File:Gymnosperm Leaves Single Needled Pinus (36445773426).jpg

cross section: Pinus needle magnification: 40x

Pinus needles may be circular, semicircular or triangular in cross section and are structurally adapted to tolerate xeric stress and freezing.

A single layered epidermis of lignified cells is covered by a heavy waxy cuticle. Numerous sunken stomata are scattered over the entire epidermal surface. They are bounded by pairs of subsidiary cells, and open internally to a substomatal cavity and externally to a respiratory cavity or vestibule.

The underlying hypodermis consists of several layers of thick-walled sclerenchyma, particularly well-developed at ridges.

Deep to the hypodermis lies an undifferentiated photosynthetic mesophyll. Cells are distinctively lobed with infolded cell walls. They are filled with chloroplasts and starch grains that may be difficult to see because of an accumulation of dark staining tannins and resins. A few resin canals can be seen close to the hypodermal side of the mesophyll.

A central vascular bundle is wrapped in a single layered endodermis with a well-defined casparian strip whose end walls may become heavily suberized with age.

Deep to the endodermis lies a multilayered parenchymous pericycle with two vascular bundles separated by an often indistinct band of sclerenchymal cells. The pericycle also contains transfusion tissues of protein rich albuminous cells which abut and assist the phloem in the transport nutrients and elongated tracheidal cells which abut and assists the xylem in the transport of water.

Most species possess two conjoint, collateral vascular bundles, surrounded and supported by the tissues of the pericycle. Xylem tracheids lie towards the adaxial surface and phloem sieve tubes towards abaxial surface of the leaf. Xylem vessels and fibers, and phloem companion cells are absent in Gymnosperms.

While vascular bundles are closed some cambium may persist near the base of the needle.