The Coryphantha tubercle. Development, identification and water conduction

The groove appears as the plant matures

Coryphantha produces globular, ovoid or elongated stems divided into tubercles. On the upper side of a mature tubercle, a groove runs from the spine-producing zone towards the axil, the depression at its base. Flowers form near the stem apex. Dicht and Lüthy describe young stages in which the groove is short or not yet visible. Identifying a seedling therefore requires considering which parts of its adult arrangement have already developed. [Dicht and Lüthy 2005, pp. 5–12]

Central spines also appear at different rates. Adult plants of some species have strong spines that seedlings do not yet produce. In Coryphantha werdermannii, juvenile armature remains composed of pale radial spines for a long time, and central spines form later. The monograph also documents variation in adult armature between populations. The number of spines counted on one young areole describes a particular stage. [Dicht and Lüthy 2005, pp. 8–12, 169–170]

Variation in dimensions and growth form

In 2016, Vázquez-Benítez and coauthors brought together 467 plants examined in the field and 1373 herbarium records. The sample covered 48 taxa under the authors' treatment. They compared 28 vegetative and reproductive characters, including 17 quantitative and 11 qualitative characters. Simultaneous analysis of several characters separated many groups well, but plant dimensions in other groups overlapped strongly. [Vázquez-Benítez et al. 2016, pp. 24–26, 38–42]

A clear example concerned Coryphantha georgii. Plants at exposed sites had a flattened globular stem 4–5 cm tall, while plants in shaded places were more cylindrical and exceeded 10 cm. This was a comparison of observed plants in different microhabitats. Growth form provides information about conditions and development, while stem colour and tubercle arrangement contribute to identification. [Vázquez-Benítez et al. 2016, pp. 41–42]

Layers beneath the stem surface

A 2023 anatomical study included three adult plants each identified as Coryphantha clavata, Coryphantha cornifera and Coryphantha radians. Beneath the single-layered epidermis, or outer skin, lay a hypodermis of two or three cell layers. Palisade and storage parenchyma were distinguished in the cortex. Vascular bundles also occurred outside the central cylinder, in the cortical parenchyma. Xylem vessels were surrounded by wide-band tracheids with ring-shaped or spiral wall thickenings. [Avila-Escobar et al. 2023, pp. 234–238]

Material identified as C. clavata had mucilage cavities in the cortex and rounded and prismatic crystals in the hypodermis. No mucilage cavities were found in the other two groups, and their crystals formed druse clusters. Historical C. radians is now included in C. cornifera. The study nevertheless retained two separately identified samples, from San Luis Potosí and Querétaro. They differed in hypodermis type and the position of stomata at the stem base. These differences describe the particular material from two sites. [Avila-Escobar et al. 2023, pp. 234–239]

Plants of the World Online treats Coryphantha radians as a synonym of Coryphantha cornifera. Linking the names in the atlas preserves the ability to find both historical identifications. When citing anatomical results, the groups of material named by the authors can still be distinguished. [2-WWW]

The vessel pathway into the tubercle

In 2016, Terrazas and coauthors compared tissues from 17 adult plants collected at natural sites during the rainy season. They belonged to six groups named in the publication as C. bumamma, C. ottonis, C. radians, C. glanduligera, C. clavata and C. erecta. Conducting elements in the central stem cylinder were compared with those in tubercle bundles. Near the areoles, bundles lay beneath the palisade parenchyma. [Terrazas et al. 2016, pp. 2–6 PDF, Table 1]

In all six groups, vessel elements in the tubercles had smaller diameters than elements in the central cylinder. In the three C. clavata plants examined, mean diameter was 31,50 ± 1,04 µm in the cylinder and 10,08 ± 1,28 µm in the tubercles. Values were reported as means with standard errors. Cell measurements were made on anatomical material. Their numbers in the preparations do not correspond to the number of independent plants. [Terrazas et al. 2016, pp. 2, 6 PDF, Table 1–2]

A model of restricted flow

Narrowing of the vessels formed the basis of a hydraulic constriction model. The authors proposed that the difference between the axial pathway and the pathway into tubercles may restrict water flow in the outer stem. They also considered the coexistence of vessels and wide-band tracheids in relation to water storage and the capacity of tissues to change volume. This is an interpretation of structure. The study did not measure actual flow, conductivity or changes in tubercle hydration during drought. [Terrazas et al. 2016, pp. 6–9 PDF, discussion]

The names of the groups studied remain relevant when applying the results. Coryphantha elephantidens subsp. bumamma (C. bumamma in the study) was represented by two plants. The result for this sample describes the subspecies and the material used in the comparison. [2-WWW]