- - AGRICULTURE CORE CURRICULUM - - (CLF6000) Advanced Core Cluster: ORNAMENTAL HORTICULTURE (CLF6150) Unit Title: PHOTOSYNTHESIS AND RESPIRATION ____________________________________________________________________________ (CLF6154) Topic: CELLULAR FUNCTION IN PLANTS Time Year(s) 1 hour 3 / 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson, the student will be able to: Learning Outcome #: (B-4) - Describe the functions of various plant cells (e.g., leaf cell, root cell, cambium cell). Special Materials and Equipment: None References: Cooper, Elmer L. (1990). AGRISCIENCE: FUNDAMENTALS AND APPLICATIONS. Albany, NY: Delmar Publishers. Hartmann, H. T., Flocker, W. J., & Kofranek, A. M. (1981). PLANT SCIENCE: GROWTH, DEVELOPMENT, AND UTILIZATION OF CULTIVATED PLANTS. Englewood Cliffs, NJ: Prentice-Hall. Evaluation: Unit Exam TOPIC PRESENTATION: CELLULAR FUNCTION IN PLANTS A. The Function of Cells: 1. How Plant Cells are Organized: a. Plant cells grouped together performing a general function are referred to as tissues. Examples of tissues are the palisade chlorenchyma mentioned in the lesson on photosynthesis (CLF6151) and xylem which transports water and nutrients through a plant. b. Various tissues, in turn, make up specific, more complex, organs such as stems, roots, and leaves. Organs differ in the organization of the tissues, and the details of their structure. c. These can combine to constitute systems such as the plant's vascular system or reproductive system. 2. Plant Tissues and Cell Function: To best understand the function of plant cells, we will consider the role of different tissues. Plant organs are too complex and far removed from individual cellular function, and the individual types of cells found within a plant, and their respective actions, are too numerous to detail here. But by looking at the different types of plant tissues, we can see how plant cells work together to perform important functions for plant growth and development. There are two main types of tissue, meristematic and permanent. B. Meristem: Meristematic tissue is the most fundamental tissue. It gives rise to all other tissues in the plant. Meristems produce the cells and developmental signals (hormones) that lead to tissues and organs. 1. Meristematic cells are typically very small and thin-walled. 2. Meristems are regions of the plant where cells divide repeatedly. a. Between cell divisions, the cells of the meristem grow enough to regain their original size. b. Zones of growth and differentiation occur beside each meristem. Here, cells expand with little or no division. 3. Cell differentiation occurs after cells are produced by the meristem. a. In differentiation, cells acquire special traits that equip them for particular tasks in the mature plant organ. b. To choose their path of differentiation, cells depend on information from their own genes as well as influences from the environment. 1) When cells divide, each cell gets a full set of genes, with all the information needed to build a whole plant. 2) Each cell reads and uses only a fraction of the information stored in the genes. 3) Signals that choose which genes to read can include chemicals produced by the cell itself (hormones), as well as influences from neighboring cells, and environmental factors including temperature, light, and gravity. 4. Meristems are usually classified by their locations in the plant. a. Apical meristems occur at the tips (apices; singular "apex") of roots and stems, and generate all the new branching growth of the plant. The apical meristem located at the axillary buds will often remain dormant until stimulated by hormonal signal to start producing new cells. b. Lateral meristems occur along the length of roots and many stems and young leaves, often as a cylindrical layer near the surface. 1) In roots, a lateral meristem called the pericycle initiates new branch roots. 2) Woody plants have lateral meristems called cambiums that add to the girth of the organ, making wood and bark, and generate the cells that become vascular tissue (xylem and phloem). c. Other meristematic tissues include sub-apical meristem and intercalary meristem. C. Permanent Tissues: These are mature tissues in which the cells have differentiated completely, having developed from some kind of meristem. We can divide permanent tissues into roughly three types: vascular tissue, epidermal tissue, and ground tissues. 1. Vascular Tissues: Cells in the vascular tissue are specialized to stiffen the plant, and carry water, nutrients, and photosynthates between organs. a. There are two kinds of vascular tissue, xylem and phloem. 1) Xylem is vascular tissue that carries water and minerals through the plant. As xylem cells differentiate, they become elongated and their side walls thicken and harden with lignin. Finally they lose their end walls and die, so the result is like a dead, wooden pipe. These conducting chains of dead cells are called vessels. 2) Phloem is vascular tissue that carries foods (as a solution in water) through the plant. Its conducting units are chains of living cells called sieve tubes. These cells lose their nucleus as they mature. b. The two kinds of vascular tissue occur side by side, making up units called vascular bundles in young stems, and a vascular cylinder in roots and woody stems. c. In addition to conducting cells, vascular tissue contains fiber cells that add strength to the tissue. 2. Epidermal Cells: Epidermal tissue consists of one or two layers of cells that form a protective covering over the whole plant. Epidermal cells occur at the surface of organs and appear as platelike cells with thick, waxy outer walls that resist damage and dehydration. While most epidermal tissue simply protects the plant from exposure to the drying outside environment, certain epidermal cells have specialized functions. These include: a. Guard cells in leaves, which control the stomatal openings. b. Root hairs, which are adapted to absorb water into the roots. 3. Ground tissues include a collection of cell types which manufacture and store food, add bulk to organs, and provide packing or insulation for vascular tissue. a. Most cells of ground tissue are rounded and thin-walled, with large vacuoles (parenchyma cells). b. Some parenchyma cells specialize in food and water storage, but those near the surface of stems and leaves develop green chloroplasts. Ground tissue conducts most of the plant's photosynthesis. c. Sclerenchyma is made up of hard, thick-walled cells, used to build supportive tissue and hard protective structures such as bark and hard seed-shells. 12/13/90 MH/sg #%&C