Water and Its Relation to Fresh Produce 273
provide a very efficient, low-resistance pathway for water movement because of
their open crosswalls.
The absence of membranes in the tracheary elements and the perforations in the
walls of the vessel permit water to move freely through the water-filled capillaries
in response to a pressure gradient. Taiz and Zeiger (1998) described the movement
of water to the leaves through the xylem of the vascular bundle that terminates in a
very fine and intricate network of veins throughout the leaves. Water from the xylem
is drawn into the cells of the leaf and along the cell walls. The negative pressure
that causes water to move up through the xylem occurs at the surface of the cell
walls in the leaf, and the cell walls act like very fine capillary wicks soaked with
water. The moist mesophyll cells within the leaf are in direct contact with the
atmosphere through the intercellular air spaces (Figure 9.2). As water evaporates
from the leaf surface, more water is withdrawn from air spaces in the cells, creating
a negative pressure that drives the movement of water through the xylem system.
FIGURE 9.2Sketch of a section through a leaf showing the various types of cells.