[From Rick Marken (2016.06.08.1610)]
Vyv Huddy (8.06.2016 21.01 GMT)--
VH: A few years back a group in Japan identified chemicals involved in the rapid folding of the trap. A causal theory party line seems to be described here:
VH: I began to wonder how PCT would explain this. When a hair is moved inside the trap the chemical is released, this chemical immediately begins to decay fairly quickly. However, successive touches of the hair (e.g. by a fly) in a short enough period (30 secs) mean that the chemical accumulate to a threshold level that "activates" an "action potential" that results in trap closure.
VH: So I was wondering if the way it accumulates seems be a negative feedback process rather like a water clock. Instead of the reference value being water at a particular level, the organisation of the plant is such that the chemical is never be _above_ a particular level.
RM: I think you're on the right track. I think the first thing to think about when doing a PCT analysis of behavior is what's the controlled variable?. And I think the best way to go about trying to guess at what the controlled variable might be is to try to see the situation from the point of view of the controlling system (the fly trap in this case). This is how I figured out what the grey lag goose is likely to be controlling when it moves an egg back into its nest (<https://urldefense.proofpoint.com/v2/url?u=http-3A__www.mindreadings.com_ControlDemo_Goose.html&d=CwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=-dJBNItYEMOLt6aj_KjGi2LMO_Q8QB-ZzxIZIF8DGyQ&m=xsdUgijleUU7PePVHvEDV8XAZlXCp0_q8Mi02HmeBeE&s=ixXBeoF42HipmTULkwm3wG7qEB7hLn6FkqCu2_eFrT8&e=>http://www.mindreadings.com/ControlDemo/Goose.html).
RM: So looking at it from the fly traps perspective, all I experience is how much bending is going on in my hair cells. So my guess is that the fly trap is controlling for keeping its hair cells from being bent. The bending is apparently perceived in terms of the amount of a chemical that is produced in the hair cells when they are bent. The reference for this perception is the "threshold" concentration of this chemical that causes the trap to close. We can call it 0; the trap is controlling for zero amount of that chemical, which is equivalent to zero bending of the hairs.
RM: A fly entering the trap is a disturbance to this variable. And the reaction to this disturbance is closing the trap. It looks like S-R but that's only because closing the trap is part of the output that compensates for the disturbance to the straightness of the hair cells (the other part is the digestive chemicals, if necessary). If the trap closes on the fly, the fly continues to be a disturbance until it is digested and the hair cells can return to normal and the trap reopens; if the fly escapes then the disturbance is gone immediately and the hair cells gradually return to normal and the trap opens with no digestive output necessary.
RM: That's my guess,anyway: The controlled perception is the chemical concentration in the hair cells and the controlled quantity is the bend of the hair cells. It's a very slow loop; the trap-shutting part of the loop is pretty fast but the digestion component takes quite a while. So the fact that the hair cells are being kept unbent (controlled) would be more easily seen using time lapse photography so that the disturbing fly would be seen as a rather transient event. Actually, control loops in plants, such as tropisms, are generally very slow and are best seen using time lapse photography.
RM: By the way, this seems to be a case where a control system deals with the sensory consequences of a disturbance (the chemical secreted as a result of the hair cell bend) by destroying the disturbance itself -- digesting it.
VH: The mechanism of the trap reopening seems to be a elastic property of the leaf returning it to a more stable state rather as a plastic cup would. I guess this is part of the feedback function?
RM: I think you're right about the elastic property of the leaf doing the reopening. I would see it as part of the output function rather than the feedback function though. There seems to be a current obsession about feedback functions which I hope will abate soon. Feedback functions are just the physical connection from output to controlled variable so you can't possibly know what the feedback function in a control loop is until you know what variable is under control. And once know what variable is under control you know what is most essential to understanding the behavior of the system -- you can easily determine, for example, how any variation in disturbances and/or characteristics of the feedback function will affect the behavior of the system.
VH: As would unfortunate fly entering into the trap? And getting stuck on sticky surface inside the leaf as well? These are "environmental" properties that happen to be part of the organism? So the plant could be seen as a closed loop control system?
RM: It is a closed loop control system, in fact, not theory, because it is controlling; it is acting to protect a variable (probably the perceived bend of the hairs) from disturbances (insects entering the trap). But research is required to determine _exactly_ what variable(s) are actually being controlled. Studying control using the TCV is not just a matter of coming up with a plausible verbal description of the variable a system is controlling. It's a matter of doing cleverly designed experiments to get a precise, quantitative measure of the variable under control. This will take some ingenuity with the Venus fly trap; but that's what PCT research will be like. I tried to show what precise, quantitative PCT research would be like in my book DRoP (<https://urldefense.proofpoint.com/v2/url?u=http-3A__www.amazon.com_Doing-2DResearch-2DPurpose-2DExperimental-2DPsychology_dp_0944337554&d=CwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=-dJBNItYEMOLt6aj_KjGi2LMO_Q8QB-ZzxIZIF8DGyQ&m=xsdUgijleUU7PePVHvEDV8XAZlXCp0_q8Mi02HmeBeE&s=urn9z3owwQvHjPUUbHpwdC2EU5_qSZ9onTSxdM7cn6Y&e=>http://www.amazon.com/Doing-Research-Purpose-Experimental-Psychology/dp/0944337554), particularly Chapter 4. It would be great if more than 3 people in the world were actually doing this research;-)
VH: I'm not remotely knowledgeable about this topic so am risking looking a bit silly here ... but my curiosity of how about how PCT would explain this got the better of me ... so that side of the conflict won and I'm at the end of this email and hitting send.
RM: It was a great question. And you don't look silly at all; you look like someone who is very curious, something I greatly admire about a person.
Richard S. Marken
Author, with Timothy A. Carey, of <https://urldefense.proofpoint.com/v2/url?u=http-3A__www.amazon.com_Controlling-2DPeople-2DParadoxical-2DNature-2DBeing_dp_1922117641_ref-3Dsr-5F1-5F1-3Fs-3Dbooks-26ie-3DUTF8-26qid-3D1449541975-26sr-3D1-2D1&d=CwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=-dJBNItYEMOLt6aj_KjGi2LMO_Q8QB-ZzxIZIF8DGyQ&m=xsdUgijleUU7PePVHvEDV8XAZlXCp0_q8Mi02HmeBeE&s=euA_czNl2gD92AamaOf-DuNcVdea41sufdLjEZUFWfE&e=>Controlling People: The Paradoxical Nature of Being Human.