How do pine needles grow in clusters called fascicles?

In pine trees (Pinus), needles grow in clusters known as fascicles through the development of specialized, determinate structures called short shoots.12

While pines possess both long shoots and short shoots, the needlelike adult leaves are borne specifically at the tip of these short shoots. These short shoots develop in the axils of deciduous scale leaves found on the tree's branches. Each fascicle typically contains two to five needles, though this can vary from one to eight depending on the species. These bundles are retained on the tree for several years—sometimes up to 20—before being shed. Unlike the branches of many other plants, these short shoots almost never grow out into full branches.21