Companion cells move sugar and amino acids into and out of the sieve elements.
In "source" tissue such as leaf companion cells use transmembrane proteins to take up sugar and amino acids by active transport.
Movement of sugars in the phloem begins at the source, where sugars are loaded (actively transported) into a seive tube.
Loading sets up a water potential gradient that facilitates movement of sugar.
In dicot stem, cambium is present between xylem and phloem, such vascular bundles are called open.
In monocot stem, the cambium is absent, such vascular bundles are called closed.
Cambium are the meristematic cells which produces secondary xylem and phloem.
Phellem, phellogen and phelloderm are collectively called periderm.
Ground tissue includes all tissues except epidermis and vascular bundles.
The ground tissue comprises the bulk of the primary plant body.
Parenchyma, collenchyma and sclerenchyma cells are common in the ground tissue.
Heartwood differs from sapwood in having dead and non-conducting elements.
In old trees, the inner region that comprises dead elements with highly lignified walls is called heartwood.
Heartwood does not conduct water but gives mechanical support to the stem.
On the other hand, the peripheral region, which is lighter in colour is called sapwood.
It is involved in the conduction of water and minerals from root to leaf.
Tracheids are chief water conducting elements of xylem in gymnosperms.
They are devoid of protoplasm and hence dead.
The wall constituting the tracheids is hard, thick and lignified.
These are elongated cells with tapering ends.
Meristem is classified on the basis of position in plant bodies into lateral meristem, apical meristem and intercalary meristem.
Lateral meristem is present on the lateral sides, e.g., vascular cambium (fascicular and interfascicular cambium) and cork cambium (phellogen).
Palisade parenchyma is absent in leaves of Sorghum.
It is a monocot plant where the parenchyma tissues of the leaves are not differentiated into palisade and spongy
In barley stem, vascular bundles are closed and scattered.
They are open only for a hours in the day time and never open at night. e.g Cereals.
The annular and spirally thickened conducting elements generally develop in the protoxylem when the root or stem is maturing.
