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Reaction wood possesses altered properties and performs the function of regulating a tree's form, but it is a serious defect in wood utility. Trees usually develop reaction wood in response to a gravistimulus. Reaction wood in gymnosperms is referred to as compression wood and develops on the lower side of leaning stems or branches. In arboreal, dicotyledonous angiosperms, however, it is called tension wood and is formed on the upper side of the leaning. Exploring the biology of reaction wood formation is of great value for the understanding of the wood differentiation mechanisms, cambial activity, gravitropism, and the systematics and evolution of plants. After giving an outline of the variety of wood and properties of reaction wood, this review lays emphasis on various stimuli for reaction wood induction and the extensive studies carried out so far on the roles of plant hormones in reaction wood formation. Inconsistent results have been reported for the effects of plant hormones. Both auxin and ethylene regulate the formation of compression wood in gymnosperms. However, the role of ethylene may be indirect as exogenous ethylene cannot induce compression wood formation. Tension wood formation is mainly regulated by auxin and gibberellin. Interactions among hormones and other substances may play important parts in the regulation of reaction wood formation.

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Ethrel (2 - chloroethylphosphonic acid) applied in lanolin paste at eoneentrations of 0.01, 0.10, or i % to stems of 12-month-old Pinus halepensis seedlings greatly altered the rate of growth and anatomy of stems within 60 days. Height growth was not affeeted but growth of bark and xylem tissues was greatly increased after treatment with ethrel at 0.10 or 1%. The thick bark was the result of inereased phloem production and more intercellular spaee. Increase in the amount of xylem resulted from production of more tracheids per radial file rather than an inerease in traeheid size. The highest coneentration of ethrel applied (1 %) stimulated inerease in the amount of ray tissue and production of longitudinal resin duets in the xylem. The role of ethylene in regulating differentiation of stem tissues is discussed.

Long‐Evans Cinnamon (LEG) rats, a mutant strain originating from Long‐Evans rats, spontaneously develop hereditary hepatitis followed by hepatocellular carcinoma. The hepatic disorder in LEC rats is associated with their abnormal copper metabolism; metal‐catalyzed reactions often give rise to oxygen radicals, which may be related to the carcinogenesis. By means of high‐pressure liquid chromatography with electrochemical detection, cellular DNA damage caused by oxygen radicals can be assessed in terms of the amount of 8‐hydroxydeoxyguanosine (oh8dG). We assayed the amount of oh8dG in DNA of liver, kidneys, and brain of LEC and Long‐Evans Agouti (LEA) control rats in seven groups (n=3 to 6) aged from 5 weeks to 24 months. Control rats, a healthy sibling line, were age‐matched. The amount of oh8dG was correlated with the severity of the age‐related clinical symptoms in LEC rats. The amount was higher in LEC rats than in the controls, especially in the liver at the acute stage of hepatitis. These findings suggest that oxygen radicals may be important in the carcinogenesis that occurs in LEC rats.