From seedling to areole. Development of the Mammillaria apex

The plant from San Ángel and its current name

The material studied by López-Ruiz and co-authors in 2026 came from a line maintained in the UNAM Botanical Garden in Mexico. The maternal plants originated from Pedregal de San Ángel, an area of ancient lava flows in the southern part of Mexico City. The authors used the name Mammillaria san-angelensis. Seeds and plants from the preserved collection allowed developmental stages to be compared despite the disappearance of the natural population described in the paper. [López-Ruiz et al. 2026, pp. 1327–1329]

Plants of the World Online includes this name within Mammillaria haageana subsp. haageana (studied as M. san-angelensis). The accepted taxon has a much wider range in Mexico. Information about extinction at the natural San Ángel site concerns a specific historical population. In the following account, results apply specifically to the preserved San Ángel material, with the identification adopted in the publication. [2-WWW]

The shoot apical meristem is a region of small, dividing cells from which successive structures of the above-ground part of the plant arise. In the cactus studied, it lies within a recessed apex, protected by spines and hairs. Its size can be measured separately from the size of the whole plant. This allows growth of the source of new tissues to be compared with growth of the stem they produce. [López-Ruiz et al. 2026, pp. 1326–1329, Fig. 1–2]

How eight stages were compared

The authors distinguished stages at 1, 3, 7, 15, 30 and 90 days after germination, followed by one year and four years. Germination was counted from the emergence of the radicle. The seeds had previously been soaked for 48 hours. Seedlings were maintained in a chamber at 28°C, with twelve hours of illumination at a photon flux density of 112 µmol m⁻² s⁻¹ and twelve hours of darkness. Four-year-old plants came from a greenhouse. This schedule describes the experimental conditions and development of the batch used. [López-Ruiz et al. 2026, p. 1328]

Seedling dimensions were measured in 8–20 plants per stage, and meristems in 3–7. Longitudinal histological sections were 7 µm thick. Images were used to determine the width, height and perimeter of the meristem dome, and the area and perimeter of the entire identified meristematic zone. Plant and meristem measurements came from different individuals. The dataset is a cross-sectional comparison of successive stages, with no series of observations of one continuously growing apex. [López-Ruiz et al. 2026, pp. 1328–1329]

The first week and formation of new structures

One day after germination, the cotyledons were closed, with a flat, sometimes slightly concave meristem visible between them. Two cell layers were identified. After three days, the cotyledons began to open, small protrusions appeared at the sides of the meristem, and three layers could be distinguished. Tubercles, called podaria in the paper, develop from these protrusions. The meristem dome becomes more distinct, although its width initially remains similar to that on the first day. [López-Ruiz et al. 2026, pp. 1330–1332, Fig. 3]

After seven days, light and scanning microscopy showed the first spines. After fifteen days, at least two enlarged tubercles had visible areoles with spines. In the earliest stages, most of the visible seedling consisted of the hypocotyl, the portion of the embryonic axis below the cotyledons. Later, the part bearing successive areoles elongated, and the exposed hypocotyl segment became less and less distinct. [López-Ruiz et al. 2026, pp. 1330–1334, Fig. 3–4, Table 1]

In sections, a tubercle begins as a meristematic protrusion beside the main apex. As it elongates, the meristematic tissue becomes spatially separated. The zone at the tip of the tubercle produces spines and hairs. At the base, a region remains from which a flower can later develop. This separation of growth sites explains the dimorphic organisation of Mammillaria areoles, differentiated into two parts. [López-Ruiz et al. 2026, pp. 1333, 1337, 1339–1341, Fig. 7 and 10]

Meristem size and stem size

During the first month, mean dome width fluctuated around 100–140 µm. The change was more pronounced in the ninety-day stage, and the four-year-old plants studied had the largest meristem. Their stems measured a mean of 28,6 ± 2,4 mm in width and 28,0 ± 2,8 mm in length. Dimensions in successive stages show a variable growth rhythm. Differences in conditions between the chamber and greenhouse form part of this comparison. [López-Ruiz et al. 2026, pp. 1333–1336, Tables 1–2]

The strongest reported relationship concerned dome width and stem length. It was calculated from stage means for stages in which the stem had already differentiated, therefore from the four later stages. Spearman's rank correlation coefficient was 1,00, while the relationship between dome width and stem width averaged 0,72 in the bootstrap analysis, with high variation in the estimate. Growth of both structures with age explains the agreement in their rankings. The analysis does not by itself determine which genetic mechanisms regulate their sizes. [López-Ruiz et al. 2026, pp. 1329, 1334–1336, 1338, Fig. 9]

Spines and flowers arise at different sites

The spine-producing zone appeared on the developing tubercle when it was approximately 300 µm long. Successive images showed small spine primordia, their elongation, hairs and subsequent hardening. At this site, the authors did not identify a distinct dome resembling the main shoot meristem. Spines therefore grow within a separately organised areolar zone borne on the developing podarium. [López-Ruiz et al. 2026, pp. 1333, 1337, 1340, Fig. 7]

The floral meristem was observed at the base of the tubercle in four-year-old plants. Its mean width was 135,7 ± 3,1 µm in a sample of three plants. It was much narrower than the main meristem at this stage. The images showed the beginning of development of the floral axis, lateral covering structures and stamen primordia. The full chronology of flower development remains a broader subject than the stages captured in this experiment. [López-Ruiz et al. 2026, pp. 1329, 1333, 1337–1338, Table 2, Fig. 7–8]

The botanical origin of the tubercle is still debated. Traditionally, it is associated with the leaf base, and the areole with an axillary bud. In the material studied, the authors found no structures unambiguously corresponding to a true leaf blade. The tissue positions and sequence of organ formation described are anatomical observations. The authors leave homology, the shared evolutionary origin of organs, to be tested through markers of gene expression and hormone distribution, among other approaches. [López-Ruiz et al. 2026, pp. 1340–1341]