which the plant was exposed.
(187) Light.
Fit up a Narcissus as above described in the dark
room, and after 2 or 3 hrs. take down the shutter so that
the plant is illuminated ; no sunshine must be admitted,
otherwise the temperature of the room will be affected.
The period of illumination should last 3 hours, when the
shutters should be once more closed. In this experiment
it is important to take readings of the wet- and dry-bulb
thermometers occasionally, both during the dark and the
light period.
(188) Liglit
Vines^ has shown that light has a retarding effect on
the growth of Phycomyces. A ripe sporangium is allowed
to burst in a watch-glass of water and a few drops are
placed, by means of a needle, on a thick slice of bread,
which should be previously steamed to roughly sterilise it.
The bread is placed in a saucer containing a little water
and covered with a flat-sided glass cover. It is now placed
on an apparatus by which it is kept revolving once in 30
minutes on a vertical axis, so as to avoid heliotropic
curvatures-. If the hyphse are growing vigorously read-
ings may be taken every 15 minutes for an hour, and
afterwards at intervals of 30 minutes. When sufficient
readings have been taken to indicate the course of the
growth-rate, i.e. to ascertain whether the rate is steady,
1 Sachs' Arbeiten, ii. p. 133.
2 We use a drum turning on a strong steel axis and driven by an
endless band connected with a pulley driven by clockwork.
D. A. 11
162 GROWTH. [CH. VI
or increasing or decreasing, the culture is darkened by a
thick cardboard cover placed over the glass. The rotation
should be continued and temperatures taken by a ther-
mometer of which the bulb is inside the glass vessel
covering the fungus. It is a good plan to wet the
cardboard cover on the outside, so that the temperature
during the dark period may be slightly cooler than during
the period of illumination. After half-an-hour or an hour
the dark cover is removed and the fungus allowed to grow
in light for an hour, during which its growth should be
noted once or twice. The rate of growth in the dark
should differ from that in the light by something like 20 p.c.
(189) Light
The effect of light on the growth of roots should
also be demonstrated on the apheliotropic roots of Sinapis
alba, on account of the interest of the fact in relation
to the theories of heliotropic curvature \ The seedling
mustards are supported by plugs of cotton-wool in holes
in a cork plate so that the roots dip in water.
The details of the experiment are practically the same
as in exp. 188, but the periods of light and dark should be
longer, say 2 or 3 hours.
(190) Periodicity.
A Narcissus kept for 24 hrs. in the dark room will
record its periodic changes in growth- rate. This will
probably not be evident on merely inspecting the tracing
on the drum; a curve representing growth must therefore
be drawn on a fairly large scale.
1 Francis Darwin, Ueber das WacJisthum negativ heliotropischer
Wurzeln. Sachs' Arbeiten, ii. p. 521.
CHAPTER VII.
CURVATURES.
Section A.
Section B.
Section C.
Section D.
Geotropism.
Decapitation experiments, curvatures due to in-
jury, to contact, to unequal turgidity, to
flaccidity.
Heliotropism.
Diaheliotropism, Diageotropism, epinasty, nuta-
tion of epicotyls.
Section A. Geotropism.
(191) Region of curvature coincident with region of groivth.
If the curvature is to be observed in damp air a bean-
root of not more than 15 mm. in length answers best.
It should be marked at intervals of 2 mm. and pinned
to the lid of a stoppered jar half full of water; the jar
should be occasionally inverted for a moment or two so
as to thoroughly wet the seedling and the cork. After
12 hours measure the distance between the marks,
which gives the region of greatest growth, and note the
11—2
164 REGION OF CURVATURE. [CH. VII
position of the greatest curvature. It is however better
to let the root grow in damp sawdust behind glass.
The glass wall is not vertical but slopes at an angle of
about 80° ; the advantage of the sloping wall is that the
root, in its attemi3t to grow vertically down, is closely
pressed against the glass, and is visible from the outside.
The marks must be made on the side which comes against
the glass. The figure p. 387 of Sachs' paper {Arbeiten, I.)
should be consulted.
(192) Region of curvature.
In summer any rapidly growing vertical shoots will
serve. Cabbage-shoo bs answer well, also the stems of
valerian \ They should be marked at intervals of 10
mm. and may be either fixed by means of bored and split
corks in bottles of water, or into an embankment of wet
sand in the angle of a tin box, the air being kept
thoroughly damp by wet sand sprinkled over the whole of
the bottom. At a temperature of about 20° C. curvature
will be well marked in 3 hours, when the form of curve
should be noted and the distance between the marks
measured. A strip of thin sheet-lead makes a useful
scale by which to measure the distance between the
marks, since it can be easily applied to the curved surface,
and retains its curvature. Kothert^ recommends strips
cut from the paper used for Richard's Thermograph. The
lines are said to be fine and at accurately equal distances.
