card. Make a dot upon the card opposite the tip of the filament, taking
care not to disturb the position of either. In a few minutes observe that
the filament is no longer opposite the dot. Mark its position anew, and
continue thus until a circle is completed on the cardboard. This is a
rough way of conducting the experiment. Darwin's method will be found in
the footnote.[1]

[Footnote 1: "Plants growing in pots were protected wholly from the light,
or had light admitted from above or on one side as the case might require,
and were covered above by a large horizontal sheet of glass, and with
another vertical sheet on one side. A glass filament, not thicker than a
horsehair, and from a quarter to three-quarters of an inch in length,
was affixed to the part to be observed by means of shellac dissolved in
alcohol. The solution was allowed to evaporate until it became so thick
that it set hard in two or three seconds, and it never injured the
tissues, even the tips of tender radicles, to which it 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 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 India 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 one or two minutes, the
lines would have been more curvilinear."--The Power of Movement in Plants,
p. 6.]

The use of the glass filament is simply to increase the size of the circle
described, and thus make visible the movements of the stem. All young