ROOT BROWNING IN PINUS-BANKSIANA LAMB AND EUCALYPTUS-PILULARIS SM .2. ANATOMY AND PERMEABILITY OF THE CORK ZONE

The periderm in roots of Pinus banksiana Lamb. and the polyderm in roots of Eucalyptus pilularis Sm. originate hom the pericycle. This occurs after the roots have turned brown due to deposition of tannins in the walls of cells external to the endodermis. In both species, cork cells form a continuous sheath around the vascular tissues. The cork cell walls are modified by the presence of suberin, lignin and tannin and it is the latter which imparts a brown colour to the tissue. The first layer of cork cells in both species constitutes an apoplastic barrier which prevents the fluorescent dye, berberine, from entering the vascular tissues, despite the absence of an identifiable Casparian band in the cells. Because the roots are still covered with the cortex and epidermis during early stages of periderm and polyderm formation, it is not possible to tell from the external aspect of the root when it makes a transition from the tannin zone to the cork zone.