A Caltech Library Service

The Avoidance Response of Phycomyces in a Controlled Environment


Meyer, Paul Wells (1986) The Avoidance Response of Phycomyces in a Controlled Environment. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/s6zv-r568.


If an object is placed 1 mm away from the growing zone of a Phycomyces sporangiophore growing in air, then after 2 to 6 min the sporangiophore bends away from the object, without ever touching it, at a rate of about 1°/min. The sporangiophore stops bending after about 30 min. This is called the avoidance response of Phycomyces.

How does the sporangiophore detect the object? It seems likely that a chemical mechanism is involved, since other physical stimuli (light, electric and magnetic fields) have already been ruled out.

A simple mechanism was proposed 10 years ago, in which the ambient air currents near the surface of an object modify the distribution of a hypothetical, short-lived effector gas emitted by the sporangiophore. However, the avoidance response occurs at its usual rate in the complete absence of ambient air currents. Thus, the suppression of air currents near the surface of a solid object cannot provide the signal for the response.

The avoidance rate depends significantly on the recent history of the experimental chamber, on the length of time the sporangiophore has spent inside the experimental chamber, and on other factors. By carefully controlling environmental variables, the variation in avoidance rate of different sporangiophores in successive experiments can be held to less than ±10%. This allows accurate determination of the distance dependence of the response, and accurate comparison of different types of barriers.

The rate of the response falls off above 90 % relative humidity - but does not fall to zero. Surprisingly, the sporangiophore avoids a thin, 120 µm diameter parallel wire placed 0.5 mm away at about the same rate as it avoids another sporangiophore placed at the same distance. Also, the distance dependence of the avoidance response appears to be much weaker than previously reported, and the response may depend on the chemical composition of the object, in contrast to previous reports. These findings, combined with the results of calculations presented in Appendix 3, argue strongly against the hypothesis that the barrier acts merely by reflecting a diffusible substance emitted by the sporangiophore.

The only remaining viable chemical mechanism for the avoidance response requires that the signal molecule emitted by the sporangiophore be adsorbed by the surface of the avoided object for a nonzero length of time, and not just be reflected by it. Three new versions of this hypothesis are presented which are consistent with the experimental results.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Biophysics
Degree Grantor:California Institute of Technology
Major Option:Biochemistry and Molecular Biophysics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Berg, Howard C.
Thesis Committee:
  • Berg, Howard C. (chair)
  • Brokaw, Charles J.
  • Lester, Henry A.
  • Pine, Jerome
  • Revel, Jean-Paul
Defense Date:6 December 1985
Funding AgencyGrant Number
Record Number:CaltechTHESIS:04162019-154502622
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11472
Deposited By: Mel Ray
Deposited On:17 Apr 2019 14:57
Last Modified:03 Nov 2021 22:24

Thesis Files

PDF - Final Version
See Usage Policy.


Repository Staff Only: item control page