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 Phyllotaxis

Phyllotaxis

Phyllotaxis, the regular arrangement of leaves, petals or other elements of plants, is very common in plants. There is a large literature on the subject which spans several centuries, the references at the end of this document point out some of the most influential works.

The present paper is a simple attempt to model the biological growth processes using basic physical principles. The hope is to see phyllotaxis emerge as a result of the numerical simulation. The main source for ideas is Douady and Couder's breakthrough 1992 paper which was the first to propose a model for growth which corresponded to observed patterns in real plants, for previous work had been almost entirely descriptive of these patterns.

My method is to explain the numerical experiments I tried - including the failed ones - in order to get to an understanding of how phyllotaxis arises.

First Steps

Before I read Douady and Couder's paper I set up the following model. Elements were born every T time units at a random point in a small circle of diameter 0.001 centred at the origin. They were subjected to a constant radial force c. All elements also repelled one another with a force acting to an inverse square law with constant of proportionality k. The elements moved in a viscous medium, so there motion was resisted with a force proportional to their velocity, with factor R.

If k is zero then the equations can be solved exactly since the elements only move radially. If R and c are also zero then the solution is a collection of elements that don't move - they just stay where they were born.

If k and R are set to zero, and c is non-zero (positive) then the elements move radially with distance ct2/2 from their starting point. That is they accelerate rapidly from their birth site. Introducing friction, by setting R to be positive, causes the elements to be approximately a distance ct from their birth site. That is, the value of R is not significant in prediciting the longer-term dynamics. By rendering the elements as circles whose radii are proportional to their age should result in a plant head with no gaps.

By finally introducing repulsive forces between elements (positive k), the idea was that elements would rearrange themselves into a pattern that minimised the energy of the configuration. While some parameter settings did result in spirals (examples...) the model did not reflect growing plants very well at all since the configuration was highly sensitive to the initial position of the new element.

If c is too large compared to the effect of k then there is little interaction between elements - and they just move away from the centre in random directions determined solely by birth site. Equally if the birth rate T is too large there is little interaction since previous elements are too far away to significantly affect the new element.

If k is too large then ...