Anatomy of Dicot Leaf - Sunflower

Leaves carry the same three tissue systems as roots and stems, just flattened into a blade: a dermal system of upper and lower epidermis, a ground system called mesophyll sandwiched between them, and a vascular system of veins running through the mesophyll. In a dorsiventral leaf such as sunflower, the mesophyll is asymmetric top-to-bottom: the upper (adaxial) side develops as compact, chloroplast-dense palisade parenchyma, oriented to intercept the strongest light, while the lower (abaxial) side develops as loosely-arranged spongy parenchyma, its large air spaces and an underlying respiratory cavity making gas exchange easy at the stomata that open mostly on this lower surface. Each vein running through this mesophyll is a small conjoint, collateral and closed vascular bundle: protoxylem and metaxylem sit toward the upper epidermis and phloem (sieve tubes, companion cells and phloem parenchyma - but never phloem fibres) sits toward the lower epidermis, the whole bundle wrapped in a compact bundle sheath (also called border parenchyma) that keeps the conducting tissue mechanically and physiologically distinct from the surrounding mesophyll; the xylem itself here is limited to vessels and xylem parenchyma, without tracheids or xylem fibres. Not every leaf follows this asymmetric plan: an isobilateral leaf such as Nerium instead develops palisade parenchyma on both s …