\citet[in Japanese]{Okuyama_1993} and \citet{ISI:A1995QJ03000001} suggested the unified hypothesis. In an attempt to solve the critical MFA discrepancy \citet{ISI:A1995QJ03000001} augmented the \citet{Barber_1964} cell wall model to include a S1 layer. The resulting model was the first to be able to account for generation of both tensile and compressive stresses over a wide range of MFAs, however this was only achievable using unnatural parameter values. The S1 layer introduced utilizes a constant MFA of 90 degrees, with the S2 layer varying from 0 to 60 degrees. Cell wall maturation occurred in two discrete steps, first the cellulose framework is constructed then the lignin deposition occurs. From the model they showed that with an increasing S1 layer thickness the critical MFA reduces. They found the model was unable to produce realistic tangential strains unless unnatural parameters were used.