Meetings of the Royal Botanical Society of the Netherlands

The main differential leaf and nodal anatomical characters of a number of Ilex species and their artificial hybrids has been investigated. It appears that the hybrids either share a certain character of one of the parents, completely lack such a character, are intermediate between the two parents for the two alternative character states, or show a completely new character not known to either parent. A certain pair of alternative characters may be inherited differently in different crosses involving the same parent species. Only in the case of marginal sclerenchyma strands, is this character never transferred to hybrid offspring in species combinations for which this character is differential. The lignification of the unspecialized abaxial epidermal cells is dominantly inherited in all but one hybrid involved. The development of a distinctive adaxial hypodermis is usually suppressed in hybrids of which one of the parents is lacking in this character. New characters have been found for hybrids of Ilex opaca x aquifolium where the vasculation of petiole and midrib, and the presence of periclinal subdivisions in the adaxial epidermis constitute features not known from either parent species. The possible significance of the four different ways of expression of parental characters in hybrid offspring is in the field of genetic control of anatomical characters and in the systematic and phylogenetic interpretation of present day diversity in Ilex leaf anatomy. Some characters are probably controlled by homozygous recessive genes, others by dominant genes, and the majority by multiple genes. If a significant role of hybridization is presumed for the diversification of Ilex leaf anatomy in evolution, the present results aid in our understanding of the reticulate pattern of leaf anatomical variation within the genus, and may also explain the unusual variability of some characters below the species level reported for some taxa (Baas 1975). Finally the leaf anatomical character complexes may aid the taxonomist and horticulturist in tracing parent species of putative Ilex hybrids.