Anatomy of Root, Stem and Leaf

Having covered the tissues and tissue systems individually, the chapter now looks at how those systems come together inside a real organ. The transverse-section anatomy of the dicot root, monocot root, dicot stem, monocot stem, dorsiventral (dicot) leaf and isobilateral (monocot) leaf are described in turn below, and comparing corresponding pairs — dicot vs monocot root, dicot vs monocot stem, dicot vs monocot leaf — is exactly where most of the ana …

A transverse section of a typical dicotyledonous root, read from the outside inward, shows the following layers. The outermost layer is a single row of cells called the epiblema, notable for having no cuticle at all; some of its cells are drawn out into long, unicellular root hairs that carry out absorption from the soil. Just inside the epiblema lies the cortex, several layers of ordinary parenchymatous cells whose main job is storing food and water. Once the epiblema dies off, the outermost layer of the cortex itself becomes cutinised and takes over the epiblema's protective role — this modified layer is called the exodermis. The innermost layer of the cortex is the endodermis, made of distinctively barrel-shaped cells whose radial walls carry a band of suberin called the Casparian strip (or Casparian band); this strip forces water moving through the endodermis to pass through the cell's cytoplasm rather than around it, except opposite the protoxylem points, where a few unthickened passage cells allow water to move more freely. Immediately inside the endodermis is a single layer of parenchymatous cells, the pericycle, which forms the outer boundary of the central stele (vascular cylinder). The stele itself is built from two to six separate radial vascular bundles, with the xylem arranged exarch (protoxylem toward the periphery, metaxylem toward the centre); depending on how many separate xylem/phloem groups are present, the stele is described as diarch through hexarch. Between the xylem and phloem groups of the stele lies a band of ordinary parenchyma, called the connective or conjunctive tissue. At the very centre of the stele is the …

The monocot root shares the same basic outer plan as the dicot root — epiblema with root hairs, a parenchymatous cortex, an endodermis with Casparian strips, and a pericycle bounding the stele. Two differences, however, set it apart. First, instead of the two-to-six xylem groups typical of a dicot root, the monocot root's stele contains more than six xylem groups, a condition called polyarch. Second, the pith at the centre of the stele is large and well developed, rather than the narrow pith of a dicot root. Because the monoc …

A transverse section of a typical dicot stem, illustrated using sunflower, shows the following organisation from outside in. The epidermis is a single outermost layer, multicellular in the sense that individual cells may grow out as hair-like trichomes, and it is usually coated on its outer surface with a layer of cuticle. Inside the epidermis lies the cortex, which in a dicot stem is differentiated into three distinct regions: an outer hypodermis, three to five layers of collenchymatous cells lying immediately beneath the epidermis with no intercellular spaces, providing mechanical support to the still-young stem; a general cortex, several layers of large parenchymatous cells with well-developed intercellular spaces; and an innermost endodermis of barrel-shaped cells, also called the starch sheath because of the starch grains it characteristically stores. …

The monocot stem differs from the dicot stem in several clear ways. The epidermis lacks trichomes altogether, and immediately beneath it the hypodermis is sclerenchymatous rather than collenchymatous, giving the young stem rigidity rather than flexibility right from the surface. There is no differentiation of the ground tissue into a distinct cortex, endodermis and pith as in the dicot stem — instead, the vascular bundles are numerous and lie scattered throughout an undifferentiated ground tissue of parenchyma. Each bundle is individually wrapped in its own sclerenchymatous bundle sheath. The bundles themselves are conjoint, collateral and closed — there is no cambium strip between the xylem and phloem — so a monocot stem, however many bundles it contains, can never undergo se …

The most common leaf type in dicotyledonous plants is the dorsiventral leaf, in which the mesophyll — the internal photosynthetic tissue — is clearly differentiated into two distinct layers, palisade and spongy parenchyma. Dorsiventral leaves are typically held more or less horizontally, so their two surfaces experience very different amounts of light: the upper surface, which faces the sun directly, is visibly darker than the shaded lower surface, and the internal structure of the leaf is built asymmetrically to match.

In a transverse section, the upper epidermis is a single layer of tightly packed, rectangular, barrel-shaped, parenchyma-like cells; these cells lack chloroplasts of their own and are coated on the outside by a distinct layer of cuticle, and stomata are generally absent (or very sparse) here, since this surface is not where the leaf wants to lose water. Immediately below the upper epidermis lies the mesophyll, the chloroplast-rich photosynthetic tissue sandwiched between the two epidermal layers, and it is here that the palisade/spongy differentiation appears. The palisade parenchyma sits directly beneath the upper epidermis and is made of closely packed, elongated cells standing upright, rich in chloroplasts and specialised for capturing the strong light falling on the upper surface — this is where most of the leaf's photosynthesis happens. Below the palisade layer is the spongy parenchyma, made of loosely arranged, irregularly shaped cells with substantial intercellular air spaces between them; these cells still contain chloroplasts, but their main job is facilitating gas exchange, since the air spaces connect directly to the atmosphere through the stomata on the lower surface. …

Where a leaf's two surfaces are illuminated more or less equally — because the leaf hangs vertically, or is oriented edge-on to the sun, as is common in grasses and many other monocots — the leaf develops as an isobilateral leaf rather than a dorsiventral one. Both surfaces of an isobilateral leaf are equally exposed to sunlight, both are equally green, and their internal structure is essentially symmetrical top to bottom.

In its overall plan a typical monocot leaf resembles the dicot leaf just described, but with three notable differences. First, it is amphistomatic: stomata occur on both the upper and the lower epidermis, rather than being concentrated on the lower surface alone. Second, its mesophyll is not differentiated into palisade and spongy layers — instead it is a single, more or less uniform mass of chlorenchyma cells filling the space between the two epidermal layers. Third, its veins run parallel to one another rather than in the branching, reticulate pattern typical of dicot leaves, and the vascular bundles running through the mesophyll are, as in the dicot leaf, conjoint, collateral and closed. …