Xylem Development

Written by: Michael Gilmore


Determining and identifying the different types of primary xylem development in vascular land plants is of considerable importance to botanists. There are four types of primary xylem development: exarch, endarch, centrarch, and mesarch. Understanding the difference between protoxylem and metaxylem and the position of each of these in relation to each other is critical to correctly identify the different types of primary xylem development. Protoxylem is the first part of the primary xylem that matures (Raven et al 1999) and these cells are usually smaller than the metaxylem. Metaxylem is the part of the primary xylem that differentiates after the protoxylem (Raven et al 1999) and these cells are usually larger than the protoxylem.





The photographs above show centrarch development in Selaginella sp. at 200 and 400 magnification respectively. In centrarch development there is one vascular strand and the protoxylem is located in the center surrounded by the metaxylem. This type of primary xylem development is found in certain extinct plants from the Devonian (Gifford and Foster 1989).





The photographs above show mesarch development in Ophioglossum sp. roots at 40 and 100 magnification respectively. In mesarch development there are several vascular strands and in each of these protoxylem is located in the center surrounded by the metaxylem. This type of primary xylem development is found in many species of ferns (Gifford and Foster 1989).





The photographs above show endarch development in Equisetum sp. stems at 100 and 200 magnification respectively. In endarch development the protoxylem begins its development from the innermost procambial cells located adjacent to the pith and development progresses outward. Therefore, the protoxylem is found toward the inside and metaxylem toward the outside of the stem. Endarch development is considered the most highly advanced type of primary xylem development (Gifford and Foster 1989).





The photograph above shows exarch development in Psilotum sp. aerial stems at 100 magnification. In exarch development the protoxylem begins development from the outermost edge of the procambial cylinder (the side closest to the stem) and development proceeds from the outside to the inside. Therefore, the protoxylem is found toward the outside and metaxylem toward the inside of the stem. This type of primary xylem development is considered a primitive condition in vascular land plants (Gifford and Foster 1989).



Bibliography

Gifford, E. M. and A. S. Foster. 1989. Morphology and Evolution of Vascular Plants, Third Edition. W. H. Freeman and Company. New York.

Raven, P. H., Evert, R. F., and S. E. Eichhorn. 1999. Biology of Plants, Sixth Edition. W. H. Freeman and Company. New York.