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Vegetative Xanthium plants grown under noninductive conditions were marked with India ink along the stem and photographed at three consecutive time intervals. Relative elemental rates (d(dX/dt)/dX) of stem elongation were estimated from displacement of marks during stem elongation. Young internodes elongated with constant relative elemental rates of 0.2 day to the -1. Older internodes displayed an acropetal pattern of elongation in which the basal segments of an internode stopped elongating first and the apical portion last. Nodal regions elongated with very small relative elemental rates of 0.05 day to the -1. Rates decreased as the age of the internodes and nodes increased and stopped shortly after Leaf Plastochron Index 9.

Regions of cell division and cell elongation were established during stem growth of Xanthium pensylvanicum (cocklebur). From percent mitosis it was determined that the region of cell division in a Xanthium stem of Plastochron Index (PI) 13.89 was 20 mm long, starting from the stem apex and proceeding in a basipetal direction. Measurements of cortical cell lengths demonstrated that mature cell length in the stem of the same plant was reached at about 55 mm distance from the stem apex. Between 20 and 55 mm distance from the apex, shoot growth occurred by cell elongation alone. The maximum rate of change in cell length (dC/dX) was at about 15 mm distance from the stem apex. The length of the apical growth region was a function of the age of the plants. The older the plant, the longer its apical region of elongation. Growth of Xanthium stems was due primarily to elongation of internodes; the nodal regions did not seem to elongate.