^ In winter young sunflower seedlings may be used.
- Ueher Heliotropismus . Breslau, 1894, p. 29.
CH. VIl] GRASS-HAULMS. 165
(193) Subsequent change in the form of curvature\
If specimens prepared as in exp. 192 are left
undisturbed for some hours longer, the free end of the
shoot will be carried far beyond the vertical. A record of
this movement and the subsequent return to the vertical
may be made, by using a box with one vertical glass
wall ; if the shoot is fixed (in a sand-embankment) so
that it is close to the glass, its changes of form can be
traced with a paint-brush filled with black varnish on the
glass. Sachs' large diagrams published with Vol. ill. of his
Ar'heiten should be consulted.
(194) Grass-haulms'^.
Cut a grass-haulm which is vertical and in which the
pulvinus has not curved. Mark the pulvinus on two
opposite faces by means of dots some 2 or 3 mm. apart.
Having measured the distance between the marks, push
the haulm horizontally into a sand-embankment in a tin
box so that one set of marks are above, the other below.
After 24 hours or more the haulm will have bent at the
pulvinus, when the marks must be again measured. The
lower surface will have increased greatly in length while
the upper surface has become shorter.
Note the pale colour of the lower half of the pulvinus ;
and if the pulvinus is a hairy one, note the divergence of
the hairs below and their convergence above.
1 Sachs' Collected Papers, ii. p. 967 (from Flora, 1874).
- Sachs' Collected Papers, ii. p. 958 (from the Arheiten, i.).
166 KESPIRATION. [CH. VII
(195) Noll's experiment^.
Noll's method is to fix a grass-haulm into a glass tube
so narrow that it only just fits, and to leave it horizontal
in damp air. The pulvinus cannot curve, but the lower
half of the pulvinus grows out into curious excres-
cences. We arrange the experiment differently. Cut
a groove in a block of cork about 5 mm. wide and
5 deep. Arrange 3 or 4 grass stalks on the cork so that
they lie across the groove with the pulvinus of each over
the groove. Fix them in place by two sheets of cork, one
of which pins down the stalks on one side of the groove,
while the second sheet fixes the parts of the stalks on the
opposite side. The whole arrangement is put in a tin
box half-full of wet sawdust for 5 or 6 days, when the
result should be clear.
(196) Fi^ee oxygen necessary.
The bean, which should have a short root (15 mm.), is
pinned in the vertical position to the lower surface of a
rubber cork fitting tightly into the ground neck of a
small bottle. Hydrogen is passed through the bottle for
about 2 hours to replace the contained air. The bottle is
then placed on its side so that the root is horizontal.
The tube connecting with the hydrogen-generator is kept
open while the outflow tube by which the hydrogen
current escaped from the bottle is shut ; in consequence
of this arrangement the only leakage that can occur is an
escape of hydrogen. After 24 hours, during which no
1 Sachs' Arbeiten, iii. p. 509.
CH. vii] Johnson's experiment. 167
geotropic curvature should occur, the hydrogen is replaced
by air, when the root will bend downwards.
(197) Free oxygen necessary.
The same result may be more simply obtained by
keeping seedling beans completely submerged while
control specimens are in damp air, or just touching the
surface of the water. The geotropic curvature is absent
in the case of the submerged specimens.
(198) Johnsons experiment^.
The interest of this experiment (in which a root does
external work during geotropic curvature) is now some-
what historical. Its original object was to demonstrate
" the unsatisfactory nature of the theories proposed to
account for the descent of the radicle V' i.e- to show that
the root does not bend by mere plasticity. A method
of performing the experiment is shown in Pfeffer's
Plujsiologie, Vol. ii. p. 320, fig. 36.
A similar experiment may be more simply arranged
in which the resistance is given by a spring. A pin is
driven vertically into the inside of the lid of a jar, and
from the lower end of the pin a thin copper wire projects
horizontally ; at the end of the wire a microscopic cover-
glass is cemented so that it lies horizontally. A bean is
now pinned to the lid so that its root projects horizontally
and rests on the glass-cover ; as the root curves down it
overcomes the elasticity of the wire. The cotyledons and
1 Edinh. New Phil. Journal, 1829, p. 312.
" From the title of Johnson's paper.
