State of the Pores in the Functioning Sieve Plate

RECENT work in this laboratory has confirmed that the pores in the sieve plates of functioning phloem are normally occluded, fairly densely, with slime fibrils. Stems of Helianthus seedlings and thin stolons of Saxifraga sarmentosa were plunged into briskly boiling water for 3 min before conventional fixation in glutaraldehyde and osmium, embedding and sectioning for electron microscopy, In almost all cases, the pores in the sieve plates appeared plugged with densely-staining material (Fig. 1 a and b ); where this was not obvious the micrographs strongly suggested that the section had passed between the shrunken plug and the pore wall. We regard these observations as evidence against the views often put forward that the plugging is an artefact of preparation caused either by longitudinal turgor release or by local enzyme action 1,2 .