Herbaceous dicot vine: Cucurbita

long section: Cucurbita stem magnification: 40x The epidermis is usually single layered with an outer cuticle and multicellular trichomes. The cortex consists of a conspicuous hypodermis of parenchyma cells with collections of collenchyma cells at the stem ridges. The deepest layer of the cortex is marked by a starch sheath (large barrel-shaped cells contain starch granules). Bicollateral vascular bundles have two zones of phloem and two zones of cambium on either side of the central xylem with outer facing tissues more heavily developed. The sequence of tissues in the bicollateral vascular bundle is outer phloem, outer cambium, xylem, inner cambium and inner phloem with outer more heavily developed. Walls of xylem vessels have lignin free pits and lignin thickenings that form rings, spirals, networks and solid blocks. Phloem consists of large clear sieve tubes with pitted cell walls and sieve plates and small narrow green stained companion cells. Many sieve plates have deposits of a mucoid p-protein that forms as part of the trauma response in injured phloem. Between the xylem and phloem are a few regularly arranged layers of elongated cambium cells.