Chapter 6: Anatomy of Flowering Plants

Phellogen, phellem, and phelloderm are collectively known as periderm. While plants undergo secondary growth, the outer epidermal layer and the cortical layer are ripped due to cambium. In order to replace them, the cortex cells turn meristematic, which produces the cork cambium or the phellogen, which comprises thin-walled, narrow and rectangular cells.

The phellogen sheds cells on either side. The cells which shed from the exterior give rise to the cork or phellem. The suberin accumulates in its cell wall, making it impermeable to water, while the inner cells emerge to become the secondary cortex or phelloderm, which is parenchymatous.

Dicots exhibit dorsiventral leaves. When examined, the vertical section of a dorsiventral leaf consists of three different parts, and they are

• Epidermis – It is found on the adaxial epidermis (upper surface) and the abaxial epidermis (lower surface). On the outside, the epidermis is covered with a thick cuticle. Compared to the upper surface, the abaxial epidermis comprises more stomata.
• Mesophyll – It is a tissue found in between the abaxial and adaxial epidermises. This tissue is differentiated into the palisade parenchyma and the spongy parenchyma. The palisade parenchyma is composed of tall, compactly-arranged cells, while the spongy parenchyma comprises round or oval, loosely-arranged cells possessing intercellular spaces. Mesophyll comprises of chloroplasts that carry out photosynthesis.

• Vascular system – The vascular bundles that are found in leaves are closed and conjoint, which are engirdled by thick layers of bundle-sheath cells.