Identification under the revision's treatment
The revision from 2017 describes Opuntia humifusa as a prostrate or slightly ascending shrub during the warmer part of the year, forming dense clusters. The segments are elliptic to nearly circular, dark green, without a glaucous coating, 9–15 cm long and 5,7–8,3 cm wide. The roots are fibrous. [Majure et al. 2017]
In the accepted diagnosis, long spines are absent, and the inconspicuous glochids are straw-coloured, becoming light brown or amber with age. The perianth is entirely yellow, without a red base to the inner tepals. The stigma is white, with 6–7 lobes. These characters should be assessed together, since closely related populations may show intermediate features. [Majure et al. 2017]
Flowers, fruits and seasonality
In the American revision material, flowering occurred in May and June, and also July in the north. Fruits reached 4,2–4,8 cm long, remained green or turned red or orange-red, and could persist through winter. Seeds were 3–3,5 mm long, with a relatively narrow band of the funicular covering. [Majure et al. 2017]
In winter, the segments lose turgor and develop transverse wrinkles. Physiological studies of material identified in older papers as O. humifusa linked cold acclimation with changes in hydration and the properties of water in the tissues. Assigning a specific result to the species as currently recognised requires agreement in provenance and identification of the population studied. [Majure et al. 2017] [Goldstein and Nobel 1994]
Distribution and habitat
The revision of eastern American populations places the main part of the range along the eastern edge of the Appalachians and towards the Atlantic coast. The species grows on rock outcrops, sandy coastal soils and dry uplands. Current POWO treatment gives a broader range, also including part of Mexico. The detailed habitat description in the revision concerns material from the United States. [Majure et al. 2017] [2-WWW]
Kompleks dawnych nazw
Older works used O. humifusa in a broad sense that included, among others, the present-day O. cespitosa and O. mesacantha. In its narrow circumscription, O. humifusa is a tetraploid with a reported chromosome number of 2n = 44. Analyses indicate mixed ancestry from the southeastern and southwestern lineages of the complex, while the precise sequence of crosses remains a research hypothesis. [Majure et al. 2017] [Majure et al. 2012a]
Compared with typical O. cespitosa, it is distinguished by darker, non-glaucous segments, straw-coloured glochids and uniformly yellow flowers. A name on an older cultivation label may refer to the former complex. Plant identity is established using the full set of characters. [Majure et al. 2017]
Results of overwintering experiments
Studies of material historically identified as O. humifusa describe winter water loss and changes in sugars, mannitol and mucilage. Material from Bald Knob in Virginia lost water during acclimation despite moist substrate. In a comparison of three opuntias at 30/20°C and 10/0°C, after 3–4 weeks and with four plants per species, rising sugar concentrations accompanied changes in water relations. The results refer to the identifications and populations in those studies, without automatic assignment to the present-day divisions of the complex. [Goldstein and Nobel 1994] [Loik and Nobel 1991]
A study of material from Iowa showed a seasonal reduction in nocturnal CO₂ uptake and a marked decline in hydration during winter. The data explain the biological basis of segment flattening and wrinkling. Appearance alone, however, does not establish whether a specimen is acclimating properly. Root condition, tissue firmness and the course of changes must be considered. [Koch and Kennedy 1980] [Fleischer and Schütz 1978]
Materials from the author's archive
The species also appears in Konrad Szychowski's private material archive. [Romulski b.d., file p. 8] [Sukulenty b.d.b, file p. 2, 3] [Kaktusy i Inne 2005a, file p. 3, 55] [Modrakowski 2005, file p. 46, 47]