Perfoliate leaves reduce herbivory in the shield‐bracted monkeyflower ( Mimulus glaucescens )

Many plant species have leaves that are completely fused around the stem, forming a disk or cup, a trait called perfoliation (Appendix S1: Figure S1). This morphology has long fascinated naturalists (Beal & St. John, 1887; Christy, 1923; Darwin, 1789, 1877a, 1877b; Keller, 1987). Much of this interest has been in plants with cuplike fused leaves that collect water (e.g., teasel, Dipsacus, cup plant, Silphium). Ecological hypotheses to explain the presence of perfoliate leaves include water reservoirs for periods of drought, plant carnivory via insect capture, and preventing herbivore movement via the moat that forms around the stem (Beal & St. John, 1887; Carlson & Harms, 2007; Darwin, 1789, 1877a, 1877b; Krupa & Thomas, 2019; Shaw & Shackleton, 2011; Sun & Huang, 2015). Despite the interest, these hypotheses are, at best, weakly supported (Krupa & Thomas, 2019), suggesting that water capture may be an incidental occurrence rather than an adaptive function. Further, most plants with perfoliate leaves do not collect water, though these surfaces might still restrict or prevent insect herbivore movement up and down the plant stem. I became interested in this while observing caterpillars interacting with a variety of California monkeyflowers (Mimulus). The aptly named shield-bracted monkeyflower, Mimulus glaucescens (syn. Erythranthe glaucescens), produces two leaf types: round to ovate basal petiolate leaves (hereafter basal leaves) and distinct inflorescence leaves formed after individual plants transition to flowering that completely fuse around the stem, forming round disks (hereafter perfoliate bracts, Figure 1). In several populations, I noticed that the basal leaves had heavy feeding damage, whereas the perfoliate bracts had little to no damage (Figure 1). M. glaucescens often co-occurs with other closely related monkeyflowers in the Mimulus guttatus species complex with very similar morphologies besides these specialized leaves. While at a site where M. glaucescens co-occurs with Mimulus nasutus, I also noticed differences in the distribution of feeding damage between species. To quantify the relative amount of damage received by perfoliate bracts versus basal leaves, I surveyed 24–34 randomly chosen plants among five populations of M. glaucescens (locations in Appendix S1: Table S1). On every plant, at each node (i.e., a pair of perfoliate bracts or basal leaves), I recorded the presence or absence of herbivore damage (0 or 1), the type of leaf (perfoliate or basal) and the position of the node (the first true leaf pair = 1, second leaf pair = 2, and so on). To test whether perfoliate and basal leaf pairs differed in the probability of damage, I fit a binomial mixed model with damage as the dependent variable, leaf type and population as independent variables, and individual plant ID as a random effect. All mixed models were fit using the lme4 package (version 1.1.28, Bates et al., 2015) in R (version 4.1.2, R Core Team, 2021). The probability of damage and 95% confidence intervals were predicted from mixed Received: 11 July 2022 Revised: 19 August 2022 Accepted: 29 August 2022