The wood anatomical range in Ilex (Aquifoliaceae) and its ecological and phylogenetic significance

The wood anatomy of 81 species of Ilex is described in detail. The wood anatomical range encountered is presented in a generic description (p. 196). Data on ontogenetic changes in vessel member length and number of bars per perforation are given for three species. The great amount of variation in mainly quantitative but also in some qualitative features is hardly or not related with subgeneric classification but with latitudinal and altitudinal distribution. In both the northern and southern hemisphere and in both the Old and New World, temperate and subtropical species are characterized by conspicuous growth rings, numerous narrow vessels, relatively short vessel members and few bars per perforation plate, conspicuous spiral thickenings on both vessel and fibre walls, and the fibretracheids are frequently provided with rather numerous conspicuously bordered tangential wall pits. In tropical lowland species growth rings are absent or less marked, the vessels are scanty and wide, the vessel members are long and the number of bars per perforation plate is high. Spirals are lacking or faint, or occur only in a minor part of the axial elements. The fibre-tracheids have usually few pits with more reduced borders on the tangential walls. Tropical montane species resemble the temperate ones to a great extent, but this does not apply to growth rings, spiral thickenings, and frequency and size of fibre-tracheid pits. The only wood sample of a climbing Ilex species from the tropics studied deviates from the general trend in having few bars per perforation plate. The two temperate species I. serrata and I. verticillata are exceptional in lacking spiral thickenings. In all wood characters they resemble the genus Nemopanthus (also Aquifoliaceae) very closely. Comparisons with data from literature and original observations on Prunus, Symplocos, Vaccinium. Viburnum, and to some extent also on Hydrangea, support the view that the gradual differences between temperate and tropical Ilex species conform to a general trend also present in other taxa. Therefore a major climatic influence on wood structure is indicated. This is discussed with reference to the major trends of phylogenetic wood specialization. The fact that within Ilex and Symplocos the tropical lowland species have perforations with the most numerous bars cannot be brought in agreement with the general phenomenon of a rare occurrence of scalariform perforations in tropical lowland floras. Other items such as the parallel between the absence of spiral thickenings and the presence of entire leaf margins, the lack of a clear taxonomic pattern in the wood anatomical variation in Ilex, and observations by former students of Ilex wood anatomy are also discussed.