SOLVENT VAPOR LOCK - AN EXTREME CASE OF THE PROBLEMS CAUSED BY LIGNIFIED AND SUBERIZED CELL-WALLS DURING RESIN INFILTRATION

To determine the cause of floating tissue pieces and poor resin infilt ration, sugarcane stem tissue was monitored during processing for micr oscopy. Bubbles formed within cells at the early stages of solvent rep lacement (the timing dependent on the fixation used), and increased in volume to fill the cells as resin concentration increased. It is conc luded that bubbles form by solvent cavitation (formation of a solvent vapour phase) as hydraulic pressure decreases within the cells, becaus e the thick, lignified and suberized walls are freely permeable to the solvent but not to the resin molecules. The pressure difference acros s the walls that would be required to release the vapour lock cannot b e achieved by vacuum infiltration, In more flexible, thinnerwalled, su berized and lignified tissues in maize roots, the lowering of hydrosta tic pressure in the cells results in distortion by cytorrhysis (cell c ollapse). The analogy is drawn between the cavitation and cytorrhysis of cells during resin infiltration and the same processes which occur in living plant cells during water stress. Cavitation and cytorrhysis did not occur in sugarcane or maize tissues processed by anhydrous fre eze substitution.