The genus and names in the literature
Plants of the World Online accepts 28 species of Ferocactus. The genus's natural range includes the southwestern and south-central United States and Mexico. In the same classification, F. acanthodes remains the accepted species name, and F. cylindraceus is a synonym of its nominate subspecies. Publications under both names therefore need to be assigned to one account, while retaining population information. [2-WWW] [2-WWW]
The genus includes species with different flowering seasons and spine structures. Data from one site primarily describe the plants studied. For example, experiments involving F. wislizeni and F. acanthodes (called F. cylindraceus in the study) took place in two separate Arizona populations, and the composition of insects visiting flowers differed even between these sites. [McIntosh 2005, p. 728–730]
The stem, areoles and ring of flowers
The characteristic stem has distinct longitudinal ribs. Areoles, short shoots producing spines and reproductive organs, lie along their crests. Flowers appear near the stem tip. In the two species studied in detail, the widely spreading perianth surrounds numerous stamens and a stigma raised above them. [Zimmerman and Parfitt 2003b] [McIntosh 2005]
As the flower opens, the stamens are clustered around the style. Over the following days, they move away from it. A bee collecting pollen moves over the surface of the stamens. To reach nectar secreted at the base of the floral tube, it pushes down between them. This spatial arrangement allows both ways of foraging to result in pollination. Experiments on F. wislizeni and F. acanthodes found no significant difference in the effectiveness of a single visit attributable solely to the type of reward collected. [McIntosh 2005, p. 728–731]
Reproduction dependent on pollen movement
In the Arizona populations studied, cross-pollination, the transfer of pollen between different individuals, played the main role. Self-pollination contributed very little to seed production. Flowers remained open for several days and could receive repeated visits. These resulted in fruits containing variable numbers of seeds, depending both on visit frequency and on the effectiveness of the particular insect. [McIntosh 2005, p. 728, 730–733]
The most important pollinators in the sample were three species of solitary bees associated with cactus pollen. Diadasia rinconis was particularly important. The presence of an insect was assessed together with subsequent fruit and seed production. This distinguished actual pollination service from nectar or pollen collection alone. The significance of this difference is explored in “Nectar for two partners. Ants and bees on Ferocactus”. [McIntosh 2005]
Extrafloral nectaries and organ protection
In F. wislizeni, the upper areoles produce modified spines that secrete nectar. These extrafloral nectaries lie close to buds, flowers and fruits. Ants collect the secretion and, while doing so, patrol the plant and may attack herbivores. Their importance depends on the species, the number of workers and how often they visit the cactus. [Ness 2006] [Ness et al. 2006]
Highly aggressive ants can also reduce bee visits. In F. wislizeni, protection of developing organs was observed alongside a cost resulting from pollinators spending less time in flowers. The balance of these effects concerns successive reproductive stages, from preserving a bud to the number of seeds in a surviving fruit. [Ness 2006, p. 510–513]
Growing in containers
In Poland, desert ferocacti are mainly grown in a bright position under cover. The RHS general guide for cacti recommends an airy location and a free-draining, peat-free growing medium with added grit. Pot size is matched to root size. Excess volume retains moisture for a long time. Water should drain freely after watering. [1-WWW]
Watering follows active growth and drying of the growing medium. In winter, the general RHS recommendation is a bright, cooler rest, with a night temperature of 8–10°C and minimal watering. These are conditions for caring for indoor plants. More detailed recommendations and experimental results are described under individual species. [1-WWW]
Development of rib number
Ribs develop along with the stem. In the Deep Canyon population of F. acanthodes, 56 of 100 randomly selected plants had 5, 8, 13 or 21 ribs in the middle part of the stem. New ribs formed closer to the tip, so a single stem could retain different numbers at successive heights. Variation in rib number can therefore reflect the plant's growth stage. The authors interpreted the relationship between this restructuring and stem expansion and contraction during changes in hydration in functional terms. [Robberecht and Nobel 1983]
Dew on the stem and spine wettability
Stem and spine surfaces differ in wettability. In a pilot study in Swansea in 2013–2014, dew formed on the stem of F. wislizeni (called F. wislizenii in the publication). Only a single drop was recorded on its spines. Its approximate contact angle with the surface was 120°, consistent with low wettability. Under an electron microscope, the spines showed numerous small projections. The authors presented the relationship between this microstructure and hydrophobicity as an explanation requiring further comparisons. [Malik et al. 2015, pp. 10–12]
The amount of dew retained was measured from the change in plant mass and expressed relative to its projected area as seen from above. Initially, single specimens of four species were studied, and a second specimen of each was later added. The result concerned surface condensation. The contribution of this water to the plant's physiological water supply required a separate test. The difference between stem and spine is particularly clear in this ferocactus and shows why the structure of the two surfaces is considered separately. [Malik et al. 2015, p. 3–6, 9–14]