where references to his original paper and to Errera's work on the subject
are given.
' Sachs' Arheiten, i. p. 212, Fig. 3.
^ 5 cm. deep, 20 cm. in diameter.
214 CHLOROPLASTS. [CH. VIII
dark room of which the floor is occasionally watered.
According to the same authority the air should be in such
a condition that the difference between the wet- and dry-
bulb thermometers is 1-5°— 2-0° R. (2°— 2-5° C). We
find it a good plan occasionally to squirt with water the
lower surface of the sieve.
(248) Movement of chloroplasts.
We find that the leaves of Oxalis acetosella^ give good
results. Ten or twelve leaves are taken from a plant, and
after the stalks have been cut short off beneath the
pulvini they are placed floating in water. Half of the
number in one dish are exposed to bright sunshine, the
rest remain in dull diffused light. After two hours they
may be examined by preparing surface sections of the
spongy parenchyma. In the sunned leaves the chloroplasts
are in the " profile position," that is, they lie against the
side walls of the stellate parenchyma cells, and may even
be crowded into the corners. In the shaded leaves they
are spread out and dotted over the surfaces which are
parallel to the plane of the leaf The leaves may be pre-
served in alcohol for future examination.
(249) Ghemotaxis: antliey^ozoids.
The following instructions are taken from Pfefifer's
paper in his Untersuchungen\ The prothalli which yield
the antherozoids for Pfeffer's experiments were chiefly
1 This plant is recommended by Stahl, Botanische Zeitung, 1880,
Stahl's figure of Oxalis is copied in Frank's Lehrhiich, p. 289.
2 Untersuchungen aus dem hotanischen Institut zu Tilhingen, i. 1881—
1885, p. 363.
CH. VIIl] ANTHEROZOIDS. 215
small ones oi Blechnumfraxineum and Adiantum cuneatum^.
They were grown on lumps of peat in the shade of other
plants and were used when only about a millimeter in
length, and had numerous antheridia but few or no
archegonia. They should be kept only moderately damp,
as this seems to favour the yield of antherozoids. The
prothalli having been washed for a moment are placed 3
or 4 together under a small cover-glass supported on
strips of paper, and are washed by repeatedly drawing a
current of rain water through the preparation. Distilled
water, being injurious to the antherozoids, must not be
used for the washing, the object of which is to remove any
malic acid which may be set free by the rupture or injury
of the tissues of the prothallus. The reasons for
preferring a small cover-glass are that the antherozoids
are thus confined to a smaller space, and that the water is
better oxygenated than when a large glass is used.
Capillary tubes of O'l to 0'14 mm. internal diameter and
from 7 to 12 mm. in length are closed at one end by
melting the glass, and are filled with the malic acid
solution with the help of an air-pump. The solution may
be either the free acid or a salt, for instance, sodium
malate ; the solutions should be made with rain water and
contain about 0*05 per cent.-. A capillary tube is pushed
under the cover-glass, when the antherozoids in the
neighbourhood of the opening are at once attracted to it.
Pfeffer has seen 60 antherozoids enter a tube of malic
1 The young prothalli of Ceratopteris, grown from spores sown on
bricks, give a good supply of antherozoids.
2 The strength may vary from 0-01 to 0-5 per cent.
216 BACTERIA. [CH. VIII
acid within half-a-minute from the beginning of the
experiment.
(249 a) Chemotaxis : Bacteria^.
Allow a boiled pea to decay in about 200 cc. of water
for two or three days : draw off some of the fluid from just
below the surface, transfer a drop to a slide and place on
it a small cover-glass raised on two strips of paper.
Capillary tubes, like those used in exp. 249, are made by
drawing out coarse tubes in the blowpipe flame and again
drawing out the tubes so made over a small flame.
Lengths (10 mm.) of the fine capillary tubes should be
sealed, each at one end, and filled with 2 p.c. KNO3 under
the air-pump. They may be removed from the beaker of
KNO3 with a camel-hair paint-brush, and should be
examined under the microscope to make sure that they are
full. Having been washed with a drop of distilled water
they should be pushed under the cover-glass. In about
10 minutes the open ends ought to be crowded with
Bacteria.
(249 b) Chemotaxis : pollen tubes'^.
This may be easily demonstrated with the wild
hyacinth (Scilla nutans). A thin jelly is made by adding
3 p.c. of good gelatine (for instance the " bacteriological "
gelatine sold by Baird and Tatlock) to water and warming
over a water-bath. A large drop is placed on a slide and
1 Pfeffer, Untersuchungen aus dem hot. Institut zu Tubingen, 11. 1888,
p. 582.
