Identifying traits contributing to the success of organisms growing in harsh environments has always been of great interest in evolutionary ecology. Here we focused on morphological features that could be advantageous for the survival of Croton mollis, a slender shrub occurring in the extreme white-sand habitats of the Amazonian igapos (black/clear-water rivers). We examined macro-morphological characters, as well as leaf micro-morphology using light microscopy and scanning electron microscopy. We also estimated areas of habitat suitability for C. mollis based on climatic, soil, and elevation variables using species distribution modeling (SDM) and available herbarium collections. Our results show that C. mollis presents morphological attributes similar to those of rheophytes, which may allow the species to overcome drastic seasonal changes in water level. We report the presence of five secretory structures (extrafloral nectaries, colleters, idioblasts, glandular trichomes, and laticifers) in the leaves. The size and position of the secretory idioblasts support the hypothesis of the existence of a transitional storage for secondary metabolites from idioblasts to secretory trichomes. Besides glandular trichomes, we also found stellate trichomes and trichome-like emergences in leaves. We hypothesize that these emergences could have an important role in the species’ survival due to their capacity of absorbing atmospheric moisture. Finally, the SDM supported the known habitat preferences of C. mollis and estimated a relatively wider geographic range than the currently known distribution based on herbarium records, suggesting that collecting efforts in the Amazonian region need to be increased in the future.

识别有助于在恶劣环境中生长的生物体成功的特征一直是进化生态学的极大兴趣。在这里，我们专注于可能有利于 Croton mollis 生存的形态特征，这是一种细长的灌木，出现在亚马逊河伊加波（黑色/清水河流）的极端白沙栖息地。我们使用光学显微镜和扫描电子显微镜检查了宏观形态特征以及叶片微观形态。我们还使用物种分布模型 (SDM) 和可用的植物标本收集，根据气候、土壤和海拔变量估计了 C. mollis 的栖息地适宜性区域。我们的结果表明，C. mollis 呈现出与流变植物相似的形态特征，这可能使该物种克服水位的剧烈季节性变化。我们报告了叶子中存在五种分泌结构（花外蜜腺、收集器、成虫、腺毛和乳突）。分泌性成胚细胞的大小和位置支持从成胚细胞到分泌性毛状体的次级代谢产物存在过渡储存的假设。除了腺毛，我们还在叶子中发现了星状毛和毛状体样的出现。我们假设，由于它们吸收大气水分的能力，这些出现可能对物种的生存起到重要作用。最后，SDM 支持 C. mollis 已知的栖息地偏好，并估计了比目前已知的基于植物标本馆记录的分布范围更广的地理范围，