The research team set out to test whether the conspicuously thickened stems observed in certain Bertolonia individuals represent an adaptive response to drought stress. To do so, they analyzed the stem morphoanatomy of six individuals drawn from populations occurring under contrasting environmental conditions, and compiled climatic data from the sites where the plants were collected. This combination of anatomical observation and climate characterization allowed the researchers to ask a deceptively simple question: do plants from drier places build different stems than their rainforest relatives?

The results were striking. Stem thickening was found in individuals from four of the populations studied. One of these corresponds to Bertolonia crassicaulis, a species whose very name hints at its fleshy stems, while the other three represent undescribed morphotypes that the authors refer to as ‘Boa Nova’, ‘Serra da Jiboia’ and ‘Valença’. In these thick-stemmed plants, the enlargement was driven not by woody growth or a swollen bark, but by a markedly expanded pith, the central tissue of the stem, composed of thin-walled cells with wide lumina, the open interior spaces of the cells.

That cellular architecture matters. In succulent plants across the world, water-storing tissue is typically built from large, thin-walled cells whose spacious vacuoles and lumina can hold substantial volumes of water. The pith in the thick-stemmed Bertolonia individuals occupies up to three-quarters of the total stem volume, a proportion that leaves little doubt about its functional significance. The authors describe this configuration as consistent with aquiferous parenchyma, a term for water-storage tissue, and suggest that these stems function as internal reservoirs that the plants can draw upon when rain becomes scarce.

The comparison with the two remaining populations makes the pattern even clearer. These belong to Bertolonia carmoi and Bertolonia acuminata, both collected from wetter environments. Their individuals lack stem thickening altogether and show markedly reduced pith proportions. In other words, the rainforest species build slender stems with a small central core, while their dry-edge counterparts invest heavily in an expanded, water-holding pith. The anatomical contrast mirrors the climatic contrast, which is exactly what one would expect if stem succulence were shaped by water availability.

But nature rarely offers a perfectly clean story, and the study includes an instructive wrinkle. One of the thick-stemmed morphotypes, ‘Valença’, does not come from a seasonally dry site at all. Its collection locality is relatively humid, with no dry months in the climatic record. This means that while stem thickening in Bertolonia is associated with environmental conditions related to water limitation, the trait is not exclusively restricted to seasonally dry environments. The authors highlight this nuance as evidence that ecological and evolutionary factors beyond simple drought exposure likely influence stem structural variation in the genus.

Several explanations could account for such a pattern. A trait like an aquiferous pith may be inherited from an ancestor and retained in lineages that no longer face strong selection for it, a phenomenon known as phylogenetic inertia. Alternatively, the trait could serve additional functions in humid environments, such as supporting the herb’s upright posture, buffering short-term water fluctuations that even wet forests experience, or providing mechanical reinforcement. The study’s authors also note the possible influence of evolutionary history, since closely related lineages often share anatomical tendencies regardless of the climates they currently occupy. Disentangling these possibilities will require broader sampling and a phylogenetic framework that places each morphotype within the genus’s family tree.

The findings also resonate with a broader body of work on succulence in Melastomataceae. Previous research on Merianthera burlemarxii, another member of the family, documented the evolution of an aquiferous pith and fistulae, suggesting a rare case of xerophytism, or drought adaptation, in a group otherwise known for its rainforest habits. Other studies have explored how succulent stems in seasonally dry environments manage cambial activity and wood formation, and how tissue succulence more generally may help plants carry water through an increasingly unpredictable climate. The Bertolonia results add a neotropical herbaceous example to this growing picture, showing that succulent stems can evolve even in lineages whose entire evolutionary history unfolded in the shadows of wet forests.

For the Atlantic Forest, one of the most biodiverse and most fragmented biomes on Earth, the study carries conservation implications. Transitional zones between ombrophilous and seasonally dry forests are dynamic ecotones, and the undescribed morphotypes identified here, ‘Boa Nova’, ‘Serra da Jiboia’ and ‘Valença’, suggest that significant botanical diversity in these areas may still await formal description. Understanding which traits allow plants to persist under periodic water limitation could help predict how narrowly distributed species will fare as climate change alters rainfall patterns across the region, potentially shifting the boundaries between moist and dry forest types.

What makes the study compelling is its methodological clarity. By pairing precise anatomical measurements, quantifying how much of each stem’s volume the pith occupies, with climate data from the exact collection sites, the researchers grounded a morphological observation in an environmental context. The picture that emerges is of a genus in transition: rainforest herbs experimenting with a water-storage strategy more often associated with cacti and other classic succulents. Some of these experiments track drought, others may reflect deeper evolutionary legacies. Either way, the thick stems of Bertolonia offer a vivid reminder that adaptation can emerge in unexpected tissues, in unexpected places, and that even the most familiar rainforest lineages may harbor secrets about surviving a drier world.

Subject of Research: Stem morphoanatomical adaptations to drought in the Atlantic Forest plant genus Bertolonia (Melastomataceae)

Article Title: Morphoanatomical adaptations of stems in Bertolonia (Melastomataceae): how individuals of an ombrophilous lineage survive in drier localities

Article References: Bisewski, G. C. A., Bacci, L. F., Amano, E., Amorim, A. M., Goldenberg, R., & Ziemmer, J. K. (2026). Morphoanatomical adaptations of stems in Bertolonia (Melastomataceae): how individuals of an ombrophilous lineage survive in drier localities. Plant Biosystems, 160(4), Article 226. https://doi.org/10.1007/s44473-026-00234-1

Image Credits: AI Generated

DOI: 10.1007/s44473-026-00234-1

Keywords: Bertolonia, Melastomataceae, Atlantic Forest, stem anatomy, succulence, aquiferous parenchyma, drought tolerance, water storage, pith, transitional forest, plant adaptation, Brazil