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Banksia ornata is a sclerophyll shrub, characteristic of the heaths of south-eastern Australia. Its follicles are almost entirely dependent on fire for rupture and release of seed, although a few open under ambient field conditions. At air-dry moisture content (10-12% oven-dry weight) temperatures in excess of about 75°C are required for the rupture of the abscission layer between the woody valves, the time of exposure necessary for rupture decreasing with increasing temperature. At higher moisture contents rupture is delayed. No rupture occurs when follicles are water-saturated and exposed to a temperature of 100° over long periods. Similarly, no rupture occurs at room temperature when follicles are dried under vacuum (to 1.4% oven-dry weight). After rupture, the extent of valve reflexion is greatest when follicle valves are driest. These results are interpreted as follows. Follicle rupture occurs when tension across the abscission zone exceeds its strength: moisture has a major role in affecting the tension, and temperature a major role in affecting the strength.

A laboratory approach to the study of fire resistance in trees is described and the method applied to three central Victorian eucalypts which possess different types of bark. For each species, bark thickness increases with the girth of the trunk or older branches. The fibrous bark of E. obliqua is thicker than the subfibrous bark of E. radiata, and this in turn is thicker than the decorticating bark of E. cypellocarpa for the same girth of trunk or branch. The rate of heat penetration to the cambium, however, is greatest in E. obliqua and least in E. cypellocarpa. This apparent anomaly is due to the greater inflammability of the fibrous bark. It is suggested that the fire tolerance of a species should be evaluated against fuel accumulation and the fire intensity of the site, as well as the extent of damage to various parts of the tree and the ability of these parts to recover.