Which of the following statements is false concerning the xylem?
A) Xylem tracheids and vessels fulfill their vital function only after their death.
B) The cell walls of the tracheids are greatly strengthened with cellulose fibrils forming thickened rings or spirals.
C) Water molecules are transpired from the cells of the leaves, and replaced by water molecules in the xylem pulled up from the roots due to the cohesion of water molecules.
D) Movement of materials is by mass flow; solutes in xylary sap move due to a positive turgor pressure gradient from source to sink.
E) In the morning, sap in the xylem begins to move first in the twigs of the upper portion of the tree, and later in the lower trunk.
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