Internal structure and growth of the stem

The stem provides support, conducts substances, stores food and takes part in vegetative propagation. A cross-section of a young mulberry twig shows, from outside in: one layer of epidermis, cortex parenchyma (living basic tissue), bast (phloem), cambium, wood (xylem) and the pith in the centre. In the second half of summer lenticels appear on the surface, through which living cells breathe; the surface gradually turns to cork and then bark, which shields the twig from cold, heat and microbes. In the bast, sieve tubes carry organic matter from leaves to other organs; bast fibres make the twig strong and flexible (thread and rope are made from hemp and flax fibres). Vessels in the wood carry water and mineral salts up from the roots to all organs; wood may be hard (oak, walnut, apricot) or soft (willow, poplar). The young cells between bast and wood — the cambium — divide constantly and thicken the stem.