Minds of small things · Article

Mechanical sensitivity thresholds and summation of touches in Drosera and Dionaea

In Insectivorous Plants (1875), Darwin reported that a gland on a leaf tentacle makes the tentacle bend when it is momentarily touched three or four times, but not when it is touched once or twice, even with force¹. Darwin took this threshold to spare the plant useless movement². Later authors used the same phrase for animals, in a different sense⁵⁶.

Earliest held: 1875, Darwin, C.
Most discussed in: The Animal Mind: A Textbook of Comparative…, 1908
In the library: 22 passages in 3 works
Rewritten: 2026-11-06

Darwin's observations

Darwin wrote that movement follows if a gland is momentarily touched three or four times, while one or two touches do not bend the tentacle, even when delivered with considerable force and a hard object¹. Darwin also wrote that a gland may be excited by simple touching three or four times, or by prolonged contact with organic or inorganic objects and various fluids⁴. Looking through a lens, Darwin saw a tentacle begin to bend within ten seconds⁴. When one exterior gland is excited, the surrounding tentacles remain unaffected⁴.

A filter against false alarms

Darwin argued that the threshold is useful. In a high wind the glands can hardly avoid being brushed now and then by neighbouring leaves, and the need for several touches spares the plant useless movement². The glands are likewise insensible to the weight and repeated blows of heavy rain drops³. Yet Darwin described them as exquisitely sensitive to the slightest pressure if it lasts a few seconds, and held this to serve the plant in capturing small insects. Gnats resting on the glands with their feet are quickly embraced³. The argument is about function, and no force values are given³.

Useless movement in animal studies

The phrase recurs in animal work, with another meaning. Jennings (1906) relayed Preyer's report that a starfish placed again and again in the same situation makes fewer superfluous twistings and motions⁵. Washburn (1908) wrote that in maze running, as paths are traversed at random, useless movements tend to be dropped and successful ones become organized into a system⁶. Washburn added that useless movements can themselves become organized, which delays their elimination⁷.

SourcesEach quotation was checked word for word against the passage it opens.

Movement ensues if a gland is momentarily touched three or four times ; but if touched only once or twice, though with considerable force and with a hard object, the tentacle does not bend.Darwin, C., 1875 · Insectivorous Plants · open at passage 581
The plant is thus saved from much useless movement, as during a high wind the glands can hardly escape being occasionally brushed by the leaves of surrounding plants.Darwin, C., 1875 · Insectivorous Plants · open at passage 581
The glands are insensible to the weight and repeated blows of drops of heavy rain, and the plants are thus likewise saved from much useless movement.Darwin, C., 1875 · Insectivorous Plants · open at passage 581
A gland may be excited by being simply touched three or four times, or by prolonged contact with organic or inorganic objects, and various fluids.Darwin, C., 1875 · Insectivorous Plants · open at passage 34
The number of useless movements, "superfluous twistings, feelings about, and forward and backward motions," becomes less the oftener the individual has been placed in such a situation.Jennings, H. S., 1906 · Behavior of the Lower Organisms · open at passage 684
As the paths are traversed at random, the useless movements tend to be dropped off, and the successful ones not merely to survive, but to become organized into a systemWashburn, M. F., 1908 · The Animal Mind: A Textbook of Comparative Psychology · open at passage 473
It must further be noted that useless movements not infrequently get organized into movement systemsWashburn, M. F., 1908 · The Animal Mind: A Textbook of Comparative Psychology · open at passage 476

Linked ideas

Tentacle inflection and transmission of the motor impulse in Drosera and Dionaearelated to

Treats the bending of the excited tentacle and how the movement is passed on, the response that the touch threshold gates.

Graded response, fatigue and summation in reflex and nerverelated to

Asks how response depends on stimulus size and duration, as with touches that fail singly but succeed when repeated or prolonged.

Nerve-muscle threshold, anesthesia and fatigueparallels

Threshold and summation in nerve and muscle give a comparison for a gland that needs several touches before it acts.

Carnivorous plant digestion and protein secretionrelated to

Covers what the plant does after capture, which the touch threshold helps to trigger only for real prey.

Trial and errorin tension with

Here useless movements fade through experience in animals, whereas the plant's filter is described as a fixed property of the gland.

Habit and plasticityrelated to

Washburn's point that useless movements can become organized into habits bears on how such movements are eliminated or kept.

Where it is discussedPassages matching touched three or four times, insensible to a single touch, slightest pressure prolonged, momentarily touched

1908	The Animal Mind: A Textbook of Comparative Psychology · Washburn, M. F.	19
1875	Insectivorous Plants · Darwin, C.	2
1906	Behavior of the Lower Organisms · Jennings, H. S.	1