Cactaceae are a diverse group of plants with a wide variety of morphologies and reproductive strategies. Many species have segmented stems in which terminal cladodes may be separated from main stem cladodes with varying amounts of resistance. Previous results demonstrated that lignified xylem cells in tensile portions of stem joints provide the main resistance to separation of cladodes within cactus plants. The purpose of the present study was to determine if stem joints of Opuntia laevis would produce additional lignified xylem cells in response to additional externally applied stresses. Normal average stress levels, which accompany the addition of a new cladode, were applied to 12 plants. In contrast, double the average stress levels were applied to 13 other plants. After exposure to the two stress regimens for 6 months, the amount and location of lignified xylem cells in joint segments were similar for both stress treatments. So, although the results support the hypothesis that lignified xylem cells act as the main resistance to stress at joints of cladodes, doubling the normal amount of applied stress was insufficient to alter the amount or location of lignified xylem cells in stem joints. These results indicate that normal amounts of lignified xylem cells can resist up to two times the normal amount of stress for 6 months without producing additional lignified xylem cells.

中文翻译：

---

压缩/拉伸应力和木质化细胞作为仙人掌（仙人掌科）枝叶之间关节中的阻力成分：对施加压力的反应

仙人掌科植物种类繁多，具有多种形态和繁殖策略。许多物种都有分段的茎，其中末端分支可能与主茎分支分开，具有不同程度的阻力。先前的结果表明，茎节拉伸部分中的木质化木质部细胞提供了仙人掌植物内枝叶分离的主要抵抗力。本研究的目的是确定仙人掌的茎关节是否会产生额外的木质化木质部细胞以响应额外的外部施加压力。对 12 株植物施加了伴随新枝条添加的正常平均压力水平。相比之下，其他 13 株植物的平均压力水平翻了一番。在暴露于两种压力方案 6 个月后，两种压力处理的关节节段木质化木质部细胞的数量和位置相似。因此，尽管结果支持木质化木质部细胞是枝条关节处应力的主要抵抗力的假设，但将正常施加的压力加倍不足以改变干关节中木质化木质部细胞的数量或位置。这些结果表明，正常量的木质化木质部细胞可以在 6 个月内抵抗高达正常量两倍的压力，而不会产生额外的木质化木质部细胞。将正常施加的压力加倍不足以改变干关节中木质化木质部细胞的数量或位置。这些结果表明，正常量的木质化木质部细胞可以在 6 个月内抵抗高达正常量两倍的压力，而不会产生额外的木质化木质部细胞。将正常施加的压力加倍不足以改变干关节中木质化木质部细胞的数量或位置。这些结果表明，正常量的木质化木质部细胞可以在 6 个月内抵抗高达正常量两倍的压力，而不会产生额外的木质化木质部细胞。