Polarity of Xylem Formation in Isolated Stem Segments of Pinus silvestris

The culture of isolated stem sogments was used as a technique to study polarity of xylem formation in Pinus silvestris L. Cambial activity was greatest at the apical or the basal end, whichever received the complete fresh medium, but decreased more abruptly with increasing distance when the basal end was so supplied. Whether gradients of activity increased basipetally or acropetally also depended upon the end to which the auxin and the sugar components were supplied. Ring-barking of segments, then cultur-ing them with apical supply of medium to the xylem, resulted in inhibition of xylem production below, and above, the ring. Cambial activity at the apical ends of such segments was expressible as a logarithmic function of the uninterrupted length of contiguous extraxylary tissues. The involvement of polarity phenomena in control of cambial activity in isolated stem segments is confirmed.