Conceptual Overview

There are two types of vascular, or conducting, tissues within which substances move--usually in opposite directions, xylem and phloem. Xylem serves as water conducting tissue(ascending transport), and phloem serves as the pathway for photosynthate movement (descending and ascending transport). Both xylem and phloem form a continuous branched system penetrating the whole plant body. Xylem and phloem consist of vascular (conducting) elements, and often also contain parenchyma, sclerenchyma, and secretory tissues.

Most commonly, xylem and phloem tissues are contiguous and form either tiers or individual vascular bundles. Both differentiate from a specialized meristem, the procambium. In most cases vascular bundles are collateral--that is, their xylem and phloem are arranged side-by-side. In monocotyledons, all cells of the procambium differentiate into vascular tissues whereas in gymnosperms and most dicotyledons a layer of dividing cells, the vascular cambium, remains between xylem and phloem and produces secondary vascular tissues which contribute to the growth of plant organ in thickness, or secondary growth. Therefore, vascular bundles of monocotyledons where no cambium is formed are termed closed and contain only primary vascular tissues, while vascular bundles of gymnosperms and dicotyledons have continuing developmental potential and are referred to as open. These designations are not of consequence in secondary xylem or in secondary phloem.

Differentiation from the procambium of primary vascular tissues is asynchronous. Protoxylem and protophloem are first to mature, followed by metaxylem and metaphloem. The protoxylem cells constitute the smaller part of primary xylem and have a relatively small cell diameter; they may be crushed during later growth of the organ forming a protoxylem lacuna. The metaxylem differentiates after growth in length is complete, so it is not crushed.

Water-conducting cells of both primary and secondary xylem are tracheary elements. They are of two types--tracheids and vessel members, both of which are dead at maturity and have variously sculptured lignified cell walls. Tracheids are long, narrow cells extending in the direction of water movement. They possess bordered pits through which water passes across pit membranes where their tapered ends overlap each other from one cell to the next. By contrast, the two adjacent end walls of vessel members may be partially or wholly destroyed during development. Through these perforations, water moves by bulk flow along a continuing series of end-to-end cells that form a multicellular tube-like vessel.

The perforated portion of a wall in a vessel member is known as a perforation plate. It may be simple with only one opening, or complex with several variously shaped openings--scalariform (ladder-like), or reticulate (net-like). The secondary lateral walls of tracheary elements may be more or less continuous, interrupted only by pits. The pits are variously arranged and are bordered in common walls with adjacent tracheary elements (intervessel pittings). In walls with parenchyma cells, simple or half-bordered pits are found. Conversely, the secondary wall may appear as isolated rings (annular secondary thickenings),spirals (helical thickenings), ladder-like (scalariform thickenings) or nets (reticulate thickenings).

Subunits:		Feature:
Vascular Bundles		Video: The Evolution of Vessel Elements in Angiosperms
Lateral Walls of Tracheary Elements
Perforation Plates of Vessel Members