RESUMEN
Hybridization and introgression between cork oak (Quercus suber) and holm oak (Q. ilex) have traditionally been reckoned as undesirable processes, since hybrid individuals lack the profitable bark characteristics of cork oak. Nevertheless, a systematic and quantitative description of the bark of these hybrids at the microscopic level, based on a significant number of individuals, is not available to date. In this work we provide such a qualitative and quantitative description, identifying the most relevant variables for their classification. Hybrids show certain features intermediate between those of the parent species (such as phellem percentage in the outer bark, which was approximately 40% as a mean value for hybrids, 20% in holm oak and almost 99% in cork oak), as well as other unique features, such as the general suberization of inactive phloem (up to 25% in certain individuals), reported here for the first time. These results suggest a relevant hybridization-induced modification of the genetic expression patterns. Therefore, hybrid individuals provide a valuable material to disentangle the molecular mechanisms underpinning bark development in angiosperms.