Phloem in carrot calluses.

Translocation of (14)C-labelled photosynthate across 2 cm long carrot calluses was not detectable even 6 hours after a 30 min (14)CO2 pulse. This is consistent with the discontinuous nature of the phloem as a whole, although small strands of contiguous, well differentiated sieve cells with companion cells were readily seen in electron micrographs.