In another case 6/7 of the particles of cinder, glass, and thread,
placed on separate glands, were carried towards, or actually to, the
centre; in another case 7/9, in another 7/12, and in the last case only
7/26 were thus carried inwards, the small proportion being here due, at
least in part, to the leaves being rather old and inactive.
Occasionally a gland, with its light load, could be seen through a
strong lens to move an extremely short distance and then stop; this was
especially apt to occur when excessively minute particles, much less
than those of which the measurements will be immediately given, were
placed on glands; so that we here have nearly the limit of any action.
I was so much surprised at the smallness of the particles which caused
the tentacles to become greatly inflected that it seemed worth while
carefully to ascertain how minute a particle would plainly act. [page
27] Accordingly measured lengths of a narrow strip of blotting paper,
of fine cotton-thread, and of a woman s hair, were carefully weighed
for me by Mr. Trenham Reeks, in an excellent balance, in the laboratory
in Jermyn Street. Short bits of the paper, thread, and hair were then
cut off and measured by a micrometer, so that their weights could be
easily calculated. The bits were placed on the viscid secretion
surrounding the glands of the exterior tentacles, with the precautions
already stated, and I am certain that the gland itself was never
touched; nor indeed would a single touch have produced any effect. A
bit of the blotting-paper, weighing 1/465 of a grain, was placed so as
to rest on three glands together, and all three tentacles slowly curved
inwards; each gland, therefore, supposing the weight to be distributed
equally, could have been pressed on by only 1/1395 of a grain, or .0464
of a milligramme. Five nearly equal bits of cotton-thread were tried,
and all acted. The shortest of these was 1/50 of an inch in length, and
weighed 1/8197 of a grain. The tentacle in this case was considerably
inflected in 1 hr. 30 m., and the bit of thread was carried to the
centre of the leaf in 1 hr. 40 m. Again, two particles of the thinner
end of a woman s hair, one of these being 18/1000 of an inch in length,
and weighing 1/35714 of a grain, the other 19/1000 of an inch in
length, and weighing of course a little more, were placed on two glands
on opposite sides of the same leaf, and these two tentacles were
inflected halfway towards the centre in 1 hr. 10 m.; all the many other
tentacles round the same leaf remaining motionless. The appearance of
this one leaf showed in an unequivocal manner that these minute
particles sufficed to cause the tentacles to bend. Altogether, ten such
26