The extent of vertical migration of 33P in thalli of the heathland lichen Cladonia portentosa was investigated under
field conditions. 33P-labelled orthophosphate was introduced into the bottom 25 mm of podetia cut to a length of
50 mm from the apices. The distribution of label was scanned using a molecular imager immediately after
incubation, and after growing for 8 wk and 6 months. Differences in the relative distribution of label between
podetia harvested at the beginning and the end of the experiment showed that there had been a significant
migration of 33P upwards out of the labelled 25 mm stratum towards the apex. This was confirmed by statistically
significant changes in the median (md) and the 90 percentile of total relative distribution of
33P label. In a control treatment in which label was introduced into the apical 25 mm of podetia, which were then grown inverted (top
down), no upward movement of label was detected. By contrast, a statistically significant reduction in the md of
the distribution indicated migration downwards towards the thallus apex. The results are consistent with the
hypothesis that P is recycled within podetia of mat-forming lichens, migrating from degrading basal regions
upwards to the growing apices following a source–sink relationship.