Instant Notes: Plant Biology

seasonally varying environments. The center of the trunk of many species is the

heartwood, where the vascular system no longer functions. It provides struc-

tural support and is often darker and impregnated with tannins (Topic J5), but

may be completely removed without killing the tree. Around the heartwood lies

thesapwoodthat contains functional phloem and xylem. Vascular raysradiate

from the center of the trunk. These are formed of long lived parenchyma cells

which conduct nutrients and water across the trunk, crossing both phloem and

xylem, and are responsible for secretions into the heartwood.

The outermost layers of the trunk are known as bark. Bark includes the corky

tissuesproduced by the cork cambium and the underlying layers outside the

vascular cambium, including the secondary phloem.

36 Section C – Vegetative anatomy

Vascular cambium

Secondary phloem Heartwood

Secondary xylem

Bark

Sapwood

Rays

Cork cambium

Cork

Fig. 3. A transverse section of a woody stem, showing the major layers of heartwood, sapwood and bark.