PLANT ANATOMY - Yu.I. Korniievskyi - 2017

Lecture Notes

Lecture No. 3. MORPHOLOGY AND ANATOMY OF VEGETATIVE ORGANS. THE ROOT

Lecture Outline

1. ROOT Morphology.

2. Root metamorphosis.

3. Water/140.html">Anatomical Structure of root crops.

4. Additional Functions of the root.

5. Mycorrhiza and bacteriorhiza.

6. Anatomical STRUCTURE OF THE root.

Vegetative Organs—the root and the SHOOT—provide Nutrition, individual life, and vegetative propagation of plants. The morphology of these organs and their anatomical structure, namely the specific arrangement of Tissues, correspond to the physical laws and physiological needs of both the organs and the entire plant Organism. A distinction is made between axial vegetative organs—the root and the stem—and the lateral vegetative organ, the leaf.

The root (radix) is mostly an orthotropic, radially symmetrical axial organ capable of branching and prolonged apical growth. As an underground organ, the root evolved later than the shoot in lycophytes. It is characteristic of all modern vascular plants except for mosses, parasitic plants (such as toothwort and dodder), hemiparasites (such as mistletoe), and certain aquatic vascular plants (such as bladderwort and salvinia). The root differs from the shoot in lacking leaves, nodes, and internodes; the root apex is protected by a root cap, and intercalary growth is absent. Roots perform the following functions:

· absorb and transport water and mineral nutrients from the soil;

· anchor plants in the substrate;

· store nutrients;

· synthesize certain Organic compounds (Amino Acids, growth Hormones, Alkaloids);

· secrete mucus, organic acids, amino acids, and carbon dioxide into the environment, which promote microflora development and the assimilation of sparingly soluble compounds;

· ensure vegetative regeneration when adventitious buds are present;

· connect the plant with other soil organisms;

· participate in soil formation.

1. Root morphology

Depending on their origin, the following types of roots are distinguished: primary (taproot), lateral, and adventitious.

The primary (taproot) develops from the embryonic root during seed germination and grows downward (positive geotropism). Lateral roots of the first order develop from the pericycle on the taproot, followed by lateral roots of the second order, and so on. Adventitious roots can form on stems, leaves, and their modifications from the pericycle, parenchyma, or cambium; these roots also exhibit positive geotropism. Occasionally, adventitious roots appear on older roots.

In shape, roots are most frequently cylindrical, long, thick (cord-like) or thin (thread-like), and less commonly conical, fusiform, tuberous, or of other shapes.

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2. Root Metamorphoses - Root Tubers

3. Anatomical Structure of Root Tubers

Root tubers develop from the primary root, with the hypocotyl also participating in their formation. A root tuber typically comprises: the HEAD, a shortened stem portion bearing leaves; the neck, the thickest part formed through the expansion of the hypocotyl; and the true root proper, from which lateral roots originate. The length of the neck can vary significantly across different plant species.

Root tubers are classified as monocambial (possessing a single cambium layer) and polycambial (possessing multiple cambium layers). In some monocambial root tubers, the secondary cortex occupies the major portion of the organ and serves as the site for reserve product accumulation (carrot type); in others, the secondary cortex is reduced, and the xylem occupies most of the volume, storing reserve substances (radish type). The polycambial root tuber is characteristic of beets. In this structural type, a diarch primary xylem occupies the center, flanked by two secondary xylem regions separated by radial parenchyma zones. A cambium ring surrounds the xylem, with secondary phloem regions adjoining it externally. Thus, the Secondary structure resembles that of other root tubers. However, secondary modifications are followed by tertiary changes. Around the secondary phloem at the root periphery, divisions of pericycle and phloem Cells give rise to a layer of parenchymal cells. Within this layer, a single row of cells divides via tangential walls, transforming into a new cambium layer that deposits xylem inward and phloem outward in the form of collateral bundles separated by thin-walled parenchyma layers. Simultaneously, a new ring of cambial cells forms within the peripheral parenchyma layer.

Fig. 1. Root tubers

A - Petroselinum sativum Hoffn. (carrot type); B - Raphanus sativus L. (radish type); C - Beta vulgaris (beet type): 1 - periderm, 2 - cortical storage parenchyma, 3 - secondary phloem, 4 - schizogenous essential oil canal, 5 - cambium, 6 - secondary xylem, 7 - accessory cambial rings, 8 - open collateral bundles, 9 - medullary ray storage parenchyma, 10 - primary xylem.

Root tubers, or root tubercles, represent thickened lateral or adventitious roots (such as in dahlia, lesser celandine, and butterfly orchid). Similar to root tubers formed from taproots, they feature well-developed storage parenchyma, lack mechanical elements, and possess very small vascular bundles.

4. Additional Root Functions

5. Mycorrhiza and Bacteriorhiza

Mycorrhiza (fungus-root). Mycorrhizae can be: ectotrophic, when fungal hyphae are located exclusively on the outside of the root; ecto-endotrophic, when hyphae partially penetrate the root cells; and endotrophic, when hyphae reside entirely within the root cells. Ectotrophic mycorrhizae are more common in trees and shrubs, whereas endotrophic forms typically occur in herbaceous plants. The fungus colonizing the plant root obtains organic nutrients from the host's tissues while supplying water and dissolved mineral salts from the soil in return. Enzymes present in the fungal cells mineralize soil organic matter, thereby facilitating its uptake by the plant.

