Sieve-plate pores in tobacco and bean.

Tobacco and bean plants were wilted and then fixed as whole plants with formaldehyde-glutaraldehyde for electron microscopy. In some tobacco plants the sieve-plate pores were large, with little callose. Light slime plugs were present, but there was no compaction of P-protein in the pores. Some pores in wilted bean plants were also unplugged. In other plants of both tobacco and bean the sieve-plate pores were plugged. The pores in unwilted control plants of both tobacco and bean were invariably plugged. Tobacco plants were also cut into thin slices and then immediately fixed. In specimens prepared in this way there was little callose in the pores, and many of the pores were not plugged with P-protein. These observations provide additional evidence that sieve-plate pores may be unplugged in vivo.