Main conclusion <p>An improved method allows accurate quantification of the maturation strain in developing flax stems and reveals its strong correlation with the area of constitutively deposited gelatinous cell wall in phloem fibers.</p> Abstract <p>Improving experimental methods to reliably quantify the maturation strain in herbaceous stems is essential for understanding the mechanical regulation of their growth and for optimizing fiber crop properties. An improved longitudinal stem-splitting method enabled quantitative assessment of the tensile strain in developing flax stems, allowing evaluation of the contribution of phloem fibers with constitutively deposited gelatinous cell walls, while excluding the influence of xylem and parenchyma. The assessment was based on direct measurements of tissue mechanical properties using an inverse three-point bending test and on removing the turgor effects by incubation with the osmoticum. The main source of internal tension in the growing flax stem is the phloem fibers with a gelatinous cell wall, and strain values show a strong correlation with their area. The obtained values correspond to the range characteristic of tension wood, confirming the universality of the tension generation mechanism in vascular plants. This method provides a reliable means of estimating internal stresses at different stages of herbaceous stem development.</p>