<正>美洲黑杨维管形成层活动周期可分为休眠期、恢复活动期、活动期和转化期4个时期。休眠期维管形成层区约由3层细胞组成，细胞壁较厚，细胞内具有大量小液泡，其内包含一些电子密度较高的物质；细胞质内充满了游离的核糖核蛋白体，脂粒圆球形，散布在小液泡之间；内质网光滑型，多为短片状，只有近质膜处才呈网格状；线粒体内嵴不清晰，高尔基体数量较少，一般由3～5层扁平囊组成，周围未分泌形成高尔基小泡。恢复活动期维管形成层的细胞壁变薄，细胞内逐渐形成大液泡，内含物降解，细胞质的电子透明度增高。活动期维管形成层区细胞层数可达8～9层，细胞壁非常薄，细胞高度液泡化，细胞质和细胞核贴壁分布，或通过原生质索悬垂在细胞的某一位置，细胞器结构清晰，细胞质膜大量内折，与壁物质的形成有关。转化期维管形成层区细胞停止分裂，细胞壁逐渐增厚，液泡数目增多，体积减小，细胞开始积累淀粉、脂类和蛋白质等类物质。休眠期和活动期细胞超微结构特征相反，而恢复活动期与转化期细胞结构特征相似，互为可逆过程。

As the particular morphology and structure of bryophytes, the contents of poilutants in the bryophytes were taken as the measure of air pollution level. And the change of physiological and biochemical changes in two epiphytic bryophytes Haplocladiurn microphyllum and Entodon compressus in the six regions in Nanjing were studied. The results showed that with the gradual aggravates of air pollution, heavy metals and S element con tents in two epiphytic bryophytes increased, the total chlorophyll contents all decreased. Free proline content and cell membrane permeability all obviously increased, but the catalase activities in the tow bryophytes took different change patterns. And the impact of air pollution to physiological and biochemical changes in two epiphytic bryophytes reached 1% significant level except the the catalase activities. The physiological and biochemical changes in H. microphyllum was in a greater degree than in E. co,npressus, which showed that H. micro phyllum was more sensitive to air pollution.