Illustration: FIG. 58.--Complete apparatus. P, plant; S, micrometer
screw for raising or lowering the plant; C, clockwork for periodic
oscillation of plate; W, rotating wheel. V, cylindrical plant-chamber.
(From a photograph.)]
_Connecting Links._--Another puzzling difficulty lay in the fact that
the magnification actually obtained was sometimes very different from
the calculated value. This unreliability I was able to trace to the
defects inherent in thread connections, employed at first to attach the
plant to the first lever, and the first lever to the second. These
flexible connections were found to undergo a variable amount of elastic
yield. Hence it became necessary to use nothing but rigid connections.
The plant attachment, A, of triangular shape is made of a piece of
_navaldum_; its knife-edge rests on a notch at the short arm of the
lever, L. There are several notches at various distances from the
fulcrum. It will be understood how the magnification can be modified
by moving A, nearer or further from the fulcrum. The lower end of the
attachment is bent in the form of a hook. The end of the leaf of the
plant P, is doubled on itself and tied. The loop thus formed is then
slipped over the hooked end of A.
The link, C, connecting L and L′, consists of a pin pointed at both
ends, which rests on two conical agate cups fixed respectively to
the upper and lower surfaces of the levers L and L′. This mode of
frictionless linking is rigid and allows at the same time perfectly
free movement of the levers.
_The fulcrum._--The most serious difficulty was in connection with
frictionless support of the axes of the two levers. The horizontal axis
was at first supported on jewel bearings, with fine screw adjustment
for securing lateral support. Any slight variation from absolute
adjustment made the bearing either too loose or too tight, preventing
free play of the lever. When perfect adjustment was secured by any
chance, the movement of the levers became jerky after a few days. This
I afterwards discovered was due to the deposit of invisible particles
of dust on the bearings. These difficulties forced me to work out a
very perfect and at the same time a much simpler device. The lever now
carries two vertical pin-points which are supported on conical agate
cups. The axis of the lever passes through the points of support.
The friction of support is thus reduced to a minimum. The levers are
kept in place under the constant pressure of their own weight. The
excursion of the end of the recording lever, which represents magnified
movement of growth, was now found to be without jerk and quite uniform.
EXPERIMENTAL ADJUSTMENTS.
The soil in a flower pot is liable to be disturbed by irrigation, and
the record thus vitiated by physical disturbance. This is obviated by
wrapping a piece of cloth round the root imbedded in a small quantity
of soil. The lower end of the plant is held securely by a clamp. In
order to subject the plant to the action of gases and vapours, or to
variation of temperature it is enclosed in a glass cylinder (V) with
an inlet and an outlet pipe (Fig. 58). The chamber is maintained in a
humid condition by means of a sponge soaked in water. Different gases,
warm or cold water vapours, may thus be introduced into the plant
chamber.
Any quick growing organ of a plant will be found suitable for
experiment. In order to avoid all possible disturbing action of
circumnutation, it is preferable to employ either radial organs, such
as flower peduncles and buds of certain flowers, or the limp leaves
of various species of grasses, and the pistils of flowers. It is also
advisable to select specimens in which the growth is uniform. I append
a representative list of various specimens in which, under favourable
conditions of season and temperature, the rates of growth may be as
high as those given below:--
Peduncle of _Zephyranthes_ 0.7 mm. per hour.
Leaf of grass 1.10 " " "
Pistil of _Hibiscus_ flower 1.20 " " "
Seedling of wheat 1.60 " " "
Flower bud of _Crinum_ 2.20 " " "
Seedling of _Scirpus Kysoor_ 3.00 " " "
The specimen employed for experiment may be an intact plant, rooted in
a flower pot. It is, however, more convenient to employ cut specimens,
the exposed end being wrapped in moist cloth. The shock-effect of
section passes off after several hours, and the isolated organ renews
its growth in a normal manner. Among various specimens I find _S.
Kysoor_ to be very suitable for experiments on growth. The leaves are
much stronger than those of wheat and different grasses, and can bear
a considerable amount of pull without harm. Its rate of growth under
favourable condition of season is considerable. Some specimens were
found to have grown more than 8 cm. in the course of twenty-four hours,
or more than 3 mm. per hour. This was during the rainy season in the
month of August. But a month later the rate of growth fell to about
1 mm. per hour.
I shall now proceed to describe certain typical experiments which will
show: (1) the extreme sensibility of the Crescograph; (2) its wide
applicability in different investigations; and (3) its capability in
determining with great precision the time-relations of responsive
changes in the rate of growth. In describing these typical cases, I
shall give detailed account of the experimental methods employed, and
thus avoid repetition in accounts of subsequent experiments.
_Determination of the absolute rate of growth: Experiment 51._--For
the determination of the absolute rate, I shall interpret the results
of a record of growth obtained with a vigorous specimen _S. Kysoor_ on
a stationary plate. The oscillation frequency of the plate was once
in a second, and the magnification employed was ten thousand times.
The magnified growth movement was so rapid that the record consists
of a series of short dashes instead of dots (Fig. 59A). For securing
regularity in the rate of growth, it is advisable that the plant
should be kept in uniform darkness or in uniformly diffused light. So
sensitive is the recorder that it shows a change of growth-rate due to
the slight increase of illumination by the opening of an additional
window. One-sided light, moreover, gives rise to disturbing phototropic
curvature. With the precautions described the growth-rate in vigorous
specimens is found to be very uniform.
[Illustration: FIG. 59.--Crescographic records: (A) successive records
of growth at intervals of one second (magnification 10,000 times).
(_a_) Effect of temperature on a stationary plate; N, normal rate of
growth; C, retarded rate under cold; H, enhanced rate under warmth;
(_b_) record on moving plate, where diminished slope of curve denotes
retarded rate under cold (Magnification 2,000 times.)]
After the completion of the first vertical series, the recording plate
was moved 1 cm. to the left; the tip of the recorder was brought
once more to the top by the micrometer screw, S, (Fig. 58), and the
record taken once more after an interval of 15 minutes. The magnified
growth for 4 seconds is 38 mm. in the first record; it is precisely
the same in the record taken fifteen minutes after. The successive
growth elongations at intervals of 1 second is practically the same
throughout, being 9.5 mm. This uniformity in the spacings demonstrates
not only the regularity of growth under constant conditions, but also
the precision of the apparatus. It also shows that by keeping the
external condition constant, the normal growth-rate could be maintained
uniform for at least fifteen minutes. The magnified rate of growth
is nearly 1 cm. per second, and since it is quite easy to measure
0.5 mm. the Crescograph enables us to magnify and record a length of
0.0005 mm. that is to say, the sixteenth part of a wave of red light.
The absolute rate of growth, moreover, can be determined in a period as
short as 0.05 of a second. These facts will give some idea of the great
possibilities of the Crescograph for future investigations.
As the period of experiment is very greatly shortened by the method of
high magnification, I shall, in the determination of the absolute rate
of growth, adopt a second as the unit of time, and µ, or _micron_, as
the unit of length,--the micron, being a millionth part of a metre or a
thousandth part of a millimeter.
