Polar Regeneration in Excised Roots of Taraxacum officinale Weber: A Light and Electron Microscopic Study

The day 0 secondary phloem of Taraxacum officinale root segmentscontains wide bands of parenchyma alternating with thin cylindersof conducting tissue composed of discreet conducting strands.At day 1 in the inner distal phloem (and by day 2 proximally)the initially-flattened nuclei of some parenchyma cells becomerounded, more densely stainable and a few have migrated fromthe peripheral cytoplasm to a suspended position in the vacuole.Cell division occurs asynchronously and gradually extends tothe midphloem. By day 4 nodules of primary meristematic cellsoccur proximally and numerous young leaves are visible externallyat days 5–6. Distally, callusing of the phloem is moreextensive and links with that developing from the secondaryxylem. Proximally adventitious buds form and these are bothmore abundant and quicker growing than the distally locatedadventitious roots. Although proliferation is initially mainly confined to the companioncells it increasingly involves activation of the parenchymatissue. These cells undergo a cytological de-differentiationwith the daughter cells showing a progressive decrease in cellsize and vacuome (with cytoplasmic strands, perhaps indicativeof lysosomal activity, often visible in the vacuoles), accompaniedby an increase in nucleolar size and cytoplasmic density.