They transport food from the leaves to the roots
External structure of a dicot stem
Transverse section of a young dicot stem in detail
The cuticle reduces water loss through transpiration
The cuticle is transparent allowing light to pass through for photosynthesis.
The epidermis may have guard cells to allow gaseous exchange while at the same time reducing water loss through transpiration.
The hypodermis of sclerenchyma or collenchyma strengthens the stem and helps keep it upright.
The parenchyma of the cortex stores food.
The intercellular air spaces between the parenchyma cells allow for transport of water and gases.
The endodermal cells stores starch.
The sclerenchyma fibres gives strength to the plant and help to keep it upright.
Xylem transports water and mineral salts from the roots to the leaves
Phloem transports manufactured food from the leaf to the roots
Increase in thickness of the stem is called secondary growth or secondary thickness.
Secondary thickness takes place in 3 steps:
Formation of secondary xylem and secondary phloem
Formation of cork
Formation of lenticels
Parenchyma cells between the vascular bundles begin to divide. This with vascular cambium forms a complete ring of cambium
Entire cambium divides to form secondary xylem on the inside and secondary phloem on the outside.
Every year a new ring of secondary xylem is formed. These are called the annual rings. (can tell the age of the tree by counting annual rings)
The xylem cells making up the older annual rings becomes clogged and are no longer able to transport water and mineral salts. They become heartwood.
Certain parenchyma cells in the cortex begin to divide and form cork cambium
The cork cambium divides to form cork cells on its outside.
The cork cells have suberin laid down on their cell walls.