A geophyte with a replaceable stem

The underground system includes thickened roots and stem axes. Visible above-ground segments are a small part of the organism. They may dry out or detach, and the living underground part produces new growth. This arrangement separates a perennial resource store from a short-lived photosynthetic surface. [Las Peñas and Bernardello 2021]

Species differ in globose, pear-shaped or cylindrical segments, surface tuberculation, areole hairiness and central spine shape. In some, a flower terminates axis growth by developing at its tip. In P. neuquensis and P. reticulatus, flowers are lateral. [Las Peñas and Bernardello 2021, Table 2]

Flower and winged seed

The ovary and part of the flower are embedded in stem tissues. The fruit opens by shedding a lid, exposing the seeds. The funicular tissue surrounding them expands into a flat wing. This feature distinguishes the genus among opuntioids and is associated with seed transport by wind. [Las Peñas and Bernardello 2021]

Anther development in four species includes formation of microspore tetrads and release of individual pollen grains. Mature pollen has numerous pores across its surface. At release it contains a vegetative cell and a generative cell. Variation in pollen shape and wall structure provides further comparative traits. [Gotelli et al. 2009]

Diversification in dry landscapes

The 2021 analysis covered ten species known at the time and three chloroplast DNA regions. It confirmed the shared ancestry of the genus, with two main lineages. One included P. tuberosus, P. fischeri and P. reticulatus, among others. The other joined P. neuquensis, P. araucanus and P. hickenii. [Las Peñas and Bernardello 2021]

The dating model used estimated the onset of diversification of modern lineages at about 7.69 million years ago, with a 95% highest posterior density interval of 4.29–10.22 million years. The authors linked this course to the development of dry Monte and Patagonian environments. The result depends on chloroplast data and the calibration adopted. [Las Peñas and Bernardello 2021]

Propagation and resource management

Segments may root after detachment and partial placement in the substrate. Vegetative propagation preserves a clone, while sowing introduces a stage particularly dependent on temperature and moisture. For P. tuberosus, these relationships were tested experimentally, including growth of young seedlings. [Las Peñas and Bernardello 2021] [Kin et al. 2015]

In the experiment, water shortage strongly limited stem elongation and seedling mass gain. Continued viability of some plants during prolonged drying was associated with a low growth rate. The physiology of dormancy and survival and conditions for active growth are therefore separate stages of cultivation. Details and conditions are given in the P. tuberosus profile. [Kin et al. 2015]