Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Organs of Plants
3.4. Anatomical Structure of the Root

From the early Selection/3.html">Stages of development, the ROOT clearly differentiates into a root cap and four distinct zones that vary in anatomical Structure and function: 1) Cell Division; 2) elongation; 3) root hairs (absorption); and 4) branching.

The division zone is represented by meristematic tissue (the apical meristem or growing point) covered by the root cap. The root cap is a specialized structure unique to roots. It covers the apex, protecting it from mechanical damage, and facilitates root penetration into the soil through the shedding of outer-layer Cells. Lost cells are replaced by new ones produced by a specialized meristematic tissue within the cap known as the calyptrogen.

Located just beneath the root cap is the division zone, apex, or growing point. A key feature of this zone is continuous cell division, which drives the increase in root mass. The division zone is typically 3—4 mm long. Within this zone, three primary meristematic cell layers differentiate: the dermatogen, periblem, and plerome. The dermatogen is the outermost cell layer that, through histogenesis, forms the epiblema with root hairs. Deeper within lie 4–6 layers of periblem cells, which give rise to the primary cortex. The central core of the zone is the plerome, which forms the central vascular cylinder.

In the elongation zone, cells stretch longitudinally and acquire their final size and shape, driving the growth of the root. This zone is several millimeters long. The root Hair zone, or absorption zone, is characterized by cellular specialization. Here, root hairs, vessels, sieve tubes, and fundamental Tissues develop; hence, this region is also referred to as the zone of cellular specialization.

In the branching zone (maturation zone), lateral roots emerge, and other developmental changes take place. This represents the oldest region of the root.

Primary root structure. In the root hair zone, the elements of the Primary Structure—characteristic of all plants—reach their final formation. In most monocots, this structure persists throughout the plant's life, whereas in gymnosperms and dicots, it is replaced by secondary structures within 7–10 days of germination.

The primary structure comprises three genetically and functionally distinct regions: the epiblema, the primary cortex, and the central cylinder.

The epiblema consists of a single layer of living cells, the majority of which produce outward projections known as root hairs. A root hair ranges from 0.15 to 8 mm in length; its tip contains the Cytoplasm and nucleus, a large central vacuole occupies the main body of The Cell, while the remaining cytoplasm forms a thin parietal layer. A square millimeter (1 mm²) of the root hosts between 350 and 450 hairs, resulting in a vast absorptive surface area. Root hairs grow rapidly, within 30–40 hours, and have a short lifespan; at a distance of 3—8 mm from the root apex, they wither along with the entire epiblema, while new hairs continuously form in the younger region closer to the tip.

The primary cortex consists of the exodermis, mesoderm (cortex parenchyma), and endodermis. The exodermis borders the epiblema and is composed of tightly packed cells. In most plants, exodermal cell walls become suberized, allowing the exodermis to take over a protective function after the epiblema dies off. The mesoderm accounts for the largest volume of the root. It is made up of living parenchymal cells that store nutrient reserves. The innermost layer of the cortex, adjacent to the central cylinder, is the endodermis. These cells are living in young tissues, but their walls eventually become impregnated with Lignin and suberin, leading to the death of the protoplast. Endodermal Cell wall thickening begins with the Casparian strip, a localized thickening of the radial primary walls. This thickening gradually spreads to the inner tangential walls, and by the end of endodermal maturation, both the inner radial and tangential walls appear thickened. Only the walls facing the mesoderm remain unthickened. Because these cell wall thickenings make this cortical layer impermeable to the soil solution entering the central cylinder, certain endodermal cells remain alive with thin walls; these are known as passage cells.

The central cylinder (stele) of the root begins with a single, thin layer of meristematic cells called the pericycle, which gives rise to lateral roots and participates in the Formation of secondary root structures. Thick-walled parenchyma occupies the center of the vascular cylinder, surrounded by a radial vascular bundle in which primary xylem strands alternate with patches of primary phloem. The xylem consists of protoxylem and metaxylem oriented from the periphery toward the center. Protoxylem strands are positioned directly opposite the passage cells.

