在伐木后的树墩上,可以看到许多同心轮纹,一般每年形成一轮,故称年轮。年轮是怎样形成的呢?植物生长由于受到季节的影响而具有周期性的变化,在树木茎干韧皮部的内侧,有一圈细胞生长特别活跃,分裂快,能够形成新的木材和韧皮部组织,称为形成层,树干的增粗全靠它活动的结果。春夏两季、天气温暖,雨水充沛,形成层细胞的活动旺盛,细胞分裂较快,向内产生一些腔大壁薄的细胞,输送水分的导管数目多,而纤维细胞较少,这部分木材质地疏松,颜色较浅,称为早材或春材。到夏末至秋季,气温和水分等条件逐渐不适于形成层细胞的活动,所产生的细胞小而壁厚,导管的数目少,纤维细胞较多,这部分木材质地致密,颜色也深,称为晚材或秋材。每年形成的早材和晚材,便 After the logging of the tree pier, you can see many concentric rims, the general formation of a round each year, so that the annual ring. Growth of the plant is affected by the season due to the cyclical changes in the phloem stem inside the tree, a circle of cells particularly active cell growth, rapid division, to form a new wood and phloem tissue, said To form a layer, the thickening of the trunk depends entirely on its activity. Spring and summer seasons, warm weather, abundant rainfall, strong activity of the formation of layers of cells, cell division faster, inward a number of large cavity thin wall cells, the number of duct delivery of water more, and less fibrous cells, this part of the wood Loose texture, lighter colors, known as earlywood or springwood. To the late summer to autumn, temperature and moisture conditions are not suitable for the formation of stratum cell activity, resulting in small cells and wall thickness, the number of small catheters, fibroblasts more, this part of the wood texture is dense, dark color, said For the late wood or autumn wood. The formation of earlywood and latewood each year, then