Observations on Vegetative Branching in Cycads

Premise of research. Branching in cycads has received little attention, in part because of a lack of available materials for developmental study. Morphology and mature anatomy have enabled some conclusions to be made, but more observations, which are the basis for the present study, are needed. Methodology. Observations concerning external morphology were made in habitat. Some plants were sectioned with a machete and were stained with phloroglucinol so that the internal organization of the primary and secondary xylem could be observed. Pivotal results. Vegetative branching occurs via two processes, isotomous branching of the shoot apex and adventitious branching from damaged regions and leaf bases. Conclusions. From external morphology and mature stem vasculature, apical branching is isotomous, with successive isotomies occurring at right angles. There are no axillary buds. False isotomies can occur from equal adventitious branches on decapitated trunks. These pseudoisotomies can be determined by disruptions in the leaf base pattern in the area of the trunk where the pseudoisotomies occur. Adventitious branching occurs via two pathways. First, in intact plants, branches arise from meristematic tissue produced by a phellogen in the leaf bases. Second, branches arise from lateral meristems (the vascular cambium and the phellogen) in damaged stems, as well as occasionally from the pith.