Delphinidin

Primary xylem, primary phloem, and the pith, if present, make up a central cylinder called the stele in most younger and a few older stems and roots. The simplest form of stele, called a protostele, consists of a solid core of conducting tissues in which the phloem usually surrounds the xylem. Protosteles were common in primitive seed plants that are now extinct and are also found in whisk ferns, club mosses , and other relatives of ferns. Siphonosteles, which are tubular with pith in the center, are common in ferns.

Most present-day flowering plants and conifers have eusteles in which the primary xylem and primary phloem are in discrete vascular bundles, as discussed in the section "Herbaceous Dicotyledonous Stems." Flowering plants develop from seeds that have either one or two "seed leaves," called cotyledons attached to their embryonic stems . The seeds of pines and other cone-bearing trees have several (usually eight) cotyledons. The cotyledons usually store food needed by the young seedling until its first true leaves can produce food themselves.

Flowering plants that develop from seeds having two cotyledons are called dicotyledons (usually abbreviated to dicots), while those developing from seeds with a single cotyledon are called monocotyledons (abbreviated to monocots). Dicots and monocots differ from one another in several other respects.