Plant Root, Stem, and Leaf Structure: Tissues and Anatomy

Plant Root, Stem, and Leaf Structure

The root is the plant part responsible for water uptake, mineral salts absorption, and fixation of the plant to the ground. In general, it is an underground organ. The root is divided into zones: the zone of growth, formed by primary meristem, whose tip is located under the root cap or pilorhiza; and the zone of branching, where lateral roots emerge.

The stem is the part that bears buds, formed from primary meristems. It is the area of union between the leaves and the stem, and it is located in meristematic tissue, from which the internodes and axillary buds will develop.

Internal Structure of Primary Growth

Epidermis

The epidermis is the outermost layer.

Vascular Cylinder

The vascular cylinder is formed by the vascular system, which includes the xylem and phloem, along with the medulla. The vascular bundles are arranged like the spokes of a wheel, extending from the cortex toward the center of the cylinder without reaching it. Between each radius of xylem and phloem there are cambium-like tissues. The xylem vessels are the conducting vessels, located toward the center of the cylinder. In roots, there is no medulla, so the vessels are arranged differently than in stems.

Cortex

The cortex provides support and is formed by parenchymatous tissue. Its inner limit is marked by the endodermis, which surrounds the vascular cylinder.

Secondary Structure

In secondary growth, new layers of xylem form toward the interior and phloem toward the exterior. The cambium is the meristematic layer responsible for forming new vessels, xylem, and phloem. In spring, large xylem cells are formed; in autumn, smaller, thicker-walled cells are formed. These layers are observed as growth rings. The xylem is the inner part, and the phloem is part of the bark.

The Leaf

The leaf performs photosynthesis, regulates gas exchange, and controls transpiration. A typical leaf consists of a blade (limbo) and a petiole, with upper (adaxial) and lower (abaxial) faces. The blade is covered by the epidermis, with the upper and lower epidermis differing in the amount of stomata, which are more numerous on the lower side. Through the petiole, the xylem and phloem branches enter the blade and form the veins.

Leaf Structure

• Epidermis: the outermost layer, present on both the upper and lower surfaces. Both epidermal layers differ in stomatal density.
• Mesophyll: located beneath the epidermis, formed by palisade parenchyma and spongy (lacunar) parenchyma. The spongy parenchyma is located beneath the lower epidermis.

Animal Epithelial Tissues

Epithelial Tissue

Epithelial tissue is formed by cells that are closely packed and form continuous layers. Its main function is to distinguish and protect.

Covering Epithelial Tissue

It covers the external surface of the body and internal surfaces, with functions of protection, absorption, or secretion of substances.

Squamous Epithelium

Formed by flat cells.

Simple or Endothelium

Formed by a single layer of cells. It is found in the alveoli of the lungs, lining the inside of blood vessels, and forming capillaries.

Stratified or Tegumentary

Formed by several layers of cells, covering the outer surface of the body and forming internal organs. The outermost layer, called the epidermis, is formed by dead cells rich in keratin, a hardening substance. This layer is continuously renewed.

Prismatic Epithelium

Formed by elongated cells.

• Simple: formed by a single layer, lining the stomach and intestines. Cells lining the intestine have microvilli, which increase the surface area for absorption.
• Pseudostratified: appears to be formed by several layers because the nuclei are at different heights, but it is actually a single layer. It lines the airways, where cells have cilia whose movements help remove foreign particles that could enter the respiratory tract.