If _m_ be the magnifying power of the compound lever and _l_, the
average distance between successive dots in mm. at intervals of _t_
seconds then:--
the rate of growth = _l_/_mt_ × 10^{3}µ per second.
In the record given _l_ = 9.5 mm.
_m_ = 10,000.
_t_ = 1 second.
Hence the rate of growth = 9.5/10,000 × 10^{3}µ per sec.
= 0.95µ per sec.
Having demonstrated the extreme sensitiveness and reliability of the
apparatus, in quantitative determination, I shall next proceed to show
its wide applicability for various researches relating to the influence
of external agencies in modification of growth. For this two different
methods are employed. In the first of these methods, the records are
taken on a stationary plate: of these the record is at first taken
under normal condition, the subsequent series being obtained under
the given changed condition; the increase or diminution of intervals
between successive dots, in the two series, at once demonstrates the
stimulating or depressing nature of the changed condition.
In the second method, the record is taken on a plate moving at an
uniform rate by clockwork. A curve is thus obtained, the ordinate
representing growth elongation and the abscissa the time. The increment
of length divided by the increment of time gives the absolute rate of
growth at any part of the curve. As long as the growth is uniform, so
long the slope of the curve remains constant. If a stimulating agency
enhances the rate of growth, there is an immediate upward flexure in
the curve; a depressing agent, on the other hand, lessens the slope of
the curve.
I shall now give a few typical examples of the employment of the
Crescograph for investigations on growth: the first example I shall
take is the demonstration of the influence of variation of temperature.
_Stationary method: Experiment 52._--The records, given in Fig. 59_a_,
were taken on a stationary plate. The specimen was _S. Kysoor_; the
Crescographic magnification was two thousand times, and the successive
dots at intervals of 5 seconds. The middle series, N, was at the
temperature of the room. The next, C, was obtained with the temperature
lowered by a few degrees. Finally H was taken when the plant-chamber
was warmed. It will be seen how under cooling the spaces between
successive dots have become shortened, showing the diminished rate of
growth. Warming, on the other hand, caused a widening of intervals
between successive dots, thus demonstrating an enhancement of the rate
of growth.
Calculating from the data obtained from the figure we find:--
The absolute value of the normal rate 0.457µ per second.
Diminished rate under cold 0.101µ " "
Enhanced rate under warmth 0.737µ " "
_Moving plate method: Experiment 54._--This was carried out with a
different specimen of _S. Kysoor_, the record being taken on a moving
plate (Fig. 59_b_). The first part of the curve here represents the
normal rate of growth. The plant was then subjected to moderate
cooling, the subsequent curve with its diminished slope denotes the
depression of growth. The question of influence of temperature will be
treated in a subsequent Paper of the present series in much greater
detail.
[Illustration: FIG. 60--Horizontal record shows absence of growth in
a dead branch; physical expansion on application of warmth at arrow
followed by horizontal record on attainment of steady temperature.
(Magnification 2,000 times)]
_Precaution against physical disturbance: Experiment 54._--There may
be some misgiving about the employment of such high magnification: it
may be thought that the accuracy of the record might be vitiated by
physical disturbance, such as vibration. In physical experimentation
far greater difficulties have, however, been overcome, and the problem
of securing freedom from vibration is not at all formidable. The whole
apparatus need only be placed on a heavy bracket screwed on the wall
to ensure against mechanical disturbance. The extent to which this has
been realized will be found from the inspection of the first part of
the record in figure 60, taken on a moving plate. A thin dead twig was
substituted for the growing plant, and the perfectly horizontal record
not only demonstrated the absence of growth movement but also of all
disturbance. There is an element of physical change, against which
precautions have to be taken in experiments on variation of the rate of
growth at different temperatures. In order to determine its character
and extent, a record was taken with the dead twig, of the effect of
raising the temperature of the plant-chamber through ten degrees. The
record (Fig. 60) with a magnification of two thousand times shows that
there is an expansion during the rise of temperature, and that the
variable period lasted for a minute, after which there was a cessation
of physical movement, the record becoming once more horizontal. The
obvious precautions to be taken in such a case, is to wait for several
minutes for the attainment of steady temperature. The movement caused
by physical change abates in a short time whereas the change of rate of
growth brought about by physiological reaction is persistent.
DETERMINATION OF LATENT PERIOD AND TIME-RELATIONS OF RESPONSE.
[Illustration: FIG. 61.--Time-relations of response of growing organ
to electric stimulus of increasing intensities applied at the short
horizontal lines. Successive dots at intervals of 2 seconds.]
_Experiment 55._--In the determination of time-relations of responsive
change in growth under external stimulus, I shall take the typical case
of the effect of electric shock from a secondary coil of one second’s
duration. Two electrodes were applied, one above and the other below
the growing region of a bud of _Crinum_. The record was taken on a
moving plate, the magnification employed being two thousand times, and
successive dots made at intervals of two seconds. It was a matter of
surprise to me to find that the growth of the plant was affected by
an intensity of stimulus far below the limit of our own perception.
As regards the relative sensitiveness of plant and animal, some of
my experiments show that the leaf of _Mimosa pudica_ in a favourable
condition responds to an electric stimulus which is one-tenth the
minimum intensity that causes perception in a human being. For
convenience I shall designate the intensity of electric shock that
is barely perceptible to us as the unit shock. When an intensity of
0.25 unit was applied to the growing organ, it responded to it by a
retardation of growth. Inspection of Fig. 61 shows that there is a
flexure induced in the curve in response to stimulus, the flattening
of the curve denoting retardation of growth. The latent period, in
this case, is 6 seconds. The normal rate was restored after 5 minutes.
The intensity of shock was next raised from 0.25 unit to one unit. The
second record shows that the latent period is reduced to 4 seconds,
and a relatively greater retardation of growth was induced by the
action of the stronger stimulus. The recovery of the normal rate was
effected after the longer period of 10 minutes. I took one more record,
the intensity being three units. The latent period was now reduced
to 1 second, and the induced retardation was so great as to effect a
temporary arrest of growth.
TABLE X.--TIME-RELATIONS OF RESPONSIVE GROWTH-VARIATION UNDER ELECTRIC
SHOCK (_Crinum_).
+------------+--------------+---------------+----------------+
|Intensity of|Latent period.| Normal rate. | Retarded rate. |
|stimulus. | | | |
+------------+--------------+---------------+----------------+
| 0.25 unit. | 6 seconds. |0.62 µ per sec.|0.49 µ per sec. |
| 1 " | 4 " |0.62 " |0.25 " |
| 3 " | 1 " |0.62 " |Temporary arrest|
| | | |of growth. |
+------------+--------------+---------------+----------------+
It is thus found that growth in plants is affected by an intensity of
stimulus which is below human perception; that with increasing stimulus
the latent period is diminished and the period of recovery increased;
and that the induced retardation of growth increases continuously with
the stimulus till at a critical intensity there is a temporary arrest
of growth. I shall speak later of the effect induced by stimulus above
this critical point.
