B组织
叶脉（vein）叶片上可见的脉纹.由贯穿在叶肉内的维管束或维管束及其外围的机械组织组成.为叶的输导组织与支持结构.它一方面为叶提供水分和无机盐、输出光合产物,另一方面又支撑着叶片,使能伸展于空间,保证叶的生理功能顺利进行.叶脉在叶片中呈有规律地分布,通过叶柄与茎内的维管组织相连.
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