Figure 1.

(a) (Left) A sealed and turgid seedpod of Acaccia caven. (Right) An open and desiccated seedpod. k₁, k₂ stand for the meridional and the longitudinal curvatures at the saddle point. During desiccation the shortening of some external fibres at the back could increase k₂ and by consequence as the product k₁ k₂ is conserved for an isometry k₁ should decrease; it would thus actuate the opening of the seedpod. (b) A folded Goursat surface behaving similarly to the seedpod. (R₁=0.58, α₁=α₂=0.7752, b=0.5, c=10, d=30, ϵ=sign(u_jun,2−u), v₀=0.94). (Left) Rest state h=0. (Right) Deformed state h=0.05. u (grey dashed line) and v (black dashed line) are the two coordinates lines of the system presented in §3a crossing at the saddle point. (Online version in colour.)