While this basic structure is common to all plants, certain variations exist regarding the number of pericycle layers. For instance, in the mulberry and some legumes, the pericycle is multiseriate (multi-layered), whereas in arborescent monocots—such as palms or tropical epiphytes—a velamen forms on the root surface as a multi-layered absorptive and protective tissue.

Secondary root structure. Particularly important structural changes occur in dicots and gymnosperms. In these plants, a secondary meristem—the interfascicular and intrafascicular cambium—forms from the phloem parenchyma and pericycle, driving Secondary Growth. These changes originate in the central cylinder. The cambium begins to produce secondary xylem and phloem elements, which in herbaceous plants form collateral vascular bundles. As secondary xylem expands, it pushes the primary xylem deep into the core of the root. Meanwhile, secondary phloem pushes the primary phloem toward the periphery, where it eventually becomes obliterated. Concurrently, sections of the pericycle located between the xylem strands give rise to the phellogen (cork cambium), which forms the periderm. Conversely, the portions of the pericycle opposite the primary xylem strands form interfascicular cambium, which produces ray parenchyma rather than conducting tissues. The expansion of the vascular cylinder and The formation of the periderm lead to the shedding of the primary cortex in a process known as "root shedding" or "molting." Following the sloughing of the primary cortex, the secondary cortex develops, establishing the Secondary structure. The secondary xylem constitutes the bulk of the mature root, featuring large vessels, relatively few thickened tracheids and libriform fibers, and well-developed thin-walled xylem parenchyma. The phloem region consists of sieve tubes with companion cells, cambiform cells, and phloem parenchyma.

In woody plants, during the Cytology/cytology/16.html">Early stages of transition from primary to secondary structure, the interfascicular and intrafascicular cambia fuse to form a continuous cambial ring. Functioning throughout the growing season, this cambial ring produces a ring of secondary phloem toward the periphery and a ring of secondary xylem toward the center. At the end of the growing season, cambial activity pauses, resuming the following year. Typically, much more xylem is deposited than phloem, meaning secondary xylem makes up the vast majority of a woody root's mass. This wood displays annual growth rings, the formation of which is driven by seasonal dynamics: in spring, the cambium produces wide-vessel xylem with fewer fibers, whereas toward the end of the growing season, it produces narrow-vessel xylem dominated by mechanical support tissues, giving latewood a darker appearance in cross-section.

Root crops possess a specialized secondary structure adapted for storing nutrient reserves, which results in significant thickening of specific root regions. Three MAIN TYPES OF root crop anatomy are recognized: the radish type, the carrot type, and the beet type.

Radish roots are characterized by nutrient accumulation primarily within the xylem, while the secondary phloem remains poorly developed. In addition to radishes, horseradish and turnip share this structural type. In carrot roots, nutrients are stored predominantly in the phloem parenchyma, with a narrower xylem core located centrally. Parsley, celery, and parsnip roots exhibit this same structural pattern.

Beet root crops thicken through a different mechanism. The Introduction/19.html">Primary structure of a beet root features a diarch vascular bundle and is short-lived. A primary cambial ring develops from the parenchyma of the primary phloem, producing a thin layer of phloem outwardly and a layer of secondary xylem inwardly. A phellogen arises from the pericycle to form the periderm, causing the primary cortex to slough off. This initial secondary growth does not lead to substantial root thickening. Instead, the major increase in girth is driven by the formation of 8–12 successive accessory (additional) cambial rings. These extra cambial rings are of phloem-pericycle origin and form external to the main cambium. They produce small vascular bundles with a well-developed xylem component alongside robust radial parenchyma rays where nutrient reserves are stored. The proliferation of these accessory cambial rings is triggered by leaf activity, with The Development of every two leaves stimulating the formation of one additional cambial ring.