[Illustration: FIG. 62.--Record of a single growth-pulse of
_Zephyranthes_ (Magnification 10,000 times.)]
_Experiment 56._--As a further example of the capability of the
Crescograph, I shall give the record of a single pulse of growth
obtained with the peduncle of _Zephyranthes Sulphurea_ (Fig. 62). The
magnification employed was 10,000 times, the successive dots being
at intervals of one second. It will be seen that the growth pulse
commences with a sudden elongation, the maximum rate being 0.4 µ per
sec. The pulse exhausts itself in 15 seconds, after which there is
a partial recovery in the course of 13 seconds. The period of the
complete pulse is 28 seconds. The resultant growth in each pulse is
therefore the difference between elongation and recovery. Had a very
highly magnifying arrangement not been used, the resulting rate would
have appeared continuous. In other specimens, owing probably to greater
frequency of pulsation and co-operation of numerous elements in growth,
the rate appears to be practically uniform.
_Advantages of the Crescograph._--There is no existing method which
enables us to detect and measure such infinitesimal movements and their
time-relations. The only attempt made in measuring minute growth has
been by observing the movement of a mark on a growing plant through
a microscope. The magnification available in practice is about 250
times. The observation of the movement would itself be sufficiently
fatiguing. But a simultaneous estimate of the time-relations of
rapidly fluctuating changes would prove so bewildering, that accurate
results from this method would be altogether impossible. A 1/12″
objective gives a linear enlargement of about 1,200 times. But the
employment of this objective is impracticable in the measurement of
growth elongation of an ordinary plant. With the Crescograph, on the
other hand, we obtain a magnification which far surpasses the highest
powers of a microscope, and it can be used for all plants. It does not
merely detect growth but automatically records the rate of growth and
its slightest fluctuation. The extreme shortness of time required for
an experiment renders the study of the influence of a single factor
at a time possible, the other conditions being kept constant. The
Crescograph thus opens out a very extensive field of inquiry into the
physiology of growth; and the discovery of several important phenomena
mentioned in this Paper is to be ascribed to the extreme sensitiveness
of the apparatus, and the accuracy of the method employed.
MAGNETIC AMPLIFICATION.
The magnification obtained with two levers was, as stated before,
10,000 times. It may be thought that further magnification is possible
by a compound system of three levers. There is, however, a limit to
the number of levers that may be employed with advantage, for the
slight overweight of the last lever becomes multiplied and exerts
very great tension on the plant, which interferes with the normal
rate of its growth. The friction at the bearings also becomes added up
by an increase in the number of levers, and this interferes with the
uniformity of the movement of the last recording lever. For securing
further magnification, additional material contact has, therefore, to
be abandoned. I have recently been successful in devising an ideal
method of magnification without contact. The movement of the lever of
the Crescograph upsets a very delicately balanced magnetic system.
The indicator is a reflected spot of light from a mirror carried by
the deflected magnet. Taking a single lever with the lengths of two
arms 125 mm. and 2.5 mm. respectively we obtain a magnification of 50
times. The magnetic system gives a further magnification of 20,000
the total magnification being thus a million times. This was verified
by moving by means of a micrometer screw the short arm of the lever
through 0.005 mm. The resulting deflection of the spot of light at a
distance of 4 metres was found to be 5,000 mm. or a million times the
movement of the short arm. It is not difficult to produce a further
magnification of 50 times by attaching a second lever to the first. The
total magnification would in this case be 50 million times.
A concrete idea of this will be obtained when we realise that by the
Magnetic Crescograph a magnification can be obtained which is about
50,000 times greater than that produced by the highest power of a
microscope. This order of magnification would lengthen a wave of sodium
light to about 3,000 cm. I am not aware of any existing method by which
it is possible to secure an amplification of this order of magnitude.
The application of this will undoubtedly be of great help in many
physical investigations, some of which I hope to complete in the near
future.
Such an enormous magnification cannot be employed in ordinary
investigations on growth, for the moving spot of light indicating
rate of growth, passes like a flash across the screen. But it is of
signal service in my investigations on growth by the Method of Balance,
to be described in a future Paper. The principle of this method
consists in making the spot of light, which is moving in response to
growth, stationary, by subjecting the plant to a compensating movement
downwards. The slightest variation caused by an external agent would
make the spot of light move either to the right or to the left,
according to the stimulating or depressing character of the agent.
It will be understood, how extremely sensitive this method is for
detection of the most minute variation in the normal rate of growth.
THE DEMONSTRATION CRESCOGRAPH.
Before proceeding with accounts of further investigations, I shall
describe a form of Magnetic Crescograph with which I have been able to
give before a large audience demonstration of a striking character on
various phenomena of growth. The magnification obtained was so great
that I had to take some trouble in reducing it. This was accomplished
by the employment of a single, instead of a compound system of two
levers. The reflected spot of light was thrown on a screen placed at
a distance of 4 metres, and this gave a magnification of a million
times; it is obvious that an increase of the distance of the screen
to 8 metres would have given a magnification of 2 million times. As
it was, even the lower magnification was far too great for use with
quick growing plants like _Kysoor_. I, therefore, employed the slower
growing flower bud of _Crinum_. It will be seen from Table X that the
normal rate of growth of the lily is of the order of 0.0006 mm. per
second. The normal excursion of the spot of light reflected from the
Crescograph exhibiting growth was found to be 3 metres in five seconds
or 60 cm. per second. This is a million times the actual rate of growth
of the _Crinum_ bud. As it is easy to measure 5 mm. in the scale, it
will be seen that with the Demonstration Crescograph it is possible to
detect the growth of a plant for a period shorter than a hundredth part
of a second.
_Experiment 57._--A scale 3 metres long divided into cm. is placed
against the screen. A metronome beating half seconds is started at
the moment when the spot of light transits across the zero division;
the number of beats is counted till the index traverses the 300 cm.
At the normal temperature of the room (30°C.), the index traversed
300 cm. in five seconds. The plant chamber was next cooled to 26°C. by
the blowing in of cooled water vapour; the time taken by the spot of
light to traverse the scale was now 20 seconds, _i.e._, the growth-rate
was depressed to a fourth. Under continuous lowering of temperature
the growth-rate became slowed down till at 21°C. there was an arrest
of growth. Warm vapour was next introduced, gradually raising the
temperature of the chamber to 35°C. The spot of light now rushed across
the scale in a second and a half, _i.e._, the growth was enhanced to
more than three times the normal rate. The entire series of the above
experiments, on the effect of temperature on growth, was thus completed
in the course of 15 minutes.
SUMMARY.
A description is given of the High Magnification Crescograph, which
enables an automatic record of growth magnified ten thousand times. The
absolute rate of growth can be easily determined from the data given in
the record.
A magnification of a million times is obtained by the employment of
Magnetic amplification. An increment of growth so minute as a millionth
part of a mm. or 0.00000004 inch may thus be detected. It is also
possible to detect the growth of a plant for a period shorter than a
hundredth part of a second.
