Young trees of balsa (Ochroma pyramidale) and papaya (Carica papaya) were repotted at approximately 45deg; from the vertical or as vertical controls. Leaning trees of both species reoriented toward the vertical. The apical region curved upward within hours and the basal region, with extensive secondary growth, began to reorient within 2 days. The anatomical basis of this change was investigated. No gelatinous (tension) fibers formed, but eccentric growth (more secondary tissue on the upper side) occurred, and a characteristic reaction anatomy was found which is described for each species. Statistically significant differences in longitudinal surface contraction or expansion, phloem ray dilation, and phloem ray cell lengths were found between upper and lower sides of reoriented stems. The possible role of the separation of primary phloem fiber groups and differential growth of phloem ray cells in axis movement is discussed.
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