168 PINOT's experiment. [CH. VII
base of the root should be kept damp by a strip of filter-
paper hanging over the seed like a rider and dipping into
the water below.
(199) Pinot's experiment^.
This experiment is the same in principle as the last,
but the resistance is supplied by the descent of the root
into mercury. The arrangement of the experiment is
shown in Sachs' paper on the growth of roots l A shallow
dish of 6 — 8 cm. in diameter is filled with mercury to a
depth of 2 — 3 cm., on which is a layer of water 5 — 6 mm.
in thickness. A split cork is firmly jammed like a rider
on the edge of the dish, and to it a bean is pinned so that
the root lies horizontally in the water just touching the
surface of the mercury. The whole arrangement is
covered with a bell-jar and left for 24 — 48 hours. Another
way of fixing the bean, which we find convenient, is
simply to support the pin, on which the cotyledons are
impaled, in a clamp attached to a small heavy stand.
We usually keep the cotyledons wet with a strip of filter-
paper dipping into the water.
(200) Knight's experiment^.
Sachs figures^ an apparatus which any one can con-
struct for himself, and by which it may be demonstrated
^ Ann. Set. Nat., series 1, T. xvii. 1829 (Bibliography, p. 94). See
also Hofmeister in Pringsheim's Jahrbilcher, Vol. iii. p. 105.
2 Arbeiten, i. p. 452, fig. 14.
3 Phil. Trans. 1806.
^ Physiologic (French Trans. ), p. 124.
CH. vii] knight's experiment. 169
that the geotropic parts of plants bend in relation to
centrifugal force.
Another simple plan is to use a water-wheel driven by
a strong fine jet of water directed against the wheel from
the water-tap. The wheel should stand in a sink fitted
with a cover; in this way, — with the help of the spray
from the wheel — the experimental plants are kept
thoroughly damp.
We use an apparatus designed by Mr H. Darwin.
A disc covered with a thick layer of cork is attached to a
horizontal axis turning on bicycle ball-bearings. It turns
with ease and is driven at considerable velocity by an
endless band from a turbine. The experimental plants
are kept damp by a bell-jar which is not attached to
the revolving disc, but fits by its broad ground edge
against a fixed vertical metal plate, through which the
axis passes. The space in which the plants rotate is
not therefore absolutely closed, but the air can be kept
sufficiently damp for practical purposes. The most serious
drawback to the apparatus is that the plants are
subjected to a current of air produced by their own
rotation. This evil has been fairly well overcome by a
four-armed fan attached to the disc, and dividing the
space inside the bell into four compartments ; as the fan
rotates, the air within the bell-jar is carried round with
the plants.
To use the apparatus it is only necessary to pin
seedling beans so that the root lies tangentially ; each
bean must be fixed on two pins firmly driven into the
cork. They should be fixed near the circumference of the
170 knight's experiment. [CH. VII
disc, but it must be remembered that the roots will curve
away from the centre of rotation ; allowance must
therefore be made for their growth in that direction.
Roots curve perfectly well even when covered by a layer
of wet sponge pinned completely over them, an arrange-
ment which insures their being kept damp.
The scape of Taraxacum or cabbage-shoots may be
used for apogeotropic curvature. Each shoot is fixed in a
bored cork through which, and through the contained shoot,
two strong pins are forced into the cork.
It is well to devote one quarter of the space inside the
bell-jar to a piece of dripping wet sponge pinned firmly to
the cork ; this serves to keep the air moist.
The apparatus should be made to turn at 600 or 700
revolutions a minute which gives a centrifugal force equal
to about six times gravity^ when the experimental plants
are at a convenient distance from the centre.
In from 12 to 24 hours a good result should be
obtained.
In Pfeffer's laboratory an instrument is used^ which
has the advantage of giving a high centrifugal force
without excessive rapidity of rotation. The rotating body
being 150 cm. in diameter, it is possible to fix plants at
various distances, up to 75 cm. from the centre of rotation,
and thus to experiment with a graduated series of stimuli
at the same time.
^ It is convenient to keep a table from which the centrifugal force
can at once be calculated from the revolutions per minute and the
distance of the plant from the centre of rotation.
2 See Fr. Schwarz, in Untersuchungen aus dem botan. Institut zu
Tubingen, i. 1881, p. 57.
CH. VIl] SUDDEN CURVATURE. 171
(201) Sudden curvature^.
When a growing shoot is prevented from curving
apogeotropically, the gravitation stimukis nevertheless
produces some change, so that when freed from constraint
the shoot suddenly bends upwards.
