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Plant Biology - Tissues

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    Study Guide Plant Biology – Tissues 1. Growth and Development The words growth and development are often used as if they mean the same thing. In botany, however, they describe different but closely related processes that help shape a plant as it matures. 1.1 What Is Development? Development refers to the overall process by which a plant becomes mature. For example, when a fertilized egg slowly turns into a fully grown tree, development is taking place. Development includes three main processes: • Growth – an increase in size • Morphogenesis – the formation of shape and structure • Differentiation – the specialization of cells Plant development is controlled by genetic instructions inherited by the cells and by environmental factors such as light, water, and nutrients. Together, these determine how the plant forms its tissues and organs. 1.2 What Is Growth? Growth is an irreversible increase in size of cells or plant organs. This happens in two main ways: 1. Cell division – new cells are produced 2. Cell enlargement – cells become bigger Cell enlargement occurs because: • The cell wall becomes more elastic • The vacuole increases in size and water content Growth can occur in two patterns: • Determinate growth – growth stops after reaching a certain size • Indeterminate growth – growth continues for an unlimited time Most plants show indeterminate growth , meaning they can keep growing throughout their lives.

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    Study Guide 1.3 Differentiation: How Cells Become Specialized Differentiation is the process by which unspecialized (generalized) cells become specialized cells with specific structures and functions. Although all cells formed in meristems have the same genetic makeup , they become different because: • Certain genes are turned on (expressed) • Other genes are turned off (repressed) A cell’s location also matters. For example: • Root cells do not form in flowers • Petals do not grow on roots So, both gene activity and position determine what type of cell develops. 1.4 Dedifferentiation: When Cells Change Back Under special conditions, mature plant cells can reverse their specialization . This process is called dedifferentiation . Dedifferentiation usually happens when a plant is injured, such as: • Broken branches • Leaves damaged by insects To repair itself, the plant causes nearby parenchyma cells to dedifferentiate and then divide. These cells form new tissues that replace the damaged ones or perform similar functions. 1.5 How Plant Growth Is Different from Animal Growth Plants and animals grow in very different ