School of Sciences, Lautoka Campus BIO509 Botany Lecture 28: Plant cells and tissues

Three major parts of a plant – roots, stems and leaves. Are composed of tissues – groups of cells performing a similar function. Tissues can be classified according to its structure, origin or function.

Learning outcomes Know the meristems present in plants and where they occur. Know the different types of tissues and their functions.

Meristematic tissues Meristematic tissues comprises regions of permanent growth and is called meristems. Not found in animals, only in plants Two types of meristem - Apical meristem found at tips of roots and stems - Lateral meristem that increase girth of roots and stems

Apical meristems Apical meristem comprises the meristematic tissues found at the tips of roots and stems. Regions of actively growing cells. The tissue increases in length as the cells divide.

Apical Meristems Produces three kinds of primary meristem in primary growth. - Protoderm, ground meristem, and procambium These three meristems develop into primary tissues - Epidermis, ground tissue, and the cambium

Lateral meristems Lateral meristem increases the girth (secondary growth). - Vascular cambium - Cork cambium Produces secondary tissues that function in support and conduction (secondary growth). When cell divides, the girth of the stem increases.

Vascular cambium Cork cambium Produces secondary xylem and phloem. Cork cambium is also a cylinder the length of the stem and root. Lies outside vascular cambium and produces bark, a secondary tissue.

Tissues produced by meristems

Simple tissues Simple tissues comprise a single cell type, make up the ground tissues. Three types of ground tissues are parenchyma, collenchyma, and sclerenchyma. Most abundant type of cell in plant (e.g. cortex, pith). More or less spherical in shape. Have large vacuoles often used for storage of water & food. Ground tissues are found in all major parts of higher plants.

Parenchyma Tissue composed of parenchyma cells, are most abundant of the cell types and are found in all major parts of higher plant. Normally spherical shaped when produced. Later they assume various shaped as the cells push up against each other and cells end with an average of 14 sides.

Several types of parenchyma Aerenchyma of aquatic plants have small spaces between them for aeration and/ or floatation. Chlorenchyma have numerous chloroplasts and function mainly in photosynthesis. May live a long time and can divide on their own and produce things like scar tissue upon injury to plant

Collenchyma Simple ground tissue typically underneath epidermis. Thicker walls, pliable and strong, to provide support (e.g. celery strings) for organs. Long-lived and contain cytoplasm

Sclerenchyma Third type of simple ground tissue. Tissue consists of cells with thick walls generally impregnated with lignin. Function in support for the plant. Two types include sclerids and fibers. The hardness of nut shells and other stone fruits is due to sclerids.

Complex tissues Complex tissues are made up of two or more kinds of cells. Two most complex tissues in plants are xylem and phloem that function in transport of water and soluble food (sugars). The complex tissues in woody plants are produced by the vascular cambium and are referred to as vascular tissues. Other complex tissues include the epidermis and periderm.

Tracheids Vessel members Xylem Xylem consists of a combination of parenchyma cells, fibres, vessels, traceids and ray cells. Vessels are long tubes made up of individual cells called vessel elements that are open at each end, often with bar-like plates extending across the opening. Traceids are like vessel elements that have thick secondary walls and are tapered at each end. Traceids have no openings at ends but pairs of pits where they contact another and serve in water transport. Traceids vessel elements

Phloem Conducts dissolved food materials produced by photosynthesis. Composed of two types of cells (sieve tube members and companion cells) without secondary thickening. Sieve tube members are long and tubular and form sieve tubes when laid end to end, unlike vessel elements, they have end plates (sieve plates) with small pores.

Epidermis Epidermis is the outer most layer of all young plant organs. Epidermis tissue is made mostly of parenchyma or similar cells, but other specialized ones as well. Mostly one cell layer thick. Most cells produce a fatty substance called cutin on both sides of outer wall that forms the cuticle on the outside.

Epidermis may include guard cells that open and close and border the pores called stomata. Stomata are opening in the epidermal surface that allow for the exchange of gases. Usually found on the lower leaf surface

Periderm Periderm replaces epidermis after secondary growth. Constitutes the outer bark and made up of rectangular and boxlike cork cells. Fatty substance, suberin, secreted into walls as waterproofing and protection from drying and freezing. It also has a group of parenchyma cells loosely arranged that make up the lenticels that function in gas exchange.

Secretory cells and tissues Occur in various places in plants. Use to secret waste products or substances harmful to cytoplasm, often into cavities (e.g., citrus oils). Often derived from parenchyma cells. Secretory substances include flower nectar, citrus oils, mint aromatics, sundew mucilage, latex, resins.

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