28 DROSERA ROTUNDIFOLIA. Cuap. II, 
the tentacles to move in a conspicuous manner. The 
smallest particle which was tried, and which acted 
plainly, was only +55 of an inch (-203 millimetre) in 
length, and weighed the ;;3,, 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 
