The xylem is the vascular tissue that is responsible for the conduction of water and nutrients from the roots up to the shoots and leaves. In contrast, the phloem is the other vascular tissue and is responsible for the translocation of metabolic products in various parts of the plant. The xylem may be categorized as primary xylem or secondary xylem. These two types of xylems are distinguished based on their origin. The primary xylem is produced from primary growth. The procambium gives rise to the primary xylem.
The primary xylem may be differentiated into two main parts: protoxylem and metaxylem. Both protoxylem and metaxylem contain tracheids, vessels, and parenchyma. They are produced from the procambium. Nevertheless, the protoxylem is the first xylem that develops (thus, the name). It consists of narrow vessels. The vessels are formed of cells that are relatively smaller than those of metaxylem. Nevertheless, the protoxylem has more parenchyma cells than metaxylem. In young plants, the protoxylem in stems has very little secondary wall material. In the stem, it is arranged towards the center whereas in the root, it is towards the periphery.
In a young stem, the protoxylem consists of tracheary element with annular or spiral thickenings, hence, capable of stretching or elongating (for stem growth). As the stem gets older, the stem stops elongating and the tracheary elements become more filled in.
Word origin: Greek prôtos (“first”) + xylem, from Greek xúlon (“wood”)