Biology · Ch 6 — Anatomy of Flowering Plants
Anatomy of the Dorsiventral (Dicot) and Isobilateral (Monocot) Leaf
Anatomy of the Dorsiventral (Dicot) and Isobilateral (Monocot) Leaf
A leaf is built, from top to bottom, of an upper epidermis, a middle layer of photosynthetic ground tissue called the mesophyll (through which the vascular bundles, or veins, run), and a lower epidermis -- but exactly how the mesophyll is organised, and how stomata are distributed across the two epidermal surfaces, differs sharply between a typical dicot leaf and a typical monocot leaf, and this difference has a clear functional explanation.
The dorsiventral (dicot) leaf. Most dicot leaves are held more or less horizontally, so their upper surface faces the sun directly while their lower surface faces mostly shade -- and their internal structure is built asymmetrically to match. Both the upper and lower epidermis are single-layered and cuticle-covered, but stomata are concentrated mainly on the lower epidermis (such a leaf is called hypostomatic), which helps limit water loss from the sun-facing upper surface. The mesophyll beneath the upper epidermis is differentiated into two distinct layers: immediately below the upper epidermis lies the palisade parenchyma, made of tall, columnar cells packed closely together with their long axes vertical (perpendicular to the leaf surface) and carrying a dense concentration of chloroplasts -- an arrangement that captures the strong, directly incident light on the upper surface very efficiently. Below the palisade layer lies the spongy parenchyma, made of loosely arranged, irregularly shaped cells with large intercellular air spaces between them, carrying comparatively fewer chloroplasts; this loose arrangement instead favours rapid gas exchange (carbon dioxide reaching every photosynthesising cell, oxygen and water vapour leaving) rather than light capture, feeding the stomata concentrated on the lower surface just beneath it. Running through the mesophyll are the leaf's veins -- vascular bundles, generally conjoint, collateral and (in the smaller veins) closed, each enclosed by a parenchymatous bundle sheath, with xylem oriented toward the upper (adaxial) side of the bundle and phloem toward the lower (abaxial) side, matching water's upward-then-outward path from the stem and food's path back the other way. …
What this figure shows. Two transverse-section leaf diagrams side by side. The dorsiventral dicot leaf (left) shows an upper and a lower epidermis, both cuticle-covered, with stomata drawn chiefly in the lower epidermis; immediately below the upper epidermis a compact layer of tall, columnar, chloroplast-dense palisade parenchyma cells arranged with their long axes vertical; below that a loosely arranged layer of irregularly shaped spongy parenchyma cells with large visible air spaces between them; and, running through the mesophyll, a vascular bundle (a vein) shown in section with a parenchymatous bundle sheath around it, xylem toward the upper (adaxial) side and phloem toward the lower (abaxial) side. The isobilateral monocot leaf (right) shows stomata in roughly equal numbers on both the upper and lower epidermis; a single uniform layer of loosely packed, isodiametric chlorenchyma cells filling the whole mesophyll with no separation into palisade and spongy layers; a vein in section similar to the dicot leaf's; and, in the upper epidermis directly above one of the larger veins, a group of unusually large, thin-wa …