2 Moliseh, Sitzb. d. k. AJcad. d. Wiss. in Wien^ ii. 1893 ; and
Miyoshi, Flora, 1894, p. 76.
CH. VIIl] POLLEN TUBES. 217
into this is stirred a quantity of pollen. Then the stigma
and one or two ovules are inserted at various places in the
jelly while it is still fluid. Care must be taken to avoid
air-bubbles. The slide is then placed in a saturated at-
mosphere, preferably in darkness. After a few hours, in
warm weather, the pollen tubes will be seen under the
microscope directed towards the stigma, the cut-end of the
style and the ovules.
Plantago media is also a good plant for the experiment,
and various species of Reseda work well if 1 p.c. of cane-
sugar be added to the gelatine.
The gelatine soon becomes mouldy and fresh material
must be made up, or else the old must be kept sterile by
repeatedly heating to 80° C. in a water-bath.
The most perfect demonstration of chemotaxis may be
made by mounting Scilla pollen with a stigma in a drop of
10 p.c. cane-sugar on a slide, and arranging a damp chamber
round it with blotting-paper and a cover-slip. The slide is
arranged on the microscope and must not be shaken or
moved. After a few hours the cover-slip is taken off and
the slide examined. The pollen of Narcissus Tazetta may
be used in the same way in 7 p.c. sugar.
(249 c) Ghemotaocis : pollen tubes.
A preparation of pollen in jelly is made as described in
exp. 249 B, except that the ovules and stigmas are omitted
and that a cover-glass is placed on the drop. The pollen
tubes will be observed to grow away from the edge of the
cover-slip towards the centre of the drop, i.e. from places
rich in oxygen to places poor in oxygen. Cane-sugar
218 TULIP, WARMTH. [CH. VIII
solution may also be used, of various strengths for
different plants \ e.g. Fritillaria imperialis, 15 p.c. ;
Narcissus Tazetta, 7 p.c; Vincetoxicum officinale, 15 p.c.
It should be noted^ that all pollen tubes do not exhibit
this phenomenon.
(250) Opening and closing of the tulip : temperature.
Many flowers open with a rise of temperature and
close with a fall ; the best adapted for experiment are the
crocus and tulip ^. Both of these are sensitive to slight
changes of temperature, and both are valuable because
they can be made by appropriate treatment to open and
shut at any time of the day. The crocus is the more sensi-
tive of the two, but the tulip answers extremely well, and
the following instructions apply to this genus.
It is convenient to begin the experiment on a cool,
cloudy morning, when the tulips are naturally closed.
Cut a flower and fix it vertically in a cork fitted into a
bottle of water. To one of the outer perianth segments
and to the opposite inner segment fix filaments of glass
drawn out to very fine capillary tubes. They are best
cemented with shellac varnish to the groove or line run-
ning down the centre of the outer surface of the segment.
The filaments, each of which projects 8 cm. beyond the
flower, serve as indices for noting the movements of the
segments. The simplest plan is to fix a millimeter scale
horizontally so that the distance between the points of
the indices can be read off. The tulip should be prepared
^ See Molisch, loc. cit. , where a list of suitable solutions is given.
- See Molisch, loc. cit.
^ Pfeffer, Physiologische Untersuchiingen, 1873, p. 181.
CH. VIIl] TULIP, WARMTH. 219
in a room free from sunshine, and where the temperature
is not above 15° C, — a temperature of 11° or 12° better
still.
The flower having been left to itself for 15 minutes is
placed in a temperature of about 20° C. In 5 or 10
minutes a clear increase in the reading on the scale shows
that the flower is opening.
It may now be replaced in a temperature of 10° — 12° C.
Notice that the flower continues to open for some time
and then begins to close. The same phenomenon mutatis
mutandis is to be seen on changing a low into a high
temperature. It is easy to make a tulip open, close, and
open again within one hour.
(251) Tulip: sensitiveness to small change of temperature.
Pfeffer^ has seen a crocus flower open slightly in
15 minutes during which the temperature rose by less
than 1° C. The change of temperature was produced by
opening the door between a cold and a warm room. For
class-work it is perhaps best to try rather larger changes
of temperature. A tulip, fitted with two indices as
described above, shows distinct opening in half-an-hour
when moved from a temperature of 13*5C. to a temperature
of 15*5°, closing slightly again on being replaced in a
temperature of 13° C.
(252) Crocus: mechanism of the movement.
