在植物体整个生命活动过程中,除了依赖阳光、CO_2、水份和无机盐之外,还需要有一类微量的、具有生理活性的物质来调节生长发育,这一类物质就叫植物激素。一盆正在发芽的玉米或小麦,放在一间暗室里,让光线从一侧的洞口照进去,过两天就会发现,幼苗在向光弯曲生长。盆栽的花卉也需要经常转动花盆,才能保持直立生长。在夏季,当一场狂风暴雨把庄稼刮倒之后,一夜功夫,它们便挣扎着抬起头来。过几天,便又齐刷刷地重新直立向上。又如,夏季长在植物枝条上的绿叶,用力拉扯也不易使叶柄掉下;而到了秋天,叶片却自动地随风轻轻飘落。产生这些生理现象的内在因素是什么呢? In the process of plant life, in addition to rely on the sun, CO_2, water and inorganic salts, but also need a trace amount of physiologically active substances to regulate growth and development, this type of material is called plant hormones. A pot of sprouted corn or wheat is placed in a dark room and the light is shone through the hole in one side. Within two days, the seedling will be bent towards light. Potted flowers also need to rotate the pot often to keep them growing upright. In the summertime, after a storm brought crops down, they struggled to look up one night. A few days, then again straight upright. In another example, the green leaves growing on the branches of plants in summer are not easily peeled off by hard pulling; but in autumn, the leaves gently fall on the wind. What are the internal factors that produce these physiological phenomena?