particles of hair were placed on ten glands on several leaves, and
seven of them caused [page 28] the tentacles to move in a conspicuous
manner. The smallest particle which was tried, and which acted plainly,
was only 8/1000 of an inch (.203 millimetre) in length, and weighed the
1/78740 of a grain, or .000822 milligramme. In these several cases, not
only was the inflection of the tentacles conspicuous, but the purple
fluid within their cells became aggregated into little masses of
protoplasm, in the manner to be described in the next chapter; and the
aggregation was so plain that I could, by this clue alone, have readily
picked out under the microscope all the tentacles which had carried
their light loads towards the centre, from the hundreds of other
tentacles on the same leaves which had not thus acted.
My surprise was greatly excited, not only by the minuteness of the
particles which caused movement, but how they could possibly act on the
glands; for it must be remembered that they were laid with the greatest
care on the convex surface of the secretion. At first I thought but, as
I now know, erroneously that particles of such low specific gravity as
those of cork, thread, and paper, would never come into contact with
the surfaces of the glands. The particles cannot act simply by their
weight being added to that of the secretion, for small drops of water,
many times heavier than the particles, were repeatedly added, and never
produced any effect. Nor does the disturbance of the secretion produce
any effect, for long threads were drawn out by a needle, and affixed to
some adjoining object, and thus left for hours; but the tentacles
remained motionless.
I also carefully removed the secretion from four glands with a sharply
pointed piece of blotting-paper, so that they were exposed for a time
naked to the air, but this caused no movement; yet these glands were
[page 29] in an efficient state, for after 24 hrs. had elapsed, they
were tried with bits of meat, and all became quickly inflected. It then
occurred to me that particles floating on the secretion would cast
shadows on the glands, which might be sensitive to the interception of
the light. Although this seemed highly improbable, as minute and thin
splinters of colourless glass acted powerfully, nevertheless, after it
was dark, I put on, by the aid of a single tallow candle, as quickly as
possible, particles of cork and glass on the glands of a dozen
tentacles, as well as some of meat on other glands, and covered them up
so that not a ray of light could enter; but by the next morning, after
27