The following instructions are based entirely on
Pfefifer's'^ account of the experiments, in which he showed
1 Physiologische Untersuchungen, 1873, p. 183.
2 Ibid., p. 167.
220 CROCUS, WARMTH. [CH. VIII
that when a crocus or tulip opens it does so because of
the accelerated growth on the inner faces of the segments,
and vice versa when it closes. A series of 4 or 5 minute
dots (about 1*5 mm. apart) are made with black spirit-
varnish on both surfaces along the region of curvature,
which in the crocus is the lower J or J of the perianth -
segment. The distance between the marks must be
measured with great care by means of an eye-piece
micrometer : the amounts of growth observed do not
exceed 3 p.c, and it is therefore necessary to use a
magnifying power of something like x 80, and a micro-
meter with which the distance between the marks on the
flower is about 200 divisions of the micrometer. To get
accurate measurements it is necessary to sketch each of
the varnish marks, noting on the drawing a corner or
projecting point from which the reading is taken. The
readings are easily taken on the outside of the perianth
segment, and by removing the opposite segments the
inner marks can also be observed. The readings are
assumed to have been taken on a closed flower, which is
then placed in a room warmer than the first by 6° — 7° C.
and after \ hr., during which the flower opens, the readings
are again taken. On the inner side the marked region
will have increased by 2'5 p.c, on the outer side an
increase of say 0'2 p.c. will be noted. If 'the readings are
taken first in the open flower and then in closed con-
dition, precisely the reverse is noted, namely, that the
inner side increases only a little, while the outer side
grows about ten times as much.
CH. VIIl] DAISY, LIGHT. 221
(253) Light and darkness: Bellis.
Among the flowers which close in darkness and open
when illuminated the daisy {Bellis perennis) is the most
universally accessible \
A daisy should be cut and fixed vertically in a bottle
of water, when the position of the ligulate florets must
be noted ; this may either be done by taking the angle
which the flower-head fills when looked at in profile, or by
measuring the horizontal distance between the tips of
two opposite florets. In one of our experiments a daisy
was darkened at 2 p.m., and the angle showed a diminu-
tion of 30° by 3-15.
Further experiments on the daisy are given in the
next section.
(254) Light and darkness: Trifolium.
Sleeping plants can be made to assume the nocturnal
position by darkening them in the daytime. This can
be shown in any of the common species of clover, such
as T. repens. The simplest plan is to cover a plant
(growing in the open air) with an inverted vessel made of
opaque material, scattering dry powdered soil round the
outside of the rim so as to make sure that light is
excluded. After one or at most two hours the plants
may be examined, when the leaflets will be found in the
nyctitropic position shown in fig. 40, where the left-hand
leaf is awake, the one on the right asleep : the lateral
leaflets are face to face and the terminal leaflet folded on
^ Pfeffer, Physiologische Vntersnchungen, p. 198,
222 CLOVER, SLEEP. [CH. VIII
to their edges. The experiment may also be made with
a sod of clover dug up and kept wet in a basin or even
with cut leaves in a bottle of water.
Fig. 40. Exp. 254.
From The Power of Movement in Plants.
By covering up one plant with a hollow bell-jar
containing potassium bichromate, and another with a bell
containing ammoniacal copper sulphate, it may be shown
that the orange light acts like darkness, while the blue
acts like daylight.
Finally, a few plants should be kept dark for 5 or 6
days to observe the fact that the leaflets ultimately
assume a position resembling the day-position, except that
the leaflets droop somewhat.
(255) Nyctitropic movements.
To get a general idea of the varied character of
nyctitropic movements it is best to compare the diurnal
and nocturnal positions in a selection of plants.
Trifolium has already been described ; as a contrast it
is well to examine the nocturnal positions of a trifoliate
Oxalis, such as 0. acetosella (in which the leaflets point
nearly vertically downwards at night), and of Marsilea
quadrifoliata (in which the four leaflets rise and arrange
themselves in a vertical packet). The nyctitropism of
CH. VIIl] MIMOSA, SLEEP. 223
Melilotus, with its curious right and left-handedness\ of
Cassia, in which the leaves sink and twist, and of Des-
modium gyrans, in which the vertical droop of the larger
leaflets is particularly striking, should also be studied.
Movements not produced by means of a pulvinus, but by
the growth of the leaf-stalk should be examined; for
instance the nocturnal rise of the young leaves of Nico-
tiana glauca or of the cotyledons of the cabbage and
radish (Brassica oler^acea and Raphanus sativus).
In all these cases note that the nocturnal is more
nearly vertical than the diurnal 230sition, and that when
there is close contact between neighbouring leaflets it is
generally the upper surface of the leaf that is protected.
