When tuber slices of Helianthus tuberosus, are cultured in liquid media containing minerals, thiamine, 2% sucrose, and 1 mg/l α-naphthalene acetic acid (NAA) for 14 days, 3-5% of the cells differentiate into tracheids. Addition of benzylaminopurine (BAP) at 5 mg/l, and 4% starch, which act as xylogenic stimuli, has little effect on growth but results in a yield of over 30% tracheids in the same time period. 70-80% of the tracheids appear between days 8 and 14 of culture. Addition to optimal tracheid media of gibberellic acid (GA3) or abscisic acid (ABA) inhibits tracheid differentiation by 80-90%. GA3 causes a concomitant 20-40% reduction in total cell number while ABA has no effect on growth. In media containing only NAA (lacking BAP and starch), ABA has a large stimulatory effect on the rate of cell division, producing 50% more cells than controls. Sucrose and glucose are equally effective sugars for differentiation, but use of the two in combination results in an unusual interaction leading to substantial inhibition of tracheid formation. There is some indication that the high level of BAP required in tracheid differentiation is acting only at the first regulatory step controlling formation of provascular cells, and not in later steps of cellular development.