A mineral forming in tissue
A biomineral is a solid, crystalline or amorphous product of an organism's activity, formed through local accumulation of chemical constituents. In cacti, deposits occur in different stem tissues. In 2022, De la Rosa-Tilapa, Vázquez-Sánchez and Terrazas compared their form, location and composition in the tribe Cacteae. Distinguishing deposition sites was particularly important. These were the subepidermal hypodermis, the extensive cortical parenchyma and the central pith. [De la Rosa-Tilapa et al. 2022, pp. 1–2]
Most deposits identified corresponded to calcium oxalate. Amorphous silica bodies were also present in the hypodermis of some cacti. Under a microscope, they may look similar to other bright inclusions, so identification combined morphological appearance, behaviour under polarised light and elemental analysis. Describing a mineral requires these pieces of information to be combined. [De la Rosa-Tilapa et al. 2022, pp. 3–7]
Scope of comparison and measurement method
The study included approximately 200 individuals belonging to 149 species and subspecies from 28 genera as circumscribed at the time. Sections 14 µm thick were observed under ordinary and polarised light. In addition, 34 species from 24 genera were analysed using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, SEM-EDS. These methods separated description of tissue structure and deposit shape from identification of elemental composition. [De la Rosa-Tilapa et al. 2022, pp. 2–3]
Maximum length was measured for 103 individuals. In each, 25 biominerals from a given layer in the middle of the stem were selected to reduce the influence of variation from base to apex. One quantity was the maximum length of an individual deposit, and another was the mean of these values for a sample or group. Numerous measurements from one stem describe variation within a specimen, while comparisons between plants depend on the number and representation of individuals. [De la Rosa-Tilapa et al. 2022, pp. 2–3, Table 1]
Prism, druse, spherulite
A prism has visible flat crystal faces. A druse is a crystal cluster with a spherical or star-shaped outline and projecting tips. A spherulite has a radial arrangement of elements, often with concentric bands. Under polarised light, it produces a characteristic extinction cross. Fine crystal sand consists of numerous small particles, while irregular aggregates have a less ordered outline. Silica bodies are amorphous and behave optically differently from birefringent crystals. [De la Rosa-Tilapa et al. 2022, pp. 3–6, Fig. 2, 4–6]
| Form | Tissue and example | Observed characteristic |
|---|---|---|
| Prisms | Cortex of Epithelantha micromeris | Crystals with distinct flat faces |
| Druses | Cortex of Kroenleinia grusonii (called Echinocactus grusonii in the paper) | Clusters with projecting tips |
| Crystal sand | Cortex of Mammilloydia candida (called Mammillaria candida in the paper) | Fine particles up to approximately 5 µm |
| Spherulites | Pith of Mammilloydia candida | Radial arrangement and concentric bands |
| Silica bodies | Hypodermis of some Echinocactus and Ferocactus studied | Amorphous deposits without birefringence |
The genus name and stem layer are part of the result. The Kroenleinia grusonii studied (called Echinocactus grusonii in the article) lacked hypodermal silica bodies, although these occurred in some other former Echinocactus. The same plant had druses in the cortex. Updating the name preserves the correct attribution of these two observations when current genus boundaries differ from those used in the publication. [De la Rosa-Tilapa et al. 2022, pp. 4–6, Fig. 4B, D][2-WWW][2-WWW]
Where deposits occur
Biominerals were found in at least one tissue in all 149 species and subspecies studied. The hypodermis contained them in 91, the cortex in 141 and the pith in 94. Distribution was therefore uneven between layers. Within the material studied, deposits were not observed in secondary xylem and phloem. The compilation applies to the sections examined and taxa for which specific stem material was available. [De la Rosa-Tilapa et al. 2022, pp. 3–4]
In six Mammillaria species studied, fine biominerals were also located outside cells, near the outer epidermal wall and cuticle. This distinguished their location from the numerous intracellular deposits in the hypodermis and parenchyma. The publication documents their position in mature tissue. Successive stages of formation and movement of these particles would require observations of the developing epidermis. [De la Rosa-Tilapa et al. 2022, pp. 3, 8–9, Fig. 1]
Size depends on the layer
