34.1 Plant Form: Themes with Many Variations

• An aboveground portion called the shoot system harvests light and carbon dioxide from the atmosphere to produce sugars, while a belowground portion called the root system anchors the plant and takes in water and nutrients from the soil.

  • Collectively, the root and shoot systems make up the plant body.

• Many root systems have a central taproot, as well as numerous lateral roots.

  • The root system anchors the plant in soil, absorbs water and ions from the soil, conducts water and selected ions to the shoot, obtains energy in the form of sugar from the shoot, and stores material produced in the shoot for later use.

• Rain is abundant enough in these areas to support a lush growth of herbaceous plants-seed plants at lack woody tissue- yet scarce enough to exclude trees and most shrubs.

• Although the aboveground portions of prairie plants burn during fires and die back during the winter or dry season, their root systems are perennial, meaning that they live for many years.

• Roots show a great deal of phenotypic plasticity-meaning that their form is changeable, depending on environmental conditions.

• The taproots and fibrous roots illustrated earlier do not begin to exhaust the types of roots found among plants.

  • For example, some roots are adventitious-meaning they develop from an unusual source, the shoot system instead of the root system.

  • In ivy, anchor roots-adventitious roots at growing from nodes in the shoot system-help anchor individuals to brick walls or other structures.

  • The prop roots of corn are adventitious roots that help brace individuals in windy weather.

• The pneumatophores of mangroves in the genus Avicennia are specialized lateral roots that function in gas exchange.

  • These mangroves grow in submerged habitats where fine silt is deposited, cutting off the oxygen supply to their roots.

• The thick taproot of some biennial plants, such as carrots and beets, stores carbohydrates during the first of the plants' two growing seasons.

  • These taproots are known as storage roots.

34.2 Plants Cells and Tissues Systems

• Chloroplasts are, all importantly, the site of photosynthesis.

  • Non-photosynthetic cells found in roots, seeds, flower petals, and other locations may have organelles that are similar to chloroplasts but are specialized for storing pigments, starch, oils, or proteins.

• Vacuoles store various compounds, including wastes, and in some cases, they digest wastes, as do animal lysosomes.

• A tissue is a group of cells that functions as a unit.

• The dermal tissue system consists of dermal tissue (literally, “skin” tissue).

  • This tissue, also called the epidermis, is the outermost layer of cells that forms the interface between the organism and the external environment.

• Many parenchyma cells are totipotent, meaning they retain the capacity to divide and develop into a complete, mature plant (much like embryonic stem cells in animals).

  • The totipotency of parenchyma cells is important in healing wounds and in reproducing asexually via stolons or rhizomes.

34.3 Primary Growth Extends the Plants Body

• Apical meristems are located near the tip of each root and shoot.

  • As cells in apical meristems divide, enlarge, and differentiate, root and shoot tips extend the plant body outward, allowing it to explore new space.

• The division of apical meristem cells, and differentiation of those cells, is responsible for primary growth, which is common to all plants.

  • The major consequence of primary growth is to increase the length of the root and shoot systems.

• All of the cells and tissues that are derived directly from apical meristems constitute the primary plant body.

• A group of cells called the root cap protects the root apical meristem.

34.4 Secondary Growth Widens Shoot and Roots

• In trees and other woody plants, secondary growth increases the width of roots and shoots.