Plant Biology: Roots, Stems, and Leaves

• Plants are stationary organisms that require energy from food to live.
• Plants manufacture their own food from simple raw materials they absorb from their environment.
• Sunlight energy is used in chemical reactions to produce food in plants.
• Because plants (and algae) make their own food, they are called producers.

Vascular Plants

• The major organs of a vascular plant are the root, the stem, and the leaf.
• These organs play a part in the food-making process and other activities of the plant.

The Root

• A root is composed of the growing tip, root hairs, conducting tissue, and storage tissue.
• The growing tip contains growing tissue, the cells divide and add new cells to the root.
• The root increases in size via division and enlargement of cells.
• The very tip of the growing tissue is covered by a protective layer of cells called the root cap.
• Root hairs are small projections grow out from outermost cells of the root, between particles of soil.
• Each cell with a root hair has a large vacuole full of water and dissolved substances.
• Conducting tissue is located near the center of the root.
• There are two types of conducting-tissue cells: xylem and phloem.
• Xylem cells conduct liquids upward.
• Phloem cells conduct liquids downward.
• Mature xylem cells are hollow, because they contain no cytoplasm and water passes easily through them.
• Storage tissue is between the epidermis and the central conducting tissue.
• Storage tissue cells possess large vacuoles and granules in their cytoplasm.
• Granules are composed of carbohydrates and proteins.
• The functions of the root are absorption, anchorage, conduction, and storage.
• Root hairs absorb water and dissolved mineral salts from the soil.
• Water enters the root hairs by osmosis; minerals enter the root hairs mainly by diffusion.
• Being underground, roots anchor the base of the plant securely in the soil.
• Substances that have entered root hairs diffuse from cell to cell within the root tip until substances reach the xylem which conducts the substances upward.
• The phloem conducts food downward from the upper parts of the plant to the cells of the root tip.
• When plants make more food than they can use during a period of time, the excess food usually passes into the storage-tissue cells of the root.
• At times when the plant lacks food, the storage-tissue cells release the stored material for the plant's use.
• Roots store plant food, therefore they are a source of food for people and other animals (beet, carrot, sweet potato, and turnip).

The Stem

• A stem is composed of conducting tissue, growing tissue, and storage tissue.
• Conducting tissue consists mainly of cells of xylem and phloem, which are connected to the xylem and phloem cells of the root.
• Growing tissue consists of cells that divide and add new cells to the length and width of the stem.
• A stem's length is increased by growth at the terminal buds (special areas at the tips of the branches).
• Other cells that continually divide add thickness to the stem.
• Some plants, such as the rose bush, possess a growing layer and live for many years.
• Woody plants, like trees, usually show annual growth rings in their stems, or trunks.
• The storage tissue layer consists of cells in the center of the stem and cells that surround the conducting tissue.
• The functions of the stem are support, conduction, and storage.
• The stem supports the leaves and flowers of the plant, helping the leaves to get sunlight.
• Xylem and phloem cells make up the conducting tubes which transport materials between the roots and leaves.
• The stems of many plants serve as storage areas for food, water, and other substances.

The Leaf

• Five major regions of a leaf: upper epidermis, lower epidermis, veins, palisade layer, and spongy layer
• The upper epidermis of the leaf consists of a single layer of cells.
• These cells secrete a waxy protective coating which helps prevent water loss from the leaf.
• The cells of the lower epidermis of the leaf are like those of the upper epidermis, except for the many pairs of guard cells.
• A stoma, or pore, lies between each pair of guard cells, regulating the size of the stomata (pores).
• Veins, or vascular bundles, are composed mainly of xylem and phloem tubes, which are connected to the xylem and phloem tissues of the stem and root.
• The palisade layer lies just below the upper epidermis and consists of one or more layers of elongated, boxlike cells, which contains numerous chloroplasts that store chlorophyll.
• Chlorophyll is the green pigment (colored substance) that traps light energy needed for food-making.
• The spongy region lies between the palisade layer and the lower epidermis.
• Cells of the spongy layer are somewhat rounded and loosely arranged and contain chloroplasts for some food-making.
• Large air spaces are scattered among the cells, and lead to the stomata, where gas exchange occurs.
• Photosynthesis: Formation of Simple Carbohydrates is an important function carried out by leaves.
• In the presence of light, chlorophyll, carbon dioxide, and water, a leaf manufactures simple carbohydrates (glucose).
• Carbon dioxide from the air enters the leaf through the stomata.
• Water from the soil enters the root through the root hairs and reaches the leaf through the xylem.
• Both carbon dioxide and water pass into the cells containing chlorophyll.
• Glucose is formed and oxygen is given off, then passes into the air spacesof the spongy region and out of the leaf through the stomata.
• Glucose light is converted to chemical energy, stored in the glucose
• Raw materials are carbon dioxide and water
• Finished product is glucose
• By-product of the reaction is oxygen
• Chlorophyll enables this to happen acting as reaction catalyst
• When conditions are favorable, a green plant makes more glucose than it needs for its life activities and the leaf changes the glucose to starch, which is then stored.
• With the aid of numerous enzymes, the leaf can change the glucose to starch.
• Starch is a large molecule composed of a chain of many glucose sugar molecules.
• When energy is needed, enzymes change, or digest, the starch back to glucose.
• Besides making carbohydrates like starch, a green plant makes the following nutrients:
  • Proteins and Vitamins.
  • Fats and Oils.
• Respiration - plant takes in oxygen and releases carbon dioxide and water vapor.
• Transpiration - the evaporation of water from the inside of a leaf
• Roots absorb soil water, leaf transpiration releases the water

Plant Diseases

• Biotechnology is used to help develop disease resistant plants
• Genetically altered plants may pose issues for agriculture in the future