File:Herbaceous Dicot Stem Ridge Collenchyma in Cucurbita (37154312500).jpg

Cross section: Cucurbita Common name: Pumpkin/Squash Magnification: 100x

Like most herbaceous dicots, Cucurbita is capable of limited amounts of secondary growth.

The stem is characterized by distinct ridges and furrows with vascular bundles in two rings: five smaller leaf trace bundles at the ridges and five larger bundles at the furrows.

A usually uniseriate and cutinized epidermis contains occasional stomata and numerous multicellular trichomes.

The three-layered cortex widens at the ridges and narrows at the furrows; the outermost hypodermis varies in thickness, consisting of up to five layers of collenchyma at the stem ridges that tapers into photosynthetic chlorenchyma at the furrows. The middle cortical layer consists of a few layers of loosely arranged parenchyma cells and the deepest layer of a narrow parenchyma sheath, or endodermis.

A ring of sclerenchyma defines the perimeter of the steele, overlying a deeper ring of parenchyma that follows the outer contour of the vascular bundles. The vascular bundles are bi collateral; the central xylem is bound by an inner and outer cambium, and topped by a larger outer and a smaller inner phloem. Sieve tubes, companion cells and phloem parenchyma are clearly evident the phloem. Xylem is well developed with numerous protoxylem evident towards the inside, and very large metaxylem to the outside of the bundles. Xylem consist primarily of large vessels and xylem parenchyma; tracheids and fibers are rare. Secondary vascular tissues are present but often difficult to distinguish from the primary. The vascular bundles are separated by zones of internal parenchyma.