Both flax and nettle feature cellulosic fiber bundles, comprising many individual cells bonded together, in the outer layer of the plant. Researchers investigating the effects of environmental conditions and mechanical damage on the tensile stiffness and strength of flax and nettle developed experimental equipment to measure the static and dynamic tensile moduli and the strength of individual fiber cells under varying environmental conditions. The breaking behavior of flax and nettle ultimate fibers conformed to that expected for brittle materials as described by the Weibull distribution function. Tensile modulus depended on environmental relative humidity. The effect of fiber damage was also significant. A correlation existed between the extent of damage (the proportion of the fiber that appeared as bright under a polarizing microscope) and the modulus. Furthermore, when the effect of damage was taken into account, a consistent relationship between modulus and relative humidity occurred. 12 refs.