The influence of external conditions on variation of rate of growth is
obtained by two methods of record. In STATIONARY METHOD, the increase
or diminution of the distance between successive dots representing
magnified rate of growth, demonstrates the stimulating or depressing
nature of the changed condition.
In the second, or MOVING PLATE METHOD, a curve is obtained, the
ordinate representing growth elongation, and the abscissa, time. A
stimulating agent causes an upward flexure of the normal curve; a
depressing agent, on the other hand, lessens the slope of the curve.
The action of external stimulus induces a variation of the rate of
growth, the time relations of which are found from the automatic record
of the growth. The latent period is shortened with the intensity of the
stimulus. A responsive variation of growth is induced by an intensity
of stimulus which is below human perception.
It is often possible to obtain record of the pulsatory nature of
growth-elongation. Thus, with the growing peduncle of _Zephyranthes_,
the growth pulse commences with a sudden elongation, the maximum rate
being 0.0004 mm. per second. The pulse exhausts itself in 15 seconds,
after which there is a partial recovery in course of 13 seconds, the
period of complete pulse being 28 seconds. The resultant growth in each
pulse is the difference between elongation and recovery.
The Magnetic Crescograph enables demonstration of principal phenomena
of growth and its variation before a large audience.
XI.--EFFECT OF TEMPERATURE ON GROWTH
_By_
SIR J. C. BOSE,
_Assisted by_
SURENDRA CHUNDER DASS, M.A.
Accurate determination of the effect of temperature on growth presents
many serious difficulties on account of numerous complicating factors.
In nature, the upper part of the plant is exposed to the temperature of
the air, while the root underground is at a very different temperature.
Growth, we shall find, is modified to a certain extent by the ascent
of sap. (See p. 189, _Expt. 70_.) The activity of this latter process
is determined by the temperature to which the roots are subjected. The
difficulty may be removed to a certain extent by placing the plant in
a thermal chamber, with arrangement for regulating the temperature
of the air. The air is a bad conductor of heat, and there is some
uncertainty of the interior of the plant attaining the temperature of
the surrounding air, unless the plant is long exposed to the definite
and constant temperature of the plant chamber. Observation of the
effects of different temperatures then becomes a prolonged process,
with the possibility of vitiation of results by autonomous variation
of growth. Reduction of the period of experiment by rapidly raising
the temperature of the chamber introduces fresh difficulties; for a
sudden variation of temperature often acts like an excitatory shock.
This drawback may to some extent be obviated by ensuring a gradual
change of temperature. This is by no means an easy process, for even
with care the rise of temperature of the air cannot be made perfectly
uniform, and any slight irregularity gives rise to sudden fluctuations
in the magnified record of growth. Another difficulty arises from the
radiation of heat-rays from the sides of the thermal chamber. These
rays, I shall in a different Paper show, induce a retardation of
growth. The effect of rise of temperature in acceleration of growth
is thus antagonised by the action of thermal radiation. This trouble
may be minimised by having the inner surface of the thermal chamber of
bright polished metal, since the radiating power of a polished surface
is relatively feeble.
The contrivance which I employ for ensuring a gradual rise of
temperature, consists of a double-walled cylindrical metallic vessel;
the plant is placed in the inner chamber, the walls of which are coated
with electrically deposited silver and polished afterwards, and at
the bottom of which there is a little water. The space between the
inner and outer cylinder is filled with water, in which is immersed a
coiled copper pipe. Hot water from a small boiler enters the inlet of
the coiled pipe and passes through the outlet at the lower end. The
water in the outer cylinder is thus gradually raised by flow of hot
water in the coiled pipe. The rate of flow of hot water, on which the
rate of rise of temperature depends, is regulated by a stop-cock. The
air of the inner chamber in which the plant is placed, may thus be
adjusted for a definite temperature. The small quantity of water in the
inner chamber keeps its air in a humid condition, since dry hot air by
causing dessication interferes with normal growth.
METHOD OF DISCONTINUOUS OBSERVATION.
[Illustration: FIG. 63.--Effect of temperature on growth, and
determination of optimum temperature.]
_Experiment 58._--High magnification records are taken for successive
periods of ten seconds, for selected temperatures, maintained constant
during the particular observation. In figure 63 is given records of
rate of growth obtained with a specimen of _Kysoor_ at certain selected
temperatures. It will be seen that the rate of growth increases with
the rise of temperature to an optimum, beyond which the growth-rate
undergoes a depression. In the present case the optimum temperature is
in the neighbourhood of 35°C.
METHOD OF CONTINUOUS OBSERVATION.
The method of observation that I have described above is not ideally
perfect, but the best that could be devised under the circumstances.
A very troublesome complication of pulsations in growth, arises at
high temperatures, which render further record extremely difficult.
Growth is undoubtedly a pulsatory phenomenon; but under favourable
circumstances, these merge practically into a continuous average
rate of elongation. At a high temperature the effect of certain
disturbing factors comes into prominence. This may be due to some
slight fluctuation in the temperature of the chamber, or to the effect
of thermal radiation from the side of the chamber. This disturbing
influence is most noticed at about 45°C, rendering the record of growth
above this point a matter of great uncertainty. It will presently be
shown that in plants immersed in water-bath growth is often found to
persist even up to 57°C.
The only way of removing the complication arising from thermal
radiation lies in varying the temperature condition of the plant, by
direct contact with water at different temperatures. This procedure
will also remove uncertainty regarding the body of the plant assuming
the temperature of surrounding non-conducting air. The disturbing
effect of sudden variation of temperature is also obviated by a
more uniform regulation of rise of temperature. The inner cylinder
containing the plant is filled with water; heat from gradually warmed
water in the outer cylinder is conducted across the inner cylinder made
of thin copper and raises the temperature of the water contained in the
inner cylinder with great uniformity. A clock-hand goes round once in
a minute; the experimenter, keeping his hand on the stop-cock, adjusts
the rate of rise of water in the inner cylinder, so that there is a
rise, say, of one-tenth of a degree every 6 seconds or of one degree
every minute. The mass of water acts as a governor, and prevents any
sudden fluctuations of temperature. The adoption of this particular
device eliminated the erratic changes in the rate of growth that had
hitherto proved so baffling.
The elongation recorded by the Crescograph will now be made up of (1)
physical expansion, (2) expansion brought about by absorption of water,
and (3) the pure acceleration of growth. The disentanglement of these
different elements presented many difficulties. I was, however, able to
find out the relative values of the first two factors in reference to
the elongation of growth. This was done by carrying out a preliminary
experiment with a specimen of plant in which growth had been completed.
It was raised through 20°C in temperature, records being taken both
at the beginning and at the end. This was for obtaining a measure of
the physical change due to temperature, and also of the change brought
about by absorption of water. I should state here that for the method
of continuous record of growth which I contemplated, the record had to
be taken for about 18 minutes. The magnification had to be lowered to
250 times to keep the record within the plate. With this magnification,
the fully grown specimen did not show in the record a change even of
1 mm. in length in 18 minutes, while the growing plant under similar
circumstances exhibited an elongation of 100 mm. or more. In records
taken with low magnification, the effect of physical change is quite
negligible.
