Plant Tissues
Types of Plant Tissues
Histology, the study of tissues, examines how groups of cells with similar structures and functions come together to form specialized tissues. In plants, tissues are categorized into simple tissues, vascular tissues, and dermal tissues.
Simple Plant Tissues
Simple tissues consist of a single type of cell that forms a uniform mass. The main types of simple plant tissues are parenchyma, collenchyma, and sclerenchyma.
Parenchyma
Parenchyma cells are versatile, living cells with roles in storage, support, photosynthesis, and phloem loading. Most of the leaf tissue, except for vascular bundles (xylem and phloem), is made up of parenchyma. In some cases, these cells, as in the epidermis, are adapted for functions such as light penetration or gas exchange. Other parenchyma cells remain undifferentiated and totipotent, capable of dividing and generating new cells. They have thin, permeable primary walls that facilitate the movement of small molecules, and their cytoplasm performs a wide range of biochemical functions such as nectar secretion or producing substances that deter herbivores. Parenchyma cells containing chloroplasts and primarily involved in photosynthesis are known as chlorenchyma cells. Parenchyma cells in structures such as potato tubers and legume cotyledons serve a storage function.
Collenchyma
Collenchyma cells are living cells at maturity, with only a primary wall and a secondary wall. These cells mature from meristem derivatives and are distinct from parenchyma as they develop a specialised cell wall. The thickest parts of the wall are usually at cell corners, where three or more cells meet. The walls of collenchyma cells in dicotyledon angiosperms are mainly composed of pectin and hemicellulose, with low cellulose content. These elongated cells provide support to growing plant parts, allowing for flexibility and tensile strength. Since their walls lack lignin, collenchyma cells provide a type of plastic support that allows them to stretch as surrounding cells grow. For example, the “strings” in celery are made up of collenchyma.
Sclerenchyma
Sclerenchyma cells are tough, supportive cells that develop thick secondary walls impregnated with lignin, making them hard and water-impermeable. They are typically dead at maturity, with their cytoplasm gone, leaving an empty central cavity. There are two main types of sclerenchyma cells: sclereids and fibres. Sclereids are hard cells that provide mechanical support and protection, often giving fruit or leaves a gritty texture. They also deter herbivory by damaging the digestive systems of small insects. Fibres, on the other hand, provide tensile strength and support in leaves and stems but are not involved in conduction of water, nutrients, or carbon compounds. Sclerenchyma fibres may have evolved as modifications of xylem and phloem cells in early land plants.
Aerenchyma
Aerenchyma consists of large air-filled cavities in the roots of some plants, allowing gas exchange between submerged tissues and the shoot above the water. This tissue forms in response to flooding, creating a low-resistance pathway for gases like oxygen and ethylene.
Vascular Plant Tissues
Vascular tissues, composed of multiple cell types, are crucial for transport and support in plants. They include xylem and phloem.
Xylem
Xylem tissue is responsible for transporting water and minerals from roots to leaves and providing mechanical support. It consists of four cell types:
Tracheids
Elongated and lignified, tracheids have tapering ends that overlap, providing mechanical strength and support. They are dead at maturity with empty lumens.
Vessel elements
These are long, tubular structures formed by the end-to-end fusion of cells. They are perforated and dead at maturity, serving as the main conducting units of the xylem.
Parenchyma
Xylem parenchyma cells are involved in storage and repair.
Fibres
These provide additional mechanical support and strength.
Phloem
Phloem is involved in transporting nutrients, particularly carbohydrates, throughout the plant. It includes five cell types:
Sieve tube cells
Elongated cells that form tubes with porous end walls, known as sieve plates. They are alive at functional maturity and facilitate the transport of food.
Companion cells
Adjacent to sieve tube cells, these living parenchyma cells assist in food conduction and contain elongated nuclei.
Parenchyma
Phloem parenchyma cells help in storage and repair.
Fibers and sclereids
Provide structural support.
Each type of plant tissue plays a specific role in the overall function and health of the plant, contributing to its growth, support, and nutrient transport.