Polar gradients of tracheid number and diameter during primary and secondary xylem development in young seedlings of pin us pinea L.

The differentiation of primary and secondary tracheids was studied quantitatively in the hypocotyl ofPimis pineaL. seedlings. The number of primary and secondary tracheids decreases and their radial diameter increases from cotyledons to roots. Rapid differentiation immediately beneath the cotyledons results in narrow tracheids, while slow development at the root end yields wide tracheids. The metaxylem elements in the root direction are the widest tracheids as a result of the longer time of their cell expansion. Differentiation of secondary tracheids starts immediately below the cotyledons, where the highest auxin concentration is expected, and continues basipetally towards the roots. These results are in accordance with the hypothesis that high levels of auxin induce narrow tracheids, whereas low levels of auxin result in wide elements.