The constraint may be applied by placing the shoots
horizontally on a shallow layer of damp sawdust, and
keeping them down with a sheet of plate-glass. Or they
may be fixed to a sheet of cork by pins crossing over the
shoot like an X, one such fastening being placed at each
end and one in the middle ; the cork must then be placed
in damp air for some 6 hours, when the plants may be
unpinned.
(202) After-effect
A turgescent shoot is fixed by means of a cork into
a bottle of water so placed that the shoot projects horizon-
tally. A needle to serve as an index is fixed in the free
end of the shoot and its position recorded on a vertical
scale. After about an hour, — or when the shoot has
begun to curve apogeotropically, — the bottle is rotated
on its axis through 180°, so that the plane of curvature
remains vertical, but what was the upper side of the
shoot is now the lower. The index will now travel
downwards over the scale, owing to the continuance of
the curvature induced by the gi-avitation stimulus. It
will finally come to rest and will at last curve up in tlie
opposite direction ^
^ Sachs' Arbeiten, i. p. 204.
2 Sachs' Collected Papers, ii. p. 966 (from Flora, 1874).
172
AFTER-EFFECT.
[CH. VII
(203) After-effects recorded on a rotating surface.
The movements described at the end of exp. 202 may
be made to record themselves in the following way.
Fig. 31. Exp. 203.
The writer (fig. 31) is made of two light pieces of wood,
the horizontal piece ends in a pointed piece of platinum
foil, while the vertical piece ends in a loop of cotton by
which it is attached to the shoot. By twisting the loop
it is easy to make the foil press lightly against the
smoked paper of a revolving drum^ ; the T piece being
light (about 0*2 gram) it does not interfere with the
geotropic action. In a typical experiment of this sort the
primary geotropic rise lasted 4 hours, the after-effect
2 hours. Then came 2 hours in which the index was
stationary ; finally it rose once more, beginning very
slowly.
1 We use the auxanometer drum, but a continuously rotating drum
would give a better result.
CH. Vll] DECAPITATED ROOTS. 173
Section B. Curvatures due to injury, contact, etc.
(204) Decapitation of roots'^.
Select 10 healthy germinating beans with straight
roots growing vertically downwards. From 5 of them cut
off 1 mm. measured from the extremity of the root-cap :
the amputation must be made by a strictly transverse
section, and the amputated point should include part of
the growing point. Place the 10 beans horizontally in
damp sawdust for 12 to 18 hours at a temperature
of 15° — 16° C. and compare the amount of geotropic
curvature.
The result is not quite constant, it may however be
safely said that decapitation prevents or greatly diminishes
geotropic curvature I
(205) Decapitation prevents the perception of the stimulus.
Place 10 beans horizontally in damp sawdust for
1^ hours ; the tips {\^ mm.) are now amputated and
the roots embedded vertically in damp sawdust. After
12 hours the roots, or most of them, will be found to be
curved laterally towards the side which was downwards
during the period (1^ hr.) during which they were kept
horizontal. This shows that amputation does not interfere
with the carrying out of an induced curvature, so that the
absence of geotropism in exp. 204 must be due to a
disturbance of the capability of being stimulated.
1 Ciesielski, Ahioartskriimmung der Wnrzel Inaug. Dissert. Breslau,
1871. Power of Movement, p. 523.
- A good deal of literature exists on this point and on the facts given
in exp. 207. References are to be found in Frank's Lehrbuch der
Botanik, i. p. 477.
174 DECAPITATED ROOTS. [CH. VII
(205 a) Pfeffers experiment
The final proof that the root-tip alone is sensitive to
the gravitation-stimulus has been given by Pfeffer^ and
by his pupil Czapek^ : the following instructions are taken
from their publications.
As material, the authors recommend Vicia faha and
Lupmus, the latter being more sensitive but also more
liable to accidental curvatures. A number of glass tubes
in which the roots are to be constrained to grow into a
desired form must first be prepared. These are L-shaped,
one end being closed, the other open ; their total length
is 3 mm. and they weigh about 30 milligrams. Czapek
says they are easily made by drawing out a thick-
walled tube of soft glass in the blowpipe : the rect-
angular bend is got by heating a very short region, care
being taken to avoid narrowing the bore by a sudden
bend. One limb of the tube is closed in the blowpipe at
1"5 mm. from the angle, the other limb broken across at
the same length, the edges being afterwards rounded in
the flame. The bore of the tube must depend on the size
of the roots used, and it is important that the tubes should
not fit too closely.
