Three shapes and a common organisation
Ozerova and Timonin compared tubercles of three representatives of Mammillaria cultivated in the greenhouse of the Main Botanical Garden of the Russian Academy of Sciences in Moscow. The material included short, hard tubercles of M. vetula (called M. vetula subsp. gracilis in the paper), longer, soft tubercles of M. decipiens (called M. decipiens subsp. albescens in the paper) and particularly large tubercles of M. longimamma. These differences in appearance allowed the researchers to test whether different shapes co-occurred with different tissue arrangements. [Ozerova and Timonin 2020a, pp. 11–12]
The historical subspecies gracilis and albescens are synonyms of M. vetula and M. decipiens, respectively, in current POWO. The publication names identify the specific specimens used in the study. Retaining them in parentheses matters because the species accepted in the database encompasses broader variation than these selected greenhouse plants. [2-WWW][2-WWW]
The paper reported tubercles up to 6 mm long and 3 mm in diameter in the vetula material, up to 20 mm and 3 mm in the decipiens material, and up to 25 mm and 8 mm in longimamma. The first were described as hard, and the other two as soft, resembling rounded leaves in outline. Despite this difference, all three had a similar basic anatomical plan. The dimensions given describe the tubercles presented in the account. [Ozerova and Timonin 2020a, pp. 11–12]
What a tubercle section shows
Under the microscope, the transverse section was rounded. Larger vascular bundles were identified inside, with broad rays of tissue between them. A network of smaller cortical bundles lay in the inner third of the cortex. The bundles supply water and nutrients, and their distribution shows that the tubercle is an organised part of a living stem. Green tissue also filled the space between them. [Ozerova and Timonin 2020a, pp. 12–14, Fig. 2–3]
Tissue containing chloroplasts is called chlorenchyma, or photosynthetic parenchyma. The authors found it in the cortex and inner zone of the tubercles studied. They did not identify distinct hydrenchyma, colourless parenchyma specialised for water storage. Water can also be stored in chlorenchyma cells. Describing the tubercles as entirely succulent here refers to the contribution of hydrated photosynthetic cells to the volume of the organ. [Ozerova and Timonin 2020a, pp. 12, 14–16]
This picture concerns the three tubercle types studied. In other parts of a cactus stem, photosynthetic and water-storage tissues may be separated. The identified absence of distinct hydrenchyma in the tubercle is therefore information about the anatomy of a specific organ. The authors link it to the substantial contribution of this structure to photosynthesis and the possibility of supplying CO₂ to deeper cells as well. [Ozerova and Timonin 2020a, pp. 12–16]
Why two section orientations are needed
The transverse section showed many cells of similar dimensions in all directions. Such cells are termed isodiametric. Their regular arrangement became apparent in longitudinal section. In the outer layer, they formed oblique, ascending rows of four or five cells. The rows ran from the surface into the tissue, with relatively straight intercellular spaces between them. [Ozerova and Timonin 2020a, pp. 12–14, Fig. 2–3]
The inner chlorenchyma had a different organisation. Its cells were more distinctly elongated, with a ratio of shorter to longer dimension of approximately 1:2–1:3, and formed longitudinal rows. Cell arrangement, bundle orientation and the course of the spaces produce a three-dimensional picture that a single transverse section does not reveal. A similar cell outline in two preparations may therefore conceal differences in the actual organisation of the tissue. [Ozerova and Timonin 2020a, pp. 12–14]
Sections for light microscopy were 10 µm thick and cut in both orientations. Scanning microscopy supplemented the observations. The material was documented by herbarium specimens Ozerova 20-2, 20-3 and 20-4 in the MHA herbarium. The paper presents an anatomical comparison, without a stated number of independent plants or statistical analysis of population variation. Its strongest basis consists of the identified tissues and their organisation in the images. [Ozerova and Timonin 2020a, pp. 11–14]
Cell columns and carbon dioxide pathways
Typical leaf palisade parenchyma consists of elongated cells. In the tubercles studied, entire rows of much shorter cells form a similar organisational orientation. Ozerova and Timonin propose the term columnar chlorenchyma for this tissue. “Column” describes the arrangement of the row rather than the shape of an individual cell. This term allows the tissue's appearance to be distinguished from the structure of a classical leaf palisade. [Ozerova and Timonin 2020a, pp. 13–16]
After passing through a stoma, CO₂ moves through intercellular spaces and then must reach the carbon-fixation site within the cell. Long, tortuous pathways increase diffusion resistance. In the anatomy discussed, the relatively straight course of spaces between cell rows provides evidence for more efficient gas supply into the tubercle. This is a functional explanation derived from structure. The authors did not measure CO₂ conductance or uptake rate in this experiment. [Ozerova and Timonin 2020a, pp. 14–16]
The structure of an individual cell also matters. In CAM metabolism, malate produced during nocturnal CO₂ fixation is temporarily stored in the vacuole. A large vacuole helps accommodate this pool. An isodiametric cell shape is suited to this function, while the columnar arrangement of neighbouring cells organises gas pathways. The authors consider vacuolar volume, space for chloroplasts and diffusion together as elements of a compromise between water management and photosynthesis. [Ozerova and Timonin 2020a, pp. 14–16]
Tubercle function and evolutionary interpretation
A tubercle increases the extent of the green stem surface and contains a substantial volume of photosynthetic tissue. In the plants studied, photosynthetic cells also occur deeper within the tubercle, near the vascular network. This structure supports the interpretation of tubercles as specialised structures participating in carbon assimilation. Their different lengths and hardness coexist with a shared tissue arrangement. [Ozerova and Timonin 2020a, pp. 11–16]
Ozerova and Timonin propose a broader hypothesis that columnar chlorenchyma facilitated the evolutionary transfer of much photosynthesis to the succulent stem. Increasing the active volume of green tissue may have compensated for leaf reduction. This proposal is based on structural analysis and its relation to known constraints on diffusion and CAM metabolism. The study of three living Mammillaria taxa provides a functional argument, while the sequence of changes in ancestors of the family remains a separate evolutionary question. [Ozerova and Timonin 2020a, pp. 16–17]
Viewing the cactus reveals the level of organisation of the entire stem. The short tubercle of M. vetula, the slender tubercle of the M. decipiens material and the long tubercle of M. longimamma allow shapes of the same structural type to be compared. Areole position defines the site of spines, and the green volume beneath it contains tissues with their own arrangement and function. The external outline of the tubercle and its anatomy are two complementary ways of describing the plant. [Ozerova and Timonin 2020a, pp. 11–14]
Cacti defeat physics: columnar chlorenchyma in tubercles of three species of Mammillaria Haw. (Cactaceae)
Ozerova L. V., Timonin A. C., Cacti defeat physics: columnar chlorenchyma in tubercles of three species of Mammillaria Haw. (Cactaceae), Wulfenia, 2020, 27:10-20
A comparison of tubercle anatomy in three Mammillaria species from greenhouse material, using transverse and longitudinal sections and light and scanning microscopy. The paper describes vascular bundles, the absence of a distinct hydrenchyma and the columnar arrangement of outer chlorenchyma cells. The discussion links this arrangement to gas diffusion, vacuole volume and CAM photosynthesis, and presents a hypothesis of its importance in the evolution of the succulent stem.
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.
