The natural durability of the central core of heartwood in trunks of many trees within the forest primeval was recognized by American pioneers who chose black locust, cedar, and chestnut for fence posts and put cypress, redwood, and longleaf pine into construction that has endured for hundreds of years. Decay of wood is caused by fungi, and heartwood in these and some other species contains substances which are toxic to fungi. Formation of heartwood is a function more of tree age than of tree size, and trees in virgin stands spent centuries in competition with their neighbors for limited amounts of nutrients and sunlight. Hence most of the lumber from virgin stands of decay-resistant species was composed of heartwood. The supply of heartwood remained reasonably abundant until 20 or 30 years ago; therefore, our own fathers could often succeed with building practices which today must be considered unsound. For much lumber cut now is from trees that have grown so fast that they consist mostly of sapwood, and no sapwood of any species has any decayresistance worth mentioning. Not even cypress. This principle is exemplified by the southern pines. The heartwood of longleaf pine (Pinus palustris Mill.) is decay-resistant (Clark, 1957; For. Prod. Lab., 1967), and much durable construction was achieved with "hard" or "fat"