(256) Nyctitropic movements : Mimosa.
In order to study the sleep movements of leaves more
12 niglit
Fig. 41. Exp. 256.
closely we employ a self-recording method. Fig. 41 is
^ Power of Movement in Plants, p. 341), fig. 140.
224 PARAHELIOTROPISM. [CH. VIII
a copy of a tracing^ made by the main petiole of Mimosa
piidica from 4 p.m. Aug. 16, until noon of the following
day. The tracing was made by means of the hanging
writer described in experiment 203, which recorded the
position of the petiole at intervals of half-an-hour on the
revolving drum used for auxanometer experiments. The
tracing only records changes in the vertical position of
the free end of the petiole, and does not give the angle
which the petiole makes with the horizon, but if a few
readings of the angle are taken, the rest can be calculated
from the known length of the petiole and writer. It will
suffice for our present purpose to know that at 7 p.m. the
petiole was roughly 15° below, and at 4 a.m. 60° above
the horizon.
The tracing shows that the leaf sank with increasing
and then decreasing rapidity from 4 p.m. to 7 p.m., when
it rose (at first slowly) until 3 a.m. It then remained
stationary until 4'30, when a fall again occurred, followed
by irregular movements continuing to noon.
(257) Paraheliotropism : Averrhoa hilimhi.
The leaves of many plants assume in bright sunshine
a more or less vertical position, which has been sometimes
called " diurnal sleep " but is now known as parahelio-
tropism. Oxalis acetosella, in which the leaves in bright sun
assume the same vertically dependent position that they
take at night, is a familiar example. Averrhoa hilimhi
1 To diminish the horizontal extension of the diagram, the horizontal
lines drawn by the writing index are reduced. The engraving is more-
over reduced by \ from the drawing so prepared.
CH. VIIl]
AVERRHOA.
225
(one of the Oxalida;) also drops its leaflets in sunshine
as it does at night. The leaves of Averrhoa, as described
in exp. 261, exhibit remarkable autonomous movements \
in which the leaflets drop rapidly through 15^ — 20',
then rise slowly to their original position, repeating the
movement once in 15 minutes or so. When sunshine
strikes the plant the leaflets fall until they make an angle
of 70° or 80° below the horizon, but this is not effected in
a single drop, but by series of rapid rises and falls as
Fig. 42. Exp. 257. From Power of Movement.
represented in fig. 42. In this diagram the numbers 0°
to 60° on the left represent the angular divergence in
1 Lynch in Linnean Soc. Journal, xvi. p. 231 ; Power of Movement in
Plants, p. 330.
D. A.
15
226 AVERRHOA. [CH. VIII
degrees of the leaflet from the vertical, those on the right
represent temperature (0°). Thus at 11 '30 the leaflet
made an angle of 52° with the vertical, i.e. an angle of 38°
below the horizon while the temperature (the dotted line) was
31'4°C. The leaflet had been slowly rising for 25 minutes,
and at BR a blind was pulled up so that the plant was
brightly illuminated, when the leaf descended in 5 steps
to the paraheliotropic position, where it executed three
rapid movements (at about 80° below the horizon), until
at SH, when the blind was pulled down, it rose for 35
minutes, to be again disturbed by sunshine at br'. In
performing this experiment the windows must be opened
when the blinds are pulled up, to equalise the temperature
as much as possible.
The observations here recorded were made as follows.
The main petiole of the leaf observed pointed straight at
the observer, being separated from him by a vertical pane
of glass. The petiole was fixed so that the pulvinus of
one of the lateral leaflets was at the centre of a graduated
arc placed close behind the leaflet. A fine glass filament,
attached to the leaflet and projecting like a continuation
of the midrib, served as an index. As the leaflet rose and
fell its angular movement was recorded, by reading at
short intervals of time the position of the index on the
arc. To avoid errors of parallax the readings were taken
by looking through a small ring painted on the vertical
glass in a line with the pulvinus and the centre of the
arc.
CH. VIIl] CIRCUMNUTATION. 227
Section B. Autonomous Movements : Periodicity.
(258) Circumnutation.
A really good method of observing circumnutation
(where the movement is small) has yet to be devised. One
of the methods described in Darwin's Power of Movement
in Plants (p. 7) is, in spite of certain faults, perhaps the
best for our present purpose.
Any rapidly growing plant will serve for observation,
for instance, a seedling cabbage or sunflower. The most
essential precaution is that the plant shall not be
subjected to lateral illumination, to insure which the
experiment ought to be conducted in a room lighted from
above. If this is not possible the plant must be in a
cylinder blackened inside, and should be illuminated by an
oblique mirror hung above the mouth of the cylinder so
as to throw the light vertically downwards. The plant
should if possible rest on a steady stone floor.