The caps are cemented to a sheet of cork, to which
seedling beans, lupins, &c. are pinned in such a way that the
tip of each root is contained in the open limb of a L tube,
and reaches nearly to the bending place. The cork plate
1 British Association, August, 1894, Annals of Botany, viii. Sept. 1894,
p. 317.
2 Jahrb. /. iviss. Bot., xxvii. 1895, p. 243.
CH. VIl] DECAPITATED ROOTS. l75
is now fixed to a klinostat and kept in slow rotation in
damp air for 8 to 12 hours. The gravitation-stimulus
being removed by the use of the klinostat, the roots grow
freely into the glass tubes and are forced to assume their
form : thus each root has a sharp rectangular bend at
1'5 mm. from its tip. If the experiment has been properly
done the tubes should fit the roots so loosely that they can
be taken off and replaced with ease: this is said to be a
condition of success.
The specimens having been removed from the cork
plate, they should be fixed (the tubes still attached) in
various positions. If the terminal I'o mm. is vertical and
the basal part of the root horizontal, no geotropic curve
occurs, although the root grows vigorously. The part of
the root within the horizontal limb of the tube is displaced
by the new growth within the tube and gradually emerges
from the tube. The fact that the growth of the root
continues horizontally seems only explicable by the
supposition that the tip of the root alone is geotropically
sensitive, and since this points vertically downwards it is
in a satisfied condition, without any tendency to curvature.
Other specimens should be fixed with the terminal I'o mm.
horizontal ; under these conditions the vertical portion of
the root outside the tube bends laterally until the tip of
the root is vertical. In this experiment the cotyledons
may be above, the main axis of the root pointing ver-
tically downwards, or vice versa, the root may be directed
upwards. For the different form of the subsequent
curvature in the two cases, Czapek's figs. 2 and o should
be consulted.
176
INJURY.
[CH. VII
Fig. 32. Exp. 206.
(206) Recovery from the effect of amputation.
If the roots in exp. 205 (p. l7o) are allowed to remain
undisturbed for 3 or 4 days, the growing point is re-
generated and the roots recover the
power of geotropic curvature. They
will grow into an S-like form, because
until the growing point is regene-
rated they continue growing horizon-
tally or obliquely in the direction
Impressed on them by the geotropic
curvature induced before amputation.
When the power of reacting to the
gravitation-stimulus is restored they
curve downwards. Fig. 82^ shows this
state of things, L indicates the side of
the seedling which was downwards while the root was
horizontally extended, AB shows the strong induced
curvature, BC the second geotropic curvature occurring
after regeneration of the root-cap.
(207) Curvature induced hy contact, injury, dx.^.
If the tip of a bean root is amputated by an oblique
cut with a razor, so that the growing point is laterally
injured, the root curves away from the injured side. The
beans should be pinned to the lid of a jar and may be
grown either in damp air or with the tips in water. It is
imjDortant that the temperature should not exceed 16° C.
A similar curvature may be produced by contact.
Minute squares (lo mm. x 1*5) of cardboard or of very
1 Poiver of Movement, fig. 195, p. 527.
~ Ibid., Chap. iii.
CH. VIl]
INJURY.
177
fine sand-paper (which adheres well) are to be fixed to the
tips of bean roots by a thin layer of strong shellac varnish
which sets hard very quickly. It is important that the
squares of card should be attached to the slope of the root-
apex, and that they should be either on the right or left of
the root-tip : if they are on the anterior or posterior face of
the root, the resulting curvature will be in the plane of
the cotyledons and therefore liable to be interfered with
by Sachs' curvature which occurs in the same plane. The
side on which the card must be placed will be understood
from Fig. 33, which represents bean roots with cards
attached, and showing various degrees of curvature.
Fig. 33. Exp. 207.
(From The Poxoer of Movement, fig. 65, p. 134.)
D. A.
12
178 UNEQUAL TURGESCENCE. [CH. VII
(208) Ciesielskis experiment^.
This experiment is of interest in relation to the
mechanism of growth-curvatures because it shows that
curvatures might conceivably arise through unequal
turgescence of the two sides of a plant-member.
Take a germinating bean with rather a long root, say
40 — 45 mm., and let it lie on the table for a minute
or two, so that it may begin to wither. Impale the seed
on a pin transversely to the plane of the cotyledons, and
fix the pin in a clamp so that the root is horizontal and
1 — 2 mm. above a surface of water. Then gently bend the
root downwards till its lower surface and the water meet.
In a few minutes the tip of the root will be seen to
be rearing itself into the air, which is due to the increased