DETERMINATION OF THE CARDINAL POINTS OF GROWTH.
The cardinal points of growth are not the same in different plants;
they are modified in the same species by the climate to which the
plants are habituated; the results obtained in the tropics may thus
be different from those obtained in colder climates. At the time of
the experiment, the prevailing temperature at Calcutta in day time was
about 30°C.
_Temperature minimum: Experiment 59._--For the determination of the
minimum, I took a specimen of _S. Kysoor_, and subjected it to a
continuous lowering of temperature, by regular flow of ice-cold water
in the outer vessel of the plant-chamber. Record was taken on a moving
plate for every degree fall of temperature; growth was found to be
continuously depressed, till an arrest of growth took place at 22°C.
(Fig. 64).
The arrested growth was feebly revived at 23°C., after which with
further rise of temperature there was increased acceleration. The
optimum point was reached at about 34°C. In some plants the optimum is
reached at about 28°C., and the rate remains constant for the next 10
degrees or more.
[Illustration: FIG. 64.--Record of effect of fall of temperature from
30°C. to arrest of growth at 22°C.]
[Illustration: FIG. 65.--Effect of rise of temperature from 53°C. to
60°C. A sudden contraction, indicative of death-spasm, takes place at
60°C.]
_Temperature maximum: Experiment 60._--For the determination of the
maximum, the temperature was raised much higher. At 55°C. growth was
found to be greatly retarded with practical arrest at 58°C. At 60°C.
there occurred a sudden spasmodic contraction (Fig. 65), which I
have shown elsewhere to be the spasm of death. This mechanical spasm
at 60°C. is also strikingly shown by various pulvinated organs. An
electric spasm of galvanometric negativity, and a sudden diminution of
electrical resistance also take place at the critical temperature of
60°C.[U]
[U] BOSE--“Plant Response,” p. 168; “Comparative Electro-Physiology,”
p. 202, p. 546.
I have described the immediate effect at the critical point. Long
maintenance at a temperature few degrees below 60°C. will no doubt be
attended with the death of the organ. Fatigue is also found to lower
the death-point.
THE THERMO-CRESCENT CURVE.
_Experiment 66._--I was next desirous of devising a method by which
an automatic and continuous record of the plant should enable us to
obtain a curve, which would give the rate of growth at any temperature,
from the arrested growth at the minimum to a temperature as high as
40°C. In order to eliminate the elements of spontaneous variation, the
entire record had to be completed within a reasonable length of time,
say about 18 minutes for a rise of as many degrees in temperature.
This gives a rate of rise of 1°C. for one minute. Separate experiments
showed that at this rate of _continuous_ rise of temperature there is
practically no lag in the temperature assumed by thin specimens of
plants. For observation during a limited range I use the slower rate
of rise at 1°C. per two minutes. But the result obtained by slower
rise was found not to differ from that obtained with one degree rise
per minute. The curve of growth is taken on a moving plate, which
travels 5 mm. per minute. Successive dots are made by the recording
lever at intervals of a minute during which the rise of temperature is
1°C. A _Thermo-crescent Curve_ is thus obtained, the ordinate of which
represents increment of growth, and the abscissa, the time. As the
temperature is made to rise one degree per minute, the abscissa also
represents rise of temperature (Fig. 66). The vertical distance between
two successive dots thus gives increment of growth in one minute for 1
degree rise of temperature from T to T′. If _l_ represents this length,
_t_ the interval of time (here 60 sec.), and _m_ the magnifying power
of the recorder, then the rate of growth for the mean temperature
(T+T′)/2 is found from the formula: rate of growth at (T+T′)/2 =
_l_/(_m × t × 60_)10^{3}µ per sec.
[Illustration: FIG. 66.--The Thermo-crescent Curve.]
TABLE XI.--RATE OF GROWTH FOR DIFFERENT TEMPERATURES.
+-------------+----------------+-------------+----------------+
| Temperature.| Growth. | Temperature.| Growth. |
+-------------+----------------+-------------+----------------+
| 22°C | 0.00 µ per sec.| 31°C | 0.45 µ per sec.|
| 23°C | 0.02 µ " " | 32°C | 0.60 µ " " |
| 24°C | 0.04 µ " " | 33°C | 0.80 µ " " |
| 25°C | 0.06 µ " " | 34°C | 0.92 µ " " |
| 26°C | 0.08 µ " " | 35°C | 0.84 µ " " |
| 27°C | 0.12 µ " " | 36°C | 0.64 µ " " |
| 28°C | 0.16 µ " " | 37°C | 0.48 µ " " |
| 29°C | 0.22 µ " " | 38°C | 0.30 µ " " |
| 30°C | 0.32 µ " " | 39°C | 0.16 µ " " |
+-------------+----------------+-------------+----------------+
I give in figure 67 a curve showing the relation between temperature
and growth.
[Illustration: FIG. 67.--Curve showing relation between temperature and
rate of growth.]
It will thus be seen that, in the course of an experiment lasting
about twenty minutes, data have been obtained which enable us to
determine the rates of growth through a wide range of temperature.
We have likewise been able by the first method to make very accurate
determinations of the temperature maximum and minimum. In short, by
adopting the methods described, the cardinal points of growth and the
rate of growth at any temperature, may be determined with a precision
unattainable by the older methods, of averages or of prolonged
observation.
SUMMARY.
Temperature induces variation in the rate of growth. In accurate
determination of the growth, the disturbing effect of radiation of heat
has not been eliminated.
A continuous record of growth under uniform rise of temperature gives
the Thermo-crescent curve, from which the rate of growth at any
temperature may be deduced.
Different plant-tissues exhibit characteristic differences in their
cardinal points of growth. In _Kysoor_, growth is arrested at the
temperature minimum of 22°C. The optimum temperature is at 34°C., after
which growth-rate declines and becomes completely arrested at 58°C. At
60°C. there is a sudden spasmodic contraction of death.
In other plants the cardinal points are different. In some plants the
optimum growth is attained at 28°C. and remains constant up to 38°C.
XII.--THE EFFECT OF CHEMICAL AGENTS ON GROWTH
_By_
SIR J. C. BOSE,
_Assisted by_
GURUPRASANNA DAS.
Chemical agents are found to exert characteristic actions on growth.
The method of investigation sketched here opens out an extended field
of investigation. The effect of a chemical substance, I find, to be
modified by (1) the strength of the solution, (2) the duration of
application, and (3) the condition of the tissue. A poisonous substance
in minute doses is often found to exert a stimulating action. Too long
continued action of a stimulant, on the other hand, exerts a depressing
effect. The influence of the tonic condition is shown by the fact
that while a given dilution of a poisonous substance kills a weak
specimen, the same poisonous solution, applied to a vigorous specimen,
actually stimulates and enhances the rate of the growth. I give below
descriptions of a few typical reactions.
