noun, plural: primary xylems
Xylem is the vascular tissue responsible for the conduction of water and nutrients from the roots to the shoots and leaves, especially of terrestrial plants. Based on the stage and origin of growth, a xylem may be classified as primary or secondary.
Primary xylem is the xylem that is formed during the primary growth from procambium of apical meristems. It differs from the secondary xylem in a way that the secondary xylem forms as result of secondary growth. Primary growth in plants is a growth in length whereas secondary growth is a growth in diameter. Thus, the primary xylem in apical shoot and root tip is seen close to the primary phloem in a vascular bundle. As the plant grows in diameter, the primary xylem is found farther from the primary phloem as secondary xylem grows next to the primary xylem. This is true though for trees or woody plants. Non-woody plants have only the primary xylem and lack the secondary xylem. The primary xylem is differentiated into two main parts: protoxylem and metaxylem. The primary xylem also has fewer fibers (if present). The walls of tracheids and vessels of the primary xylem are also less thick than those of the secondary xylem.
Word origin: Greek xúlon (“wood”)