Abstract

Although the common mahogany tree consists of only two species, Swietenia mahabaoni Jacq. (small-leaved) and S. macrophylla King (big-leaved), a third form with medium leaflets has been found in mahogany plantations. In the mahogany forests of Taiwan, these so called 'medium-leaved' mahogany trees grow more rapidly and possess higher resistance to canker disease. For some years the foresters and tree breeders have wanted to know the origin of this good form in order to select it for planting. Briscoe and Lamb (1962) have supposed that this medium-leaved mahogany may be a hybrid between the big-leaf and the small-leaf, identified from the progenies of isolated parents and of those in mixed stands. In the same year, Briscoe and Nobles (1962) reported that the medium-leaved seedlings, either from the big-leaved or small-leaved parents, grew taller than either parent species. But all the seedlings studied by them were raised from open-pollinated seeds. Their conclusions are still questionable because they did not use controlled pollination. After the unisexual flowers (not hermaphrodite as mentioned by other taxonomists) were found by Lee (1967). Yang and his coworkers, including the author, have successfully made such controlled pollinations (Yang, 1965). The present paper is aimed at an investigation of the differences between the one year old seedlings of selfed and crossed seeds of S.mahagoni with a check on open-pollinated seeds, which tends to justify the assumption of Briscoe and Lamb's report and to provide some valuable information for the breeding of mahogany trees.