Plants of 13 fibre flax varieties were tested for their photosensitivity and differences of stems structure while grown in long and short daylight. It was detected that seven varieties did not change time of flowering in short 12 hours daylight in comparison with long 19 hours daylight. The extended diversity of fibre flax reactions on photoperiod changes and its relative independence of photosensitivity degree were found. Also, weak but significant correlation was found between photosensitivity coefficient and the degree of leaves number increase (r = 0.57). At the same time photo insensitive variety Belosnezhka improved leaves number up to 34%. Numbers of fibre bundles, their individual and total area on the stem cross section were not changed or reduced. Width, height, perimeter and shape of fibre bundles on a cross sections changed multidirectional. The areas of stem, its wooden part and cavity on a cross section showed no significant differences between plants grown in long and short day conditions. Evaluated flax characters were not strictly correlated with each other except those indicating the structure of fibre bundles. For breeding it is important that their relationships are not strongly influenced by the environment.