Application of the equilibrium concept to the development of agaric fruit-bodies, with special reference to their straight downward growth in light from below

Equilibrium, a concept of dynamics, is found to be applicable to the phototropic and gravitropic growth in agaric fruit-bodies. The fruit-bodies exposed to light from below grow straight downward without bending upward, and those exposed to light from obliquely below grow first downward and then upward by negative gravitropism. The fruit-bodies exposed to light from above grow upward. Fruit-bodies growing straight downward or upward do not change the direction of growth; they are in ‘equilibria’. The straight downward growth can be regarded as an ‘unstable equilibrium’ having a higher potential, and the straight upward growth as a ‘stable equilibrium’ having a lower potential. The change in the direction of growth can be explained by the change in the potential; the upward bending in fruit-bodies that have grown obliquely downward can be regarded as a ‘transition’ from the unstable equilibrium to the stable one.