Resin duct in the context of "Epithelium"

Pinus, the pines, is a genus of approximately 110–120 extant tree and shrub species. The genus is currently split into two subgenera, subgenus Pinus (hard pines), and subgenus Strobus (soft pines). Each of the subgenera have been further divided into sections and subsections based in the past on morphology, ecology and biogeography, and more recently increasingly from chloroplast DNA sequencing and whole plastid genomic analysis. While the genetic analysis has given robust results at the higher levels, they often give conflicting results lower in the phylogenetic trees, with species allocated to different subsections (and sometimes different sections) by different studies or even within a study. Within subsections, the genetic relationships between species can be even more complex and conflicting; in one study, three samples of the very distinctive and morphologically constant Pinus lambertiana were placed in three different clades of the subsection Strobus, and similar problems with many other species with widespread nonmonophyly.

Several features are used to distinguish the subgenera, sections, and subsections of pines; the number of leaves (needles) per fascicle, whether the fascicle sheaths are deciduous or persistent, the number of fibrovascular bundles per needle (two in Pinus, one in Strobus), the position of the resin ducts in the needles (internal or external), the presence or shape of the seed wings (rudimentary or effective, articulate or adnate), and the position of the umbo (dorsal or terminal) and presence of a prickle on the scales of the seed cones.