The reagent, when in a liquid form, is locally applied on the growing
organ. The records, taken before and after the application, exhibit the
stimulatory or depressing character of the reagent. A different method
of application of the reagent is employed for plants with extended
region of growth. The specimen is then enclosed in a glass cylinder,
with inlet and outlet pipes. The cylinder is first filled with water,
and the normal rate of growth recorded. This rate remains constant for
several hours; but prevention of access of air for too long a time
affects the normal growth. After obtaining normal record, water charged
with the giving chemical agent is passed into the cylinder; and the
subsequent record shows the characteristic effect of the reagent. The
introduction of a gas into the chamber offers no difficulty.
EFFECT OF STIMULANTS.
_Hydrogen Peroxide: Experiment 62._--This reagent, as supplied by
Messrs. Parke Davis & Co., was diluted to 1 per cent. and applied to
the growing plant. Its stimulating action on growth is demonstrated in
the right hand record of Fig. 68_a_, where the rate of growth is seen
enhanced two and a half times the normal rate.
[Illustration: FIG. 68.--Effect of chemical agents: (_a_) Acceleration
of growth under H_{2}O_{2}, (_b_) Effect of NH_{3}, preliminary
acceleration followed by retardation. (_c_) Effect of ether (E) and
recovery (A).]
_Ammonia: Experiment 63._--The immediate effect of dilute vapour of
this reagent is an enhancement of growth, seen in the middle record of
Fig. 68_b_, where the rate is seen to be double the normal. Continued
action, however, caused a depression; the third record of this series
shows this, where the reduction is three-fourths of the normal rate.
EFFECT OF ANÆSTHETICS.
[Illustration: FIG. 69.--Effect of CO_{2}. (_a_) Normal record; (_b_)
immediately after application of CO_{2}, and (_c_) 15 minutes after.]
_Ether: Experiment 64._--In Fig. 68_c_, the records exhibit the effect
of introduction of ether vapour into the plant chamber, and its
recovery after the removal of the vapour. Ether is seen to depress the
rate of growth to a little more than a third of the normal rate. The
recovery is seen to be nearly complete half an hour after the removal
of the vapour.
_Carbonic Acid: Experiment 65._--The action of this gas is very
remarkable. The plant was immersed in water and normal record taken;
the plant chamber was now filled with water, charged with carbonic acid
gas. This induced a very marked acceleration of growth (Fig. 69). In a
seedling of Onion, the increase was found to be two and a half times.
In the flower bud of _Crinum_, the rate was found enhanced threefold
from the normal 0.25 µ to 0.75 µ per second. After this preliminary
enhancement, there was a depression of growth within 15 minutes of the
application, the rate being now reduced to 0.15 µ per second. These
effects were found to take place equally in light or in darkness.
ACTION OF DIFFERENT GASES.
_Coal Gas: Experiment 66._--Coal gas induces a depression. It is
curious that subjection to the action of this gas does not produce so
evil an effect as one would expect. The introduction of the gas had
reduced the growth-rate to more than half; but there was a recovery
half an hour after the introduction of fresh air.
_Sulphuretted Hydrogen: Experiment 67._--This gas not only exerts a
depressing effect, but its after-effect is also very persistent. The
plant experimented on was very vigorous and its rate of growth was
depressed to half by subjection to the action of the gas for a short
time. The record taken half an hour after the introduction of fresh air
did not exhibit any recovery.
ACTION OF POISONS.
_Ammonium Sulphide: Experiment 68._--This reagent in dilute solution
retards growth, and in stronger solution acts as a poison. The
following results were obtained with a wheat seedling under different
strengths of solution:--
Normal rate 0.30 µ per sec.
0.5 per cent. solution 0.15 µ " "
2.0 " " " 0.08 µ " "
_Copper Sulphate: Experiment 69._--The effect of a solution of this
reagent is far more depressing than the last. One per cent. solution
acting for a short time depressed the rate from 0.45 µ to 0.13 µ per
sec. Long continued action of the poisonous solution kills the plant.
SUMMARY.
The effect of a chemical agent is modified by the strength of the
solution, the duration of application and the tonic condition of the
tissue.
Dilute solution of hydrogen peroxide induces an acceleration of growth.
The action of dilute vapour of ammonia is a preliminary enhancement
followed by depression of growth.
Ether vapour depresses the rate of growth. On the removal of the vapour
there is a recovery of the normal rate.
The effect of carbonic acid is a great enhancement of the rate of
growth; after this preliminary action, growth undergoes a decline. The
effect described takes place equally in light or in darkness.
Coal gas induces a depression of the rate of growth from which there
is a recovery after the removal of the gas. The action of sulphuretted
hydrogen is far more toxic, the after-effect being very persistent.
Solution of ammonium sulphide induces increasing retardation of growth,
with the strength of the solution. Copper sulphate solution acts as a
toxic agent, retarding the rate of growth and ultimately killing the
plant.
XIII.--EFFECT OF VARIATION OF TURGOR AND OF TENSION ON GROWTH
_By_
SIR J. C. BOSE.
The movements of leaves of sensitive plants are caused by variation
of turgor in the pulvinus induced by stimulus. The down movement or
_negative_ response of _Mimosa_ is caused by a diminution or negative
variation of turgor, while the erection or _positive_ response is
brought about by an increase, or positive variation of turgor.
We shall now investigate the change induced in a growing organ in the
rate of growth by variation of turgor. Turgor may be increased by
enhancing the rate of ascent of sap or by an artificial increase of
internal hydrostatic pressure. A diminution of turgor may, on the other
hand, be produced by withdrawal of water through plasmolysis. In order
to maintain a constant terminology I shall designate an increase, as
the positive, and a diminution, the negative variation of turgor.
RESPONSE TO POSITIVE VARIATION OF TURGOR.
In experimenting with _Mimosa_ the plant was subjected to the condition
of drought, water being withheld for a day. On supplying water, the
leaf, after a short period, exhibited a positive or erectile movement
(_Expt. 12_). The delay was evidently due to the time taken by the
water absorbed by root to reach the responding organ.
[Illustration: FIG. 70.--Effect of irrigation: D. record of growth
under drought, C. acceleration after irrigation with cold water, H.
enhanced acceleration on irrigation with warm water. (_S. Kysoor_)]
_Method of Irrigation: Experiment 70._--In order to investigate the
effect of enhanced turgor on growth, I took a specimen of _Kysoor_
which had been dug up with an attached quantity of soil; this latter
was enclosed in a small bag. The plant was then securely clamped
and fixed on a stand. This precaution was taken to prevent upward
displacement by the swelling of the soil in flower pot of the plant
under irrigation. The specimen was then subjected to a condition of
drought, water being withheld for a day. The depressed rate of growth
is seen in record (Fig. 70). Ordinary cold water was now applied at the
root, the effect of which is seen in record C. Finally the record (H)
was obtained after irrigation with tepid water. It will be seen that
the spaces between successive dots, representing magnified growth at
intervals of ten seconds, are very different. While a given elongation
took place under drought in 19 × 10 seconds, a similar lengthening took
place, after irrigation with cold water, in 13 × 10 seconds, and after
irrigation with warm water in 3 × 10 seconds. Irrigation with warm
water is thus seen to increase the rate of growth more than six times.
