Question

In a vertical section of a dorsiventral leaf, the protoxylem in its middle bundle
A. Faces the dorsal epidermis of the leaf.
B. Faces the ventral epidermis of the leaf.
C. Is not distinct.
D. Is surrounded by metaxylem.

Answer 1

Hint: A dorsiventral leaf is one that has two surfaces differing from each other in appearance and structure. Protoxylem is the first part of the primary xylem that matures.

Complete answer: Dicot leaves are dorsiventral. The upper surface of the leaf is dark green whereas the lower surface is light green. Lower epidermis contains numerous stomata without chloroplast; while the upper epidermis contains high amounts of chloroplast and no stomata, thus the leaf is hypostomatic. Numerous vascular bundles are scattered in spongy parenchyma. Each bundle is conjoint, collateral, and closed. The vascular bundle is surrounded by a large parenchymatous bundle sheath or border parenchyma. Collenchyma may also be associated with bundle sheath cells. Xylem consists of protoxylem that lies toward the upper epidermis and phloem toward the lower epidermis.
Xylem consists of vessels, tracheids, wood fibers, and wood parenchyma (lies toward the upper epidermis). The xylem tracheids and vessels help in the conduction of water and minerals and mechanical supports while parenchyma helps in lateral transport. At maturity, the protoxylem lies towards the upper epidermis(dorsal) and the metaxylem toward the lower (ventral) epidermis.
So, the answer is A-Faces the dorsal epidermis of the leaf.
Additional information:
Stomatal guard cells are kidney-shaped. Leaf veins are reticulated. The mesophyll is distinguishable into palisade and spongy tissues with palisade usually restricted to the upper side. Bundle sheath cells usually lack chloroplast and are parenchymatous. The inner and the radial walls of the epidermal cells are thin. The outer walls are cutinized. A distinct layer of cuticle lies on the outside of the epidermis. The cuticle prevents excessive transpiration, helps bind epidermal cells, and protects them from mechanical injury. Hair may occur here and there. They are also covered over by a layer of cuticle. Mesophytic leaves may have stomata in the upper epidermis. Phloem consists of sieve tubes, companion cells, and some cells of phloem parenchyma (lies towards lower epidermis). The phloem companion cells and sieve tubes help in the conduction of food materials and phloem parenchyma helps in lateral conduction and storage of food materials.

Note: A leaf consists of three parts— leaf base, petiole and lamina. The protoxylem has more parenchyma cells than the metaxylem. The mesophyll is distinguishable into palisade and spongy tissues with palisade usually restricted to the upper side. Bundle sheath cells usually lack chloroplast and are parenchymatous.