The principal issue involved in moving water and minerals from roots to shoots is

• A. phloem
• B. xylem
• C. petiole
• D. epidermis

Answer: (B)

The key idea is that xylem is the tissue that moves water and minerals from roots to shoots. Xylem is made up of hollow, dead cells with thick lignified walls that form continuous tubes from the roots up through the stems and into the leaves. The ascent of water is driven mainly by transpiration in the leaves, which creates a negative pressure that pulls the water upward; cohesion between water molecules and adhesion to the walls of the xylem help sustain a continuous water column against gravity. As water moves, minerals dissolved in it are carried along to the shoots. Root pressure can contribute in some conditions, but it is not the primary mechanism. The other tissue listed, the phloem, translocates organic nutrients like sugars, not the bulk movement of water and minerals upward. The remaining options—the leaf stalk and the outer protective layer—do not serve as conduits for long-distance water transport.