Convergence of glandular trichome morphology and chemistry in two montane monkeyflower (Mimulus, Phrymaceae) species

Two distantly-related North American montane monkeyflower species, Mimulus lewisii and M. tilingii, possess glandular trichomes. In this study, we characterized the morphological and histochemical features of these glandular trichomes. For each species, we used traditional light microscopy and scanning electron microscopy (SEM) to examine morphological characteristics, and categorized the natural products on the leaf surface using histochemical and thin layer chromatography (TLC) staining techniques. We identified type VI capitate glandular trichomes on leaf surfaces in both species of monkeyflowers. These trichomes exhibited stalk-cell lengths and head-cell counts that varied across adaxial and abaxial leaf surfaces. Both species contained secretory products within the cuticle of the trichome head, which releases the subcuticular metabolites when ruptured. Histochemical tests of M. lewisii and M. tilingii indicated the presence of lipids and polysaccharides on the leaf surface. TLC plate staining further supported their presence, as well as UV active compounds and amines. The common morphology and chemistry of the glandular trichomes suggests an analogous response to similar environmental conditions in these evolutionary distant montane monkeyflowers.