Growth of photosynthetic portions of Discocactus shoots is seemingly not suppressed by cephalium formation, as vascular traces are prominent between the vascular cylinder and the circular juncture of cephalium and juvenile growth. Neither of these morphological traits has previously been documented in the Cactaceae. I therefore provide a pair of hypotheses consistent with these morphological traits and suggest ways to test these hypotheses, although do not test them myself. One hypothesis for these morphological traits is that the shoot vascular cylinder bifurcates when the cephalium first forms, with one cylindrical ‘branch’ of the vascular cambium terminating in the shoot apical meristem and the other concentric cylindrical ‘branch’ of the vascular cambium terminating in a circular meristem at the base of the terminal cephalium. A second hypothesis is that subapical development occurs very slowly surrounding a depressed shoot apical meristem in Discocactus. Vegetative portions of the shoot mature sufficiently slowly that the cephalium can be well formed even while juvenile areoles and photosynthetic internodes continue to grow and develop for several years after production by the juvenile phase of the shoot apical meristem.
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Vol. 61 • No. 2