The enhancement of the rate of growth on irrigation with cold water
took place after seventy seconds. The interval will obviously depend
on the distance between the root by which the water is absorbed and
the region of growth. It will further depend on the activity of the
process of the ascent of sap. The time interval is greatly reduced
when this activity is in any way increased. Thus the responsive growth
elongation after application of warm water was very much quicker;
in the case described it was less than 20 seconds. With regard to
application of warm water, the variation of temperature should not be
too sudden; it should commence with tepid, and end with warm water.
_Sudden_ application of hot water brings about certain complications
due to excitatory effect. As regards the persistence of after-effect of
a single application of warm water, it should be remembered that the
absorbed water gradually cools down. In an experiment with a peduncle
of _Zephyranthes_ the growth under partial drought was found to be
0.04 µ per second; application of warm water increased the growth rate
to 0.20 µ per second. After 15 minutes the growth rate fell to 0.13 µ
per second; and after an hour to 0.08 µ per second. It will be noted
that even then the rate was twice the initial rate before irrigation.
TABLE XII.--EFFECT OF IRRIGATION.
+---------------+---------------------------+------------------+
| Specimen. | Condition of Experiment. | Rate of growth. |
+---------------+---------------------------+------------------+
|_Kysoor_ |Dry soil |0.21 µ per second.|
| |Irrigation with cold water |0.30 µ " " |
| |Irrigation with warm water |1.33 µ " " |
|Peduncle of |Dry soil |0.04 µ " " |
|_Zephyranthes_ |Irrigation with warm water |0.20 µ " " |
+---------------+---------------------------+------------------+
EFFECT OF ARTIFICIAL INCREASE OF INTERNAL HYDROSTATIC PRESSURE.
Increased turgor was, next, artificially induced by increase of
internal hydrostatic pressure.
_Experiment 71._--The plant was mounted water-tight in the short limb
of an U-tube, and subjected to increased hydrostatic pressure by
increasing the height of the water in the longer limb. Table XIII
shows how increasing pressure enhances the rate of growth till a
critical point is reached, beyond which there is a depression. This
critical point varies in different plants.
TABLE XIII.--EFFECT OF INCREASED INTERNAL HYDROSTATIC PRESSURE
(_Kysoor_).
+-----------+-----------------------+-------------------+
| Specimen. | Hydrostatic pressure. | Rate of growth. |
+-----------+-----------------------+-------------------+
| | Normal | 0.18 µ per second.|
| No. I | 2 cm. pressure | 0.20 µ " " |
| | 4 cm. " | 0.11 µ " " |
| | | |
| | Normal | 0.13 µ " " |
| No. II | 1 cm. pressure | 0.20 µ " " |
| | 3 cm. " | 0.18 µ " " |
| | 4 cm. " | 0.15 µ " " |
+-----------+-----------------------+-------------------+
RESPONSE TO NEGATIVE VARIATION OF TURGOR.
I shall now describe the influence of induced diminution of turgor on
the rate of growth.
[Illustration: FIG. 71.--Effect of alternate increase and diminution of
turgor on the same specimen: N, normal rate under drought; H, enhanced
rate under irrigation with warm water; N′, normal permanent rate after
irrigation; P, diminished rate after plasmolysis (_Zephyranthes_).]
_Method of plasmolysis: Experiment 72._--Being desirous of
demonstrating the responsive growth variations of opposite signs in an
identical specimen under alternate increase and diminution of turgor,
I continued the experiment with the same peduncle of _Zephyranthes_
in which the growth acceleration was induced by irrigation with warm
water. In that experiment the growth rate of 0.04 µ per second was
enhanced to 0.20 µ per second after irrigation. A strong solution of
KNO_{3} was now applied at the root; and the growth-rate fell almost
immediately to 0.03 µ per second, or nearly to one-third the previous
rate, the depression induced being thus greater than under condition of
drought (Fig. 71).
TABLE XIV.--EFFECT OF ALTERNATE VARIATION OF TURGOR ON GROWTH
(_Zephyranthes_).
+--------------------------------+-------------------+
| Condition of Experiment. | Rate of growth. |
+--------------------------------+-------------------+
|Dry soil | 0.04 µ per second.|
|Application of warm water | 0.20 µ " " |
|Steady growth after 1 hour | 0.08 µ " " |
|Application of KNO_{3} solution | 0.03 µ " " |
+--------------------------------+-------------------+
From the series of results that have been given above, it will be seen
that employing very different methods of turgor variation, the rate
of growth, within limits, is enhanced by an increase of turgor. A
diminution or negative variation of turgor, on the other hand, brings
about a retardation or negative variation in the rate of growth. We
should, in this connection, bear in mind the fact that, growth is
dependent on protoplasmic activity, and the variation of turgor itself
is also determined by that activity.
RESPONSE OF MOTILE AND GROWING ORGANS TO VARIATION OF TURGOR.
I have already described (p. 40) the effects of variation of turgor
on the motile pulvinus of _Mimosa_. There is a strict correspondence
between the responsive movement of the leaf of _Mimosa_ and the
movement due to growth, which is summarized as follows:--
(1) _An increase or positive variation of turgor induces an erection
or positive response of the leaf of_ Mimosa, _and a positive
variation or enhancement of the rate of growth_.
(2) _A diminution or negative variation of turgor induces a fall
or negative response of the leaf of_ Mimosa, _and a negative
variation or retardation of the rate of growth_.
EFFECT OF EXTERNAL TENSION.
_Experiment 73._--The recording levers are at first so balanced that
very little tension is exerted on the plant. Record of normal growth
is taken of a specimen of _Crinum_. The tension is gradually increased
from one gram to ten grams. The table given below shows how growth-rate
increases with the tension till a limit is reached, after which there
is a retardation.
TABLE XV.--EFFECT OF TENSION ON GROWTH.
+--------------+--------------------+
| Tension. | Rate of growth. |
+--------------+--------------------+
| 0 (Normal) | 0.41 µ per second. |
| 4 grams | 0.44 µ " " |
| 6 " | 0.48 µ " " |
| 8 " | 0.52 µ " " |
| 10 " | 0.40 µ " " |
+--------------+--------------------+
SUMMARY.
Increase of turgor induced by irrigation enhances the rate of growth.
Irrigation with warm water induces a further augmentation of the rate
of growth.
The latent period for enhancement of growth depends on the distance of
growing region from the root. The latent period is reduced when the
plant is irrigated with warm water.
Artificial increase of internal hydrostatic pressure, up to a critical
degree, enhances the rate of growth.
A diminution or negative variation of turgor depresses the rate of
growth.
There is a strict correspondence between the responsive movement of
the leaf of _Mimosa_, and the movement due to growth. An increase or
positive variation of turgor induces an erection of positive response
of the leaf of _Mimosa_, and a positive variation or enhancement of the
rate of growth. A diminution or negative variation of turgor induces
a fall or negative response of the leaf of _Mimosa_, and a negative
variation or retardation of the rate of growth.
