The Power of Movement in Plants | Page 5

was applied. To the end of the glass filament an
excessively minute bead of black sealing-wax was cemented, below or
behind which a bit of card with a black dot was fixed to a stick driven
into the ground. The weight of the filament was so slight that even
small leaves were not perceptibly pressed down. another method of
observation, when much magnification of the movement was not
required, will presently be described. The bead and the dot on the card
were viewed through the horizontal or vertical glass-plate (according to
the position of the object), and when one exactly covered the other, a
dot was made on the glass-plate with a sharply pointed stick dipped in
thick Indian-ink. Other dots were made at short intervals of time and
these were afterwards joined by straight lines. The figures thus traced
were therefore angular; but if dots had been made every 1 or 2 minutes,
the lines would have been more curvilinear, as occurred when radicles
were allowed to trace their own courses on smoked glass-plates. To
make the dots accurately was the sole difficulty, and required some
practice. Nor could this be done quite accurately, when the movement
was much magnified, such as 30 times and upwards; yet even in this
case the general course may be trusted. To test the accuracy of the
above method of observation, a filament was fixed to an
* These terms are used in the sense given them by De Vries, 'Würzburg
Arbeiten,' Heft ii 1872, p. 252.
[page 7] inanimate object which was made to slide along a straight
edge and dots were repeatedly made on a glass-plate; when these were
joined, the result ought to have been a perfectly straight line, and the
line was very nearly straight. It may be added that when the dot on the
card was placed half-an-inch below or behind the bead of sealing-wax,
and when the glass-plate (supposing it to have been properly curved)
stood at a distance of 7 inches in front (a common distance), then the
tracing represented the movement of the bead magnified 15 times.
Whenever a great increase of the movement was not required, another,
and in some respects better, method of observation was followed. This
consisted in fixing two minute triangles of thin paper, about 1/20 inch

in height, to the two ends of the attached glass filament; and when their
tips were brought into a line so that they covered one another, dots
were made as before on the glass-plate. If we suppose the glass-plate to
stand at a distance of seven inches from the end of the shoot bearing the
filament, the dots when joined, will give nearly the same figure as if a
filament seven inches long, dipped in ink, had been fixed to the moving
shoot, and had inscribed its own course on the plate. The movement is
thus considerably magnified; for instance, if a shoot one inch in length
were bending, and the glass-plate stood at the distance of seven inches,
the movement would be magnified eight times. It would, however,
have been very difficult to have ascertained in each case how great a
length of the shoot was bending; and this is indispensable for
ascertaining the degree to which the movement is magnified.
After dots had been made on the glass-plates by either of the above
methods, they were copied on tracing paper and joined by ruled lines,
with arrows showing the direction of the movement. The nocturnal
courses are represented by straight broken lines. the first dot is always
made larger than the others, so as to catch the eye, as may be seen in
the diagrams. The figures on the glass-plates were often drawn on too
large a scale to be reproduced on the pages of this volume, and the
proportion in which they have been reduced is always given.*
Whenever it could be approximately told how much the movement had
been magnified, this is stated. We have perhaps
* We are much indebted to Mr. Cooper for the care with which he has
reduced and engraved our diagrams.
[page 8] introduced a superfluous number of diagrams; but they take up
less space than a full description of the movements. Almost all the
sketches of plants asleep, etc., were carefully drawn for us by Mr.
George Darwin.
As shoots, leaves, etc., in circumnutating bend more and more, first in
one direction and then in another, they were necessarily viewed at
different times more or less obliquely; and as the dots were made on a
flat surface, the apparent amount of movement is exaggerated
according to the degree of obliquity of the point of view. It would,

therefore, have been a much better plan to have used hemispherical
glasses, if we had possessed them of all