-(Variation on leaf structure of Mikania glomerata Spreng. (Asteraceae) under different light conditions). Variations in the leaf structure of Mikania glomerata Spreng. were investigated in distinct light regimes (high light, half-shade; shade; 100%, 26,4% and 13,8% of light intensity, respectively) associated to different vegetation types in the Araucaria Forest, Municipality of Castro, PR (25°50'64" S and 49°43'69" W). This study was designed to orient medicinal-plant producers to choose the best light condition for planting and to evaluate the potential use of medicinal species as an alternative to restore and to manage the understorey. After two years of the experiment, thirty seven leaves were collected from each treatment and the following parameters were analyzed: fresh and dry leaf mass, water content, leaf area, specifi c leaf area, stomata and trichome densities, lamina and petiole thickness and chlorophyll concentration. The higher mean values of fresh and dry mass, water content, leaf area and specifi c leaf area occurred in the leaves of the shade treatment, while the stomata and trichome densities, and lamina thickness presented higher values in leaves of the open-fi eld treatment. The principal component analysis showed similarities between the leaves of the half-shade and high light treatments, suggesting that low light conditions are suffi cient to stimulate leaf morphology responses. The higher mean values of leaf area and dry mass suggest a higher productivity on shade treatment. However, studies concerning growth rates, seasonal responses, and distinct nutrient conditions are necessary to complement information about more effi cient cultivation methods.Key words -guaco, leaf plasticity, light intensity, medicinal plants RESUMO -(Variação na estrutura foliar de Mikania glomerata Spreng. (Asteraceae) sob diferentes condições de luminosidade). As variações estruturais foliares de Mikania glomerata Spreng. foram investigadas em distintas condições de luminosidade (pleno sol, meia-sombra e sombra; 100%, 26,4% e 13,8% de intensidade luminosa, respectivamente) associadas a diferentes tipos de coberturas vegetais numa região de Floresta com Araucária, Município de Castro, PR (25°50'64" S e 49°43'69" W). Este estudo buscou subsidiar os produtores de plantas medicinais na escolha da melhor condição de luz para plantio, gerando informações sobre o potencial das plantas medicinais na recomposição e manejo do sub-bosque da área. Após dois anos de crescimento nos tratamentos, 37 folhas foram coletadas em cada tratamento, para a análise de massa foliar fresca e seca, teor de água, área foliar, área foliar específi ca, densidade de estômatos e de tricomas, espessura da lâmina e pecíolo e concentração de clorofi la. Os maiores valores médios de massa foliar fresca, teor de água, área foliar e área foliar específi ca ocorreram nas folhas do tratamento sombra, enquanto a densidade estomática e de tricomas e a espessura da lâmina apresentaram maiores valores nas folhas do tratamento pleno sol. A análise...