To the upper end of the hypocotyl a delicate glass
filament, 20 mm. in length, is fixed vertically by shellac
varnish, which should be thick enough to dry rapidly.
Before it is fixed the following preparations are necessary.
A minute equilateral triangle of paper (2 or 3 mm.
to the side) is pierced in the centre and is slipped
over the glass filament, pushed down to the base and
there fixed with shellac, so that it is at right angles to
the filament. At the other end of the filament a minute
bead of black sealing-wax is fixed. A sheet of glass
(2 ft. X 2 ft.) fixed horizontally about 2 ft. above the apex
of the plant serves as a medium on wliich to record the
15—5
228 CIRCUMNUTATION. [CH. VIII
movement. The head of the observer is moved until the
globule of sealing-wax is exactly in the centre of the
paper triangle, and a dot is made on the glass in line
Fig. 43. Exp. 258. From Poiver of Movement.
with these two points. The dot should be made with a
piece of hard wood cut to a sharp point like a pencil, and
dipped in Indian ink or moistened with water-colour. It
is best to hold the pointer as close as possible to the
glass until the observer has made up his mind where
CH. VIIl] TWINING PLANTS. 229
the dot is to be made, and then to bring the pointer
sharply down on the glass. A little practice is needed
to get results with small amounts of movement. The
observations should at first be made at intervals of about
10 minutes, so that the observer may get an idea of
the rapidity with which the movement under observation
is proceeding. He may thus be able to regulate the
intervals between his subsequent observations so as not to
spend unnecessary time, and yet not to fail in getting a
fair idea of the movement.
Fig. 43^ represents the circumnutation of a cabbage
seedling, from 9.15 a.m. to 8.30 a.m. on the following day,
the dots represent the actual marks made on the glass
with the sharpened wood, the lines and arrows being
added to show the course of the movement : the woodcut
is reduced to half the size of the original. No observations
were made at night : the broken line represents the change
of position which took place between the first evening
and the following morning. The tracing therefore (if the
broken line be neglected) practically represents the
circumnutation during a day of about 12 hours.
(259) Circumnutation : tivining plants-.
The observations should be made either in a greenhouse
or indoors, on Humulus lupulus (the hop) and Phaseolus
multiflorus. The basal part of the plant should be tied to
a stick stuck in the soil of the pot, and 6 inches or a foot
1 The tracing was made by a slightly different metliod to that here
described.
2 C. Darwin, Climbing Plants, Chapter i.
230 AUTONOMOUS MOVEMENTS. [CH. VIII
of the stem should project beyond the upper end of the
stick and hang over so as to be more or less horizontal. If
the flower-pot stands on a sheet of paper it is easy to note,
by means of a line drawn radially from the pot as a centre,
the direction in which the nutating shoot points at any
moment ; and thus the rate at which it swings round can
be recorded. One revolution in 2 hours is what may be
expected in a vigorous plant. Note that the hop travels
with the hands of a watch, Phaseolus against it : also, that
if they are allowed to climb up sticks they do so by
an apparent continuation of their revolving nutation.
Thus a hop which has wound spirally round a support
makes a left-handed screw, while Phaseolus is right-
handed.
(260) Autonomous movements: Trifolium.
The spontaneous variation-movements of leaves may
easily be studied in the genus Trifolium.^. They may
be observed by a modification of the method described
in exp. 258, or by Pfeffer's method, which is simpler and
on the whole gives more reliable results. Transplant a
lump of turf, containing a plant of clover, to a flower-pot :
fix a vertical stick (about as thick as a pencil) firmly into
the soil and attach the petiole of a leaf to it by two bands
of gummed paper so that the top of the petiole is level
with the top of the stick. The terminal leaflet is free to
move, and its movements are observed by fixing with
shellac-varnish a very fine glass filament along the midrib
on the upper surface so as to project 2 or 3 mm. Cut out
1 See Sachs' Physiologie (French Trans.), p. 515.
CH. VIIl] AVERRHOA. 2:^1
a C-shaped piece of cardboard and graduate the inner
edge into 5\ i.e. make 36 graduations to the semicircle.
Now attach the card to a second stick fixed in the soil
so that the pulvinus is in the centre of the arc, and so
that the glass index can travel over the graduations.
The experiment is preferably conducted with illumination
from above, but even with a side light the movements
are clearly seen.
The following table gives the readings in an experi-