The mean maximum biomineral length in the pooled compilation was approximately 25,0 µm for the hypodermis, 120,6 µm for the cortex and 134,0 µm for the pith. The spread was wide, and the values included different mineral forms and plants. The compilation indicates that large deposits were more often observed in internal tissues, while the hypodermis contained smaller deposits on average. The measurement does not describe the mass or number of deposits in the entire stem. [De la Rosa-Tilapa et al. 2022, p. 8, Table 1]
Among genus means, Kadenicarpus had particularly small biominerals, approximately 32 µm in the cortex and 31 µm in the pith. For Astrophytum, the mean in the pith was approximately 212 µm, and for Strombocactus in the cortex approximately 189 µm. These are means of maximum lengths of the deposits examined in the sampled representatives of each genus. Their scope does not automatically include all species and developmental stages encompassed by the current classification. [De la Rosa-Tilapa et al. 2022, pp. 4, 8–9, Fig. 3]
Why elemental composition matters
EDS identifies the elements present at the measurement site. Signals of calcium, carbon and oxygen predominated in most crystals examined, consistent with identification as calcium oxalates. Other elements, including magnesium, manganese and aluminium, were also detected in some deposits. The image and spectrum need to be interpreted together. Element presence does not by itself determine every detail of the structure of a mineral compound. [De la Rosa-Tilapa et al. 2022, pp. 5–9, Fig. 7]
Distinguishing calcium oxalate monohydrate from dihydrate requires the crystal structure to be established. The authors compared morphology with earlier mineralogical studies, but the EDS analysis used did not determine the number of water molecules in the lattice. This methodological difference matters when describing a particular prism or spherulite. The reliable information in this study remains its shape, position and measured elemental composition. [De la Rosa-Tilapa et al. 2022, pp. 8–9]
Biominerals in lineage identification
The shape of minerals in the hypodermis helped compare lineages formerly combined in Turbinicarpus. The Kadenicarpus and Turbinicarpus studied had prismatic forms there, and Rapicactus had druses with a concentric arrangement. The mineral character therefore did not distinguish the first two genera on its own. Together with seed structure, cortical bundles and DNA, it formed a broader dataset. The site of crystal deposition is as important as its general presence. [De la Rosa-Tilapa et al. 2022, pp. 8–9][De la Rosa-Tilapa et al. 2019, online version, pp. 2–4]
The authors proposed that more consistent morphology of hypodermal deposits corresponds to more strongly developmentally controlled mineral deposition, while cortex and pith variation may involve a greater contribution from processes induced by local conditions. This is a model interpreting the distribution of observed structures. The paper measured their morphology and elements, while functions in calcium balance, stem optics or defence against herbivores remain separate questions. The strongest result of the comparison is a set of anatomical characters useful in Cacteae systematics. [De la Rosa-Tilapa et al. 2022, pp. 8–10]
Biominerals in the Cacteae tribe: Chemical-morphological characterization, distribution and systematic perspectives
De la Rosa-Tilapa A., Vázquez-Sánchez M., Terrazas T., Biominerals in the Cacteae tribe: Chemical-morphological characterization, distribution and systematic perspectives, Flora, 2022, 294:152129, doi: 10.1016/j.flora.2022.152129
An anatomical and chemical comparison of biominerals in stems of 149 Cacteae species and subspecies representing 28 genera. The authors identify deposit shapes and distribution in tissues, compare their maximum lengths and analyse elemental composition in 34 species. The paper discusses the systematic significance of prisms, druses, spherulites and amorphous silica bodies.
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.
Stem anatomy of Turbinicarpus s.l. (Cacteae, Cactaceae) and its contribution to systematics
De la Rosa-Tilapa A., Vázquez-Sánchez M., Terrazas T., Stem anatomy of Turbinicarpus s.l. (Cacteae, Cactaceae) and its contribution to systematics, Plant Biosystems, 2019, 153(4):600-609, doi: 10.1080/11263504.2018.1527791
A comparison of stem anatomy in 35 taxa historically assigned to Turbinicarpus. It describes the epidermis, stomata, hypodermis, parenchymatous tissues, vascular bundles, xylem, phloem and periderm. The distribution of crystals and fibres in cortical bundles supports recognition of three evolutionary lineages. It also shows species-level exceptions, including bundle structure in T. alonsoi, T. lophophoroides and the current Rapicactus subterraneus.
