Mammillaria crucigera

Name and subspecies

Mammillaria crucigera Mart. is an accepted species with a natural range in Puebla and Oaxaca. POWO recognises the nominate subspecies crucigera and subsp. tlalocii. The historical combination Cactus cruciger is a synonym based on the same name. The 2006 temperature study identifies the material at species level, so the results were not divided between subspecies. [2-WWW] [Martorell and Patiño 2006, POWO, 11 October 2026. Martorell and Patiño, p. 543]

Habitat and stems of the population described

Martorell and Patiño described plants from gypsum rock faces in Oaxaca at 1050 m above sea level. The surroundings contained seasonally deciduous tropical forest, and the annual rainfall total for the area was 743.4 mm. In their observations, the species occupied cliffs or vertical parts of boulders. Plants often branched extensively, and individual stems were approximately 7 cm high and 5 cm in diameter. [Martorell and Patiño 2006, p. 543]

The rock face studied had a slope of 80° and an aspect azimuth of 283°, close to west. This position limited direct radiation in the first part of the day and changed rock-surface temperature. On the April measurement days, direct light began to reach the site after 10.52 h. The timing of sunlight is part of the characterisation of the occupied microhabitat. [Martorell and Patiño 2006, p. 544, 547]

Stem and substrate temperature

Three unbranched plants were moved to flat ground above the cliff, and three were transplanted back onto the rock face. For two days, temperature was measured in the outer 3 mm of the stem, near the apex and in the bud zone. At 13.00 h, stems on flat ground averaged 47.6°C and were 9.9°C warmer than those on the cliff. The highest reading for plants above the cliff was 51.4°C. During the same hours, the difference in substrate-surface temperatures was 14.4°C. [Martorell and Patiño 2006, p. 543–547, ryc. 2]

Beside the cliff, air temperature 1–6 cm from the substrate was more uniform than above it. On the open surface, the air became warmer closer to the ground. At night, plants on the rock were slightly warmer. The microsite therefore affected the entire daily course, with the greatest temperature separation near the daily maximum. [Martorell and Patiño 2006, p. 546–550]

Light and the scope of physiological interpretation

A model for a globose stem 5 cm in diameter predicted 23.3 MJ of direct radiation per year on flat ground and 14.4 MJ on the cliff studied, assuming a cloudless sky. The reduction of about 38.2% is a calculation of energy reaching the whole plant. The authors did not measure photosynthesis in this experiment. A possible benefit for CO₂ exchange is inferred from lower temperatures and reference to the physiology of other cacti. [Martorell and Patiño 2006, p. 544, 547–550]

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