Following severance of the root stele mature bundle-sieve tubes show a rapid wound response, plugging their sieve pores and depositing callose. Close to the blocked sieve tubes the predetermined but still immature bundle sieve tubes differentiate and consist of mature sieve elements 48 hours after wounding. Within a serially sectioned area the existence of lateral sieve pores connecting blocked bundle-sieve tubes with those which matured after wounding could be proved. Wound-sieve tubes are initiated close to the latter, linked to them by lateral sieve pores. The wound-sieve tubes elongate bidirectionally, parallel to the interrupted phloem trace, until a first (towards the cortex) deviating member is established on one end and, on the other, the length of the common course with the bundle is sufficient for assimilate transfer. Presumably, both initiation and elongation of wound-sieve tubes are guided by preexisting plasmodesmata, which later give rise to sieve pores. Eventually the deviating wound-sieve tubes are in close plasmatic contact with those bundle-sieve tubes which mature after wounding and hence, indirectly, with blocked sieve tubes.

One precondition to the restitution of translocation within blocked bundle-sieve tubes is a secondary opening of the plugged sieve pores. The reversibility of callose deposition and the structure of functional pores are discussed.

The model of sequential differentiation for channelling auxin in undifferentiated tissue (SACHS 1975) is compared with the sequential differentiation of wound-sieve tubes.