External tension within limits, enhances the rate of growth.
XIV.--EFFECT OF ELECTRIC STIMULUS ON GROWTH
_By_
SIR J. C. BOSE,
_Assisted by_
GURUPRASANNA DAS.
In plant physiology, the word ‘stimulus’ is often used in a very
indefinite manner. This is probably due to the different meanings which
have been attached to the word. An agent is said to _stimulate_ growth,
when it induces an acceleration. But the normal effect of stimulus is
to cause a retardation of growth. It is probably on account of lack of
precision in the use of the term that we often find it stated, that a
stimulus sometimes accelerates, and at other times, retards growth.
In order to avoid any ambiguity, it is very desirable that the term
stimulus should always be used in the sense as definite as in animal
physiology. An induction shock, a condenser discharge, the make or
break of a constant current, a sudden variation of temperature, and a
mechanical shock bring about an excitatory contraction in a muscle.
These various forms of stimuli cause, as we have seen, a similar
excitatory contraction of the motile pulvinus of _Mimosa pudica_. We
shall enquire whether the diverse forms of stimuli enumerated above,
exert similar or different reactions on the growing organ.
EFFECT OF ELECTRIC STIMULUS OF VARYING INTENSITY AND DURATION.
The form of stimulus which is extensively used in physiological
investigations, is the electric stimulus of induction shock which is
easily graduated by the use of the well known sliding induction coil,
in which the approach of the secondary to the primary coil, indicated
by the higher reading of the scale, gives rise to increasing intensity
of stimulus. The retarding effect of electrical stimulus on growth has
already been demonstrated in record taken on a moving plate (Fig. 61).
I shall adopt for unit stimulus, that intensity of electric shock which
induces a barely perceptible sensation in a human being. It is very
interesting to find, as stated before, that growth is often affected by
an electric stimulus, which is below the range of human perception.
[Illustration: FIG. 72.--Effects of electrical stimulus of increasing
intensities: of 0.25 unit, 1 unit, and 8 units. Short dashes represent
the moments of application of stimulus.]
_Effect of Intensity: Experiment 74._--I shall now describe a typical
experiment on the effect of intensity of stimulus in retarding the
rate of growth. The normal rate of growth of the bud of _Crinum_
was 0.35 µ per second. On the application of electric shock of unit
intensity for 5 seconds, the rate became reduced to 0.22 µ per second.
When the stimulus was increased to 2 units, the retarded rate of growth
was 0.07 µ per second. When the intensity was raised to 4 units, there
was a complete arrest of growth. In figure 72 is given records of a
different experiment which show the effects of increasing intensity of
stimulus in retardation of growth.
[Illustration: FIG. 73.--Effect of continuous electric stimulation of
increasing intensity. The last record exhibits the actual shortening of
the growing organ under strong stimulus.]
_Effect of continuous stimulation: Experiment 75._--The effect of
continuous stimulation of increasing intensity will be seen in the
record (Fig. 73), taken on a moving plate. On application of continuous
stimulus of increasing intensity an increased flexure was produced in
the curve, which denoted greater retardation in the rate of growth.
When the intensity of stimulus was raised to 3 units, there was induced
an actual contraction.
CONTINUITY BETWEEN INCIPIENT AND ACTUAL CONTRACTION.
It will thus be seen, that external stimulus of electric shock
induces a reaction which is of opposite sign to the normal growth
elongation or expansion. We may conveniently describe this effect as
‘incipient’ contraction; for under increasing intensity of stimulus,
the contractile reaction, opposing growth elongation, becomes more and
more pronounced; at an intermediate stage this results in an arrest of
growth; at the further stage, it culminates in an actual shortening
of the organ. There is no break of continuity in all these stages.
I shall, therefore, use the term ‘contraction’ in a wider sense,
including the ‘incipient’ which finds expression in a retardation of
growth.
In Table XVI is given the results of certain typical experiments on the
effect of stimulus of increasing intensity and duration.
TABLE XVI.--EFFECT OF INTENSITY AND DURATION OF ELECTRIC STIMULUS ON
GROWTH.
+--------------------------+------------+------------------+
| Duration of Application. | Intensity. | Rate of growth. |
+--------------------------+------------+------------------+
| |Normal |0.35 µ per second.|
|5 seconds |1 unit |0.22 µ " " |
| " |2 units |0.07 µ " " |
| " |4 " |Arrest of growth. |
+--------------------------+------------+------------------+
| |Normal |0.30 µ per second.|
|Continuous stimulation |0.5 unit |0.20 µ " " |
| " " |1 " |0.09 µ " " |
| " " |3 " |Contraction. |
+--------------------------+------------+------------------+
With regard to the question of immediate and after-effect of stimulus,
I find great difficulty in drawing a line of demarcation. Owing to
physiological inertia there is a delay between the application of
stimulus and the initiation of responsive reaction (latent period);
owing to the same inertia, the physiological reaction is continued
even on the cessation of stimulus. All responsive reactions are thus
after-effects in reality. The latent period is shortened under strong
stimulus, but the contractile reaction becomes more persistent. When
the stimulus is moderate or feeble, the recovery from incipient
contraction takes place within a short time. Stimulus, under certain
circumstances, is found to improve the ‘tone’ of the tissue, and as we
shall presently see bring about, as the after-effect, an enhancement of
the rate of growth.
The effect of electric stimulus is thus an incipient or actual
contraction.
SUMMARY.
In normal conditions electric stimulus induces an incipient contraction
exhibited by the retardation of the rate of growth. Growth is often
affected by an electric stimulus which is below human perception.
Under increasing intensity of stimulus, the contractile reaction
opposing growth elongation becomes more and more pronounced. At a
critical intensity of stimulus growth becomes arrested. Under stronger
intensity of stimulus growing organ undergoes an actual shortening in
length.
There is continuity between the incipient contraction seen in
retardation, arrest of growth, and contraction of the organ under
stronger stimulus.
The latent period of responsive variation of growth is shortened under
stronger stimulus, but the period of recovery becomes protracted.
XV.--EFFECT OF MECHANICAL STIMULUS ON GROWTH
_By_
SIR J. C. BOSE.
Amongst the various stimuli which induce excitation in _Mimosa_ may be
mentioned the irritation caused by rough contact, by prick, or wound.
Friction causes moderate stimulation, from which the excited pulvinus
recovers within a short time. But a prick or a cut induces a far more
intense and persistent excitation; the recovery becomes protracted, and
the wounded pulvinus remains contracted for a long period.
I shall now describe the effect of mechanical irritation on growth. For
moderate stimulus, I employ rough contact or friction; more intense
stimulation is caused by a prick or a cut.
EFFECT OF MECHANICAL IRRITATION.
[Illustration: FIG. 74.--(_a_) N, normal rate of growth; F, retarded
rate immediately after friction; A, partial recovery after 15 minutes.