The reaction anatomy of the stems of seedlings of A 1 species of hroad-leaved trees was examined. The reaction was induced by bending the stems at right angles or growing the plants horizontally. As a result of greater cambial activity on the upper side such stems developed a structural asymmetry characterized by enhanced xylem and phloem development. The structural features of the lower side were similar to those of stems grown vertically. The xylem on the upper side contained fewer vessels, more fibres, and fewer medullary rays than that on the lower side of the same stem. "Gelatinous" layers developed within the reaction fibres as well as in fibres of the phloem. Vessels, medullary ray cells, and parenchyma cells were often compressed and sometimes obliterated. Reaction fibres showed reduced wall pitting and tended to retain their contents until they lay relatively deep within the stem. There were no differences in fibre length between upper and lower sides of the stem, but comparisons were probably confounded with an age difference. Cortical cells and, in species where they occurred, canals were compressed on that side of the stem. Such cortical cells were not only smaller in cross section than those on the lower side, but were also shorter in length. This was also a feature of medullary ray cells. There was a greater accumulation of tannins in phloem and cortex on the upper side of the stem. The structure of reaction fibres was examined in ultraviolet, polarizing, and electron microscopes. In the first of these, the "gelatinous" layer was seen to consist of a series of concentric bands convoluted towards the cell lumen, the bands containing radial striations. The polarizing microscope showed that the innermost part of the wall possessed some birefringence. Both convolutions and birefringence tended to disappear as the cells matured; staining with Congo red became less intense. The heterogeneity of the "gelatinous" layer was confirmed by electron microscopic examination. The results are discussed in connection with the gravimorphic responses of displaced stems, the altered patterns of lignification, and the development of growth stresses in such stems.