1 - ecto-endotrophic mycorrhiza; 2-4 - bacteriorhiza of certain legumes: 2 - clover, 3 - lupine, 4 - soybean.

Bacteriorhiza represents a Symbiosis between plant roots and Bacteria that fix atmospheric nitrogen and convert it into soluble compounds, thereby enriching the soil. A close interaction exists between the bacteria and root cells, resulting in the synthesis of organic substances (such as Vitamins and enzymes) by the higher plant, while the bacteria utilize organic compounds produced by the root. Aggregations of nitrogen-fixing bacteria on the roots of higher plants (including legumes, rosids, ginkgo, birches, brassicas, buckthorns, oleaster family members, and grasses) form specialized bacterial nodules. A nodule consists of a bacterial infection pocket connected via Vascular Tissues to the root's phloem and xylem.

6. Anatomical Structure of the Root

Based on their Structure and Functional capabilities, four distinct histological zones are recognized in the root: Cell Division (with the root cap); elongation; absorption; and conduction and strengthening.

Roots with a Primary Structure are characterized by the presence of three tissue systems: the dermal system, the primary cortex, and the central cylinder.

The Dermal Tissues comprise the epiblema (rhizodermis), the epidermis (in the maturation zone of monocots), and the velamen (in aerial roots). The epiblema performs absorptive, secretory, and protective-covering functions; it lacks Stomata, a thick cuticle, and trichomes. The primary cortex lies beneath the epiblema and is subdivided into the exodermis, mesodermis, and endodermis. The exodermis performs protective, supportive, and permeability functions, consisting of 3–4 layers of large, tightly packed cells. The mesodermis is a multi-layered storage region of the primary cortex that performs transport and sometimes aerenchyma functions; its cells are living, large, rounded, loosely arranged, with thin or thickened walls, and filled with starch grains. The endodermis is the innermost layer of the primary cortex, serving supportive and regulatory functions. In monocots, the radial and inner tangential cell walls undergo thickening, suberization, and lignification into a horseshoe shape; interspersed among these dead endodermal cells opposite the xylem rays are living passage cells through which water and minerals enter the root. Dicotyledonous plants are characterized by lenticular thickenings known as Casparian spots or strips.

The central cylinder, or stele, occupies the axial part of the organ and consists of the xylem, phloem, and a peripheral pericycle ring from which lateral roots, haustoria, phellogen, cambium, laticifers, and other structures originate. Phloem and xylem strands alternate radially to form a radial vascular bundle. Depending on the number of xylem groups, the bundle may be di-, tri-, tetra-, penta-, hexa-, or multi-rayed (polyarch). In dicots, there are typically no more than six xylem rays, whereas monocots possess a greater number (polyarch bundle).

Fig. 2. Root with primary structure (absorption zone)

A. Monocot plant *Iris germanica* L. B. Dicot plant *Ranunculus acris* L.: 1 - epiblem, 2 - exodermis, 3 - mesodermis, 4 — endodermis (a - cells with U-thickened walls, b - passage cell, c - with Casparian strips), 5 - pericycle, 6 - radial bundle (d - phloem, e - xylem, f - sclerenchyma)

Fig. 3. Root with Introduction/11.html">Secondary structure of a herbaceous dicot plant of the fascicular type (conduction zone) – *Cucurbita pepo* L.

1 - periderm, 2 - cortical parenchyma, 3 - open collateral bundle (a - secondary phloem, b - cambium, c - secondary xylem), 4 - medullary ray, 5 - primary xylem.

Secondary root structure

It is formed in the anchorage zone due to The activity of secondary lateral Meristems – the cambium in the central cylinder and the phellogen in the primary cortex. The secondary structure of the root can be of either fascicular or non-fascicular type. Secondary transformations in the primary cortex of the root are associated with the activity of the phellogen. The protective tissue is the periderm or rhytidome. The presence of a radial vascular bundle, sclerenchyma, or primary xylem vessels in the central part of the axial organ distinguishes the root from the stem and rhizome, which possess a true pith in the center.

Structure of a dicotyledonous root:

A - general view; B - longitudinal section diagram; C-F - stages of primary growth and transition to secondary thickening; G - secondary non-fascicular structure; 1 - cell division zone protected by the root cap; 2 - elongation and differentiation zone; 3 - absorption zone; 4 - conduction and anchorage zone with lateral roots; 5 - apical meristem; 6 - procambium; 7 - ground meristem; 8 - protoderm; 9 - phloem elements; 10 - xylem elements; 11 - epiblem with root hairs; 12 - pericycle; 13 - primary cortex; 14 - primary xylem; 15 - primary phloem; 16 - cambium; 17 - secondary xylem; 18 - secondary phloem; 19 - medullary ray; 20 - periderm.