Chapter 4: Inside Roots and Leaves

In roots, newly divided cells quickly differentiate into primary tissues: the epidermis, which produces root hairs to maximize absorption; the cortex, a storage area rich in starch grains with loosely packed cells that allow air and water movement; and the central vascular cylinder of primary xylem and phloem, surrounded by the endodermis, which regulates water and mineral entry. From just inside the endodermis, the pericycle gives rise to branch roots, pushing outward through cortex and epidermis, ensuring continuity with the vascular system. At the root–stem junction, tissues transition seamlessly, with pericycle and endodermis disappearing while pith and stem bundles emerge. Older perennial roots undergo secondary growth, producing wood, inner bark, and cork, though root wood remains irregular and less commercially valuable. Leaves are described as “anatomical sandwiches,” with upper and lower epidermis enclosing the mesophyll: palisade cells densely packed for light capture, and spongy cells loosely arranged for gas exchange. Veins of xylem and phloem thread through the blade, supplying water and exporting sugars. The epidermis is coated by a cuticle or protected by trichomes, adaptations that reduce water loss and deter insects. Gas exchange occurs through stomata, bordered by guard cells that dynamically open and close to balance photosynthesis with water conservation. Stomatal density varies by species and environment—from apple leaves with ~39,000 per cm² to corn and water lilies with specialized placement for aquatic or upright growth. Capon emphasizes how guard cells regulate openings, curbing dehydration during drought, heat, or night. The chapter also explores specialized leaf anatomy in succulents, where thick tissues hold water, and adaptations in high-light environments, where cuticle or hairs reflect damaging radiation. Capon concludes with plant cell types: parenchyma (thin-walled, versatile, used in tissue culture for cloning), fibers (thick-walled, strong, sources of linen, hemp, rope, and paper), stone cells or sclereids (the hard pits of fruits), and conducting cells—sieve tubes and companion cells in phloem, and vessels or tracheids in xylem. These cells showcase remarkable specialization: xylem cells are dead but structurally vital, while phloem sieve tubes lack nuclei and depend on companion cells for control. Together, these cellular systems reveal the extraordinary engineering of roots and leaves, balancing structure, transport, protection, and photosynthesis to sustain life.