Floral bracts (bracteoles, cataphylls) are leaf-like organs that subtend flowers or inflorescences but are of non-floral origin; they occur in a wide diversity of species, representing multiple independent origins, and exhibit great variation in form and function. Although much attention has been paid to bracts over the past 150 years, our understanding of their adaptive significance remains remarkably incomplete. This is because most studies of bract function and evolution focus on only one or a few selective factors. It is widely recognised that bracts experience selection mediated by pollinators, particularly for enhancing pollinator attraction through strong visual, olfactory, or echo-acoustic contrast with the background and through signalling the presence of pollinator rewards, either honestly (providing rewards for pollinators), or deceptively (attraction without reward or even trapping pollinators). However, studies in recent decades have demonstrated that bract evolution is also affected by agents other than pollinators. Bracts can protect flowers, fruits, or seeds from herbivores by displaying warning signals, camouflaging conspicuous reproductive organs, or by providing physical barriers or toxic chemicals. Reviews of published studies show that bracts can also promote seed dispersal and ameliorate the effects of abiotic stressors, such as low temperature, strong ultraviolet radiation, heavy rain, drought, and/or mechanical abrasion, on reproductive organs or for the plants' pollinators. In addition, green bracts and greening of colourful bracts after pollination promote photosynthetic activity, providing substantial carbon (photosynthates) for fruit or seed development, especially late in a plant's life cycle or season, when leaves have started to senesce. A further layer of complexity derives from the fact that the agents of selection driving the evolution of bracts vary between species and even between different developmental stages within a species, and selection by one agent can be reinforced or opposed by other agents. In summary, our survey of the literature reveals that bracts are multifunctional and subject to multiple agents of selection. To understand fully the functional and evolutionary significance of bracts, it is necessary to consider multiple selection agents throughout the life of the plant, using integrative approaches to data collection and analysis.

中文翻译：

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被子植物花苞片的多功能性：综述

花苞片（bracteoles、cataphylls）是包覆花朵或花序的叶状器官，但并非花卉来源；它们存在于多种物种中，代表多个独立的起源，并在形式和功能上表现出巨大的差异。尽管过去 150 年来人们对苞片给予了很多关注，但我们对其适应性意义的理解仍然非常不完整。这是因为大多数苞片功能和进化的研究只关注一个或几个选择性因素。人们普遍认为，苞片经历由传粉媒介介导的选择，特别是通过与背景强烈的视觉、嗅觉或回声声对比以及通过诚实地（为传粉媒介提供奖励）或通过发出传粉媒介奖励的存在来增强传粉媒介的吸引力。欺骗性的（没有奖励的吸引，甚至诱捕传粉者）。然而，近几十年来的研究表明，苞片进化也受到授粉媒介以外的媒介的影响。苞片可以通过显示警告信号、伪装明显的生殖器官或提供物理屏障或有毒化学物质来保护花朵、水果或种子免受食草动物的侵害。对已发表研究的评论表明，苞片还可以促进种子传播并改善非生物应激源（例如低温、强紫外线辐射、大雨、干旱和/或机械磨损）对生殖器官或植物传粉者的影响。此外，授粉后的绿色苞片和彩色苞片变绿可促进光合作用，为果实或种子的发育提供大量碳（光合产物），特别是在植物生命周期或季节的后期，当叶子开始衰老时。更进一步的复杂性源于这样一个事实：驱动苞片进化的选择因素在物种之间，甚至在一个物种内的不同发育阶段之间存在差异，并且一种因素的选择可以被其他因素加强或反对。总之，我们对文献的调查表明，苞片具有多功能性，并且受到多种选择因素的影响。为了充分了解苞片的功能和进化意义，有必要在植物的整个生命周期中考虑多种选择剂，并使用综合方法进行数据收集和分析。