1. Protoxylem The PRIMARY XYLEM elements that are formed from the procambium first.

2. It is found in protoxylem and first metaxylem tracheids and vessels.

3. The degree of lignification varies from slight in protoxylem to heavy in sclerenchyma and some xylem vessels, but values of 25-30% lignin and 50% cellulose are average.

4. Annular thickening Rings of thickening laid down on the inner wall of protoxylem vessels and tracheids .

5. Exarch Denoting a stele in which the metaxylem develops to the inside of the protoxylem.