<p><i>Bertolonia</i> (Melastomataceae) is a genus endemic to the Atlantic Forest, typically associated with moist and shaded understory environments. However, some populations occur in transitional zones between ombrophilous and seasonally dry forests, where pronounced climatic seasonality and recurrent drought raise questions about their persistence under periodic water limitation. In these environments, some individuals show conspicuously thickened stems, suggesting a potential role in water storage. Here, we investigated whether stem thickening represents an adaptive response to drought stress. We analyzed the stem morphoanatomy of six <i>Bertolonia</i> individuals from populations occurring under contrasting environmental conditions and compiled climatic data from their collection sites. Stem thickening was observed in individuals from four populations: one corresponding to <i>Bertolonia crassicaulis</i> and the other three representing undescribed morphotypes referred to here as ‘Boa Nova’, ‘Serra da Jiboia’, and ‘Valença’. In these plants, stem thickening was associated with a markedly expanded pith composed of thin-walled cells with wide lumina, occupying up to three-quarters of the stem volume. This configuration is consistent with aquiferous parenchyma and suggests a role in water storage, although the morphotype ‘Valença’ occurs in a more humid environment with no dry months. The remaining two populations belong to <i>Bertolonia carmoi</i> and <i>Bertolonia acuminata</i>, both from wetter environments. Individuals from these populations lack stem thickening and show reduced pith proportions. These findings suggest that stem thickening in <i>Bertolonia</i> may be associated with environmental conditions related to water limitation, although this trait is not exclusively restricted to seasonally dry environments, highlighting the possible influence of ecological and evolutionary factors on stem structural variation.</p>