The following facts have been established f o r the spiral growth of the spore-bearing cell of Phycomyces :
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The inclination of the axis of spiral growth may be influenced by change of temperature, or may occasionally exhibit slow spontaneous change.
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The inclination varies significantly from cell to cell, cells of larger diameter having larger values of a, the a.ng1e between the longitudinal axis of the cell arid the axis of growth.
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I n the earlier stage of this cell, when the growing apex is without a spore-mass, spiral growth does not occur. The onset of spiral growth coincides with the re-initiatio n of growth after the temporary halt while the sporangium is f0rmed.l From these facts it has been concluded that spiral growth depends on a special geometrical configuration of the gro.wing zone in which certain membrane stresses are minimized by the production of an axial twist in the cells. A model illnstrating the production of spiral structure by such a 'least action' principle has been constructed (Castle, '36). Acoording to this idea, in the cell any existing degree of twist is a balanced condition resulting from the interaction of forces.
This conclusion was originally drawn from studies of particles attached to the growing zones of young and mature sporangiophores. It also follows from examination of the structural patterns of the cell wall. Spiral markings of the cuticle first begin a t the level where the sponrangium formed.