向重力性(gravitropism) ? 向重力性就是植物在重力影响下,保持一定方 向生长的特性。根顺着重力方面向下生长,称 正向重力性(positive gravitropism);茎背离重 力方向向上生长,称为负向重力性(negative gravitropism);地下茎侧水平方向生长,称为 横向重力性(diagravitropism)。 Gravitropic responses in the shoot of a young tomato plant (Lycopersicon esculerttum). The plant in (a) was placed on its side and kept in a stationary position; the plant in (b) was placed upside down and held in position in a ring stand. 感受重力最敏感的器官是 根冠 。去除根冠失去重力性。根冠中的柱细胞中含 有 淀粉体 (amyloplast),亦称平衡石(statolith)。一个细胞内有4-12个淀粉体, 每个造粉体外有一层膜,内有1-8个淀粉粒。植物体内平衡石的分布因器官而 异。根部的平衡石在根冠中,茎部的平衡石分布在维管束周围的1-2层细胞(也 称淀粉鞘)。平衡石在重力影响下,下沉在细胞底部。 Photomicrograph of a median longitudinal section of the root cap of the primary root of the common bean. Arrows point to amyloplasts sedimented near the transverse walls in the central column cells of the root cap. The amyloplasts (arrows) are visible at the bottom of each cell near the transverse walls. 1.根冠的淀粉体受到重力影响,向下运动压在内质网上;2.诱使内质网将Ca2+ 释放出来;3.Ca2+与钙调素结合,呈激活状态;4.激活钙泵和生长素泵;5.前 者将Ca2+运到细胞壁,后者将生长素运到细胞壁。生长素大部分分布在根的下 侧,Ca2+也促进生长素返回伸长区下侧。下侧生长素过多,抑制伸长区伸长, 而上侧生长素较少,生长正常上侧生长快,下侧生长慢,所以根就向重力方向 弯曲生长。 质膜 淀粉体 钙调素 生长素 钙泵 生长素泵 8.8.4向触性 (thigmotropism) A less apparent, though common, tropism is thigmotropism (from the Greek thigma, meaning "touch"), a response to contact with a solid object. One of the most common examples of thigmotropism is seen in tendrils, which are modified leaves in some species and modified stems in others. The tendrils wrap around any object with which they come in contact and so enable the plant to cling and climb. The response can be rapid; a tendril may wrap around a support one or more times in less than one hour. Cells touching the support shorten slightly and those on the other side elongate. 8.8.5感触性(感震性) Thigmonasty(seismonasty) 含羞草(Mimosa pudica)经过触 摸、摇晃、电激 或热激等的变化 情况 含羞草运动的过程 Diagrammatic representation of pulvini in Mimosa pudica. (a) Portion of rachis showing three leaflets, each with a pulvirtus at its base. (b) Transverse section through a rachis with two leaflets in the closed condition, showing longitudinal view of the pulvini. (c) Transverse section through a pulvinus showing the core of the vascular tissue surrounded by a cortex, which consists largely of thin-walled parenchyma cells. 叶枕 皮层