Strombocactus

Abstract: Aztekium and Strombocactus, monotypic genera of Mexican cacti, have been considered to be congeneric. Studies in the field and laboratory dealt with habitat, distribution, flowers, fruits, outer cell layers of the stem, spines, pollen, and seeds. Although the genera are similar, particularly with regard to habitat and seeds, there are numerous distinguishing features. These two genera are compared to each other and to eight other genera of Cactinae in a table of 30 characters. Aztekium and Strombocactus should continue provisionally to be recognized as distinct genera. Aztekium and Strombocactus, two monotypic genera of Mexican cacti, are similar in general appearance and occur in remarkably similar habitats. Buxbaum (1950) studied their gross morphology and stated that although "Strom- bocactus disciformis has much smaller seeds than Aztekium, the very similarity of the morpholog- ical types of their seeds, as well as their vege- tative organs, suggests that Aztekium is a true Strombocactus and only has subgeneric value." The verrucose seeds of both genera possess what Buxbaum (1950) called a strophiole, a corky, spongy mass of material covering the hilum.

Abstract: Background: Floral nectar is offered by the plant to its pollinators to promote cross-fertilization. Questions: Are floral nectaries morpho-anatomically similar among the taxa of Strombocactus ? What sugars do nectaries offer their pollinators? Studied species: Strombocactus disciformis subsp. disciformis , S. disciformis subsp. esperanzae and S. corregidorae, during 2016-2018. Methods: Bud flowers and flowers in anthesis from each taxon of Strombocactus , were processed to be observed in SEM; other specimens were embedded in Paraplast and sectioned to histochemical tests. The concentration of nectar was measured using a refractometer. Results: The nectaries are similar among the three studied taxa, located below the stamens and reach the upper part of the ovary forming a ring in the hypanthium. An almost flat epidermis constitutes the nectary in S. disciformis subsp. esperanzae , although in S. disciformis subsp. disciformis is slightly bulked and papillae in S . corregidorae, a nectariferous parenchyma composed of metabolically very active cells, and a subnectariferous parenchyma of larger cells, associated with vascular bundles. The nectar is secreted through nectarostomata. The concentration of nectar was 8.65 ± 3.98, 16.12 ± 4.48 and 22.09 ± 7.42 °Brix for S. disciformis subsp. esperanzae , S. corregidorae and S. disciformis subsp. disciformis , respectively. Conclusions: All taxa of Strombocactus have an annular-type nectary that secretes a low volume of nectar and together with pollen are offered as rewards. Based on other floral characteristics observed, such as the diurnal anthesis, the infundibuliform shape and the color of the flower, we propose that bees pollinate these species.

Abstract: Most cacti in subfamily Cactoideae have a thick cortex vascularized by a network of cortical bundles. It was noticed that some cortical bundles had a leaf-like feature: clusters of terminal tracheids. The objective of this research was to examine a range of species and characterize the clusters. Of the 226 species examined in 85 genera, 105 (46.7%) had clusters of terminal tracheids. Species with clusters were widely distributed throughout the subfamily. A few genera were uniform, with all sampled species having clusters (Ariocarpus, Corryocactus) or with all sampled species lacking them (Armatocereus, Mammillaria, Melocactus). Most genera were variable (e. g., Cereus, Cleistocactus) having some species with terminal tracheid clusters and other species lacking them. Where present, the clusters consisted of short, broad tracheids (not vessel elements), one or several phloem poles, but never fibers or sclereids (except Strombocactus disciformis). Clusters were most likely to occur at the apex of a rib or tubercle, especially near an areole; they often also occurred along the face of a rib; they were least common at the sinuses between ribs. Without exception, they occurred within the palisade cortex or along its base but never deep within the cortex. No ecological or functional role was obvious.