The green surface changes its structure
A pale, scaly layer may accumulate on older stems of columnar cacti, followed by a brown, more raised covering. Evans and Macri described both sets of changes in four species from Ecuador. In their terminology, scale meant wax accumulation on the surface, while bark meant a brown covering associated with restructuring of surface tissues. The field study assessed the combined proportion of surface occupied by both types of changes. [Evans and Macri 2008, pp. 475, 477–479]
The authors linked this description to earlier studies of cactus anatomy and gas exchange. Growing layers can conceal stomata, the sites of controlled gas exchange between stem and surroundings. However, the Ecuadorian experiment itself did not measure CO₂ uptake or stomatal conductance. Its direct result is a map of changes visible on the stem and their comparison with exposure direction. [Evans and Macri 2008, pp. 475–477]
Four sides of a column near the equator
The researchers compared inter-rib grooves closest to north, east, south and west. They generally assessed a portion about 20 cm long, located about one metre above the ground. They estimated the proportion of altered surface to the nearest five percentage points. Before beginning, they standardised assessment methods among observers. [Evans and Macri 2008, p. 477]
Near the equator, the illumination geometry of a vertical stem differs from that at more distant latitudes. Evans and Macri compared their data with an existing model of annual exposure of vertical surfaces. Here, the model predicted greater total radiation on eastern and western sides than on northern and southern sides. This comparison provides a testable hypothesis about the directional distribution of changes. Light dose would need to be measured separately on each assessed stem to obtain a direct dose–response relationship. [Evans and Macri 2008, pp. 476–477, 480–481]
Santa Cruz and San Cristóbal show different patterns
Jasminocereus thouarsii was studied on two islands. Data were collected from 131 plants on Santa Cruz (var. delicatus in the paper) and 128 on San Cristóbal (var. thouarsii). In the current POWO treatment, the historical variety names belong to a single species. When reading the results, they indicate two separate populations rather than interchangeable samples of the same plant. [Evans and Macri 2008, Evans and Macri, pp. 477, 480][2-WWW]
On Santa Cruz, mean groove coverage by alterations was greater on the eastern and western sides. Depending on severity class, the ratio relative to northern and southern sides was about 1,2–1,7. On San Cristóbal, it remained close to 1,1 in all classes. The authors observed more vegetation around the cacti there and suggested that it contributed to weakening the difference. They did not quantitatively measure shading. The difference between islands therefore remains a significant result, while its detailed explanation remains a hypothesis. [Evans and Macri 2008, Table 2 on p. 480 and discussion on p. 481]
The pooled result and plant selection
After pooling the four species, the authors described about 1,4 times greater coverage by alterations on eastern and western sides. The value summarises the combined sample from the Galápagos and mainland Ecuador. Individual classes in the table have their own proportions. The difference between the two Jasminocereus datasets shows that this pooled result does not describe every population studied. [Evans and Macri 2008, p. 475, Table 1 on p. 479, Table 2 on p. 480]
Plants were selected deliberately. Specimens that were heavily shaded, had no visible alterations or had brown covering over their entire surface were excluded. The latter two groups would not have provided differences between stem sides. This selection matched the research question but does not allow calculation of alteration frequency across the entire population. Increasing severity was considered through surface-coverage classes without long-term tracking of the same individuals. [Evans and Macri 2008, pp. 477–478]
Dependence on light and its biological significance
The distribution of alterations is consistent with solar exposure contributing to surface restructuring. The field study documents this relationship, while the detailed contributions of ultraviolet radiation, temperature and tissue age require other measurements. The authors discussed earlier UV-B experiments in a different cactus species. Those results constitute a separate level of evidence and do not replace an experiment on Jasminocereus populations. [Evans and Macri 2008, pp. 476, 480–481]
The apparent uniformity of a columnar stem therefore includes surfaces with different exposure histories. Their positions, shelter from neighbouring vegetation and degree of covering change form a shared physiological context. The Galápagos study shows this context particularly clearly because even one species on two islands produced different strengths of directionality. [Evans and Macri 2008, pp. 480–481]
Full text of Evans and Macri 2008. Table data were checked against images of pages 479–480. The pooled result for four species has been separated from the two Jasminocereus populations. The current scope of names was checked in POWO on 10 October 2026.
Stem surface injuries of several species of columnar cacti of Ecuador
Evans L. S., Macri A., Stem surface injuries of several species of columnar cacti of Ecuador, Journal of the Torrey Botanical Society, 2008, 135(4):475-482, doi: 10.3159/07-RA-020R1.1
A field comparison of directional changes in stem surfaces in four species from Ecuador. It includes the accumulation of wax layers and a brown covering in grooves between ribs, and an analysis of exposure relative to compass directions. Two Jasminocereus populations on Santa Cruz and San Cristóbal show different degrees of directionality. The paper also presents purposive plant selection criteria and compares the combined result with a model of annual light exposure.
Plants of the World Online — Royal Botanic Gardens, Kew
Accepted names, synonyms and distribution. A citation beside a taxon leads to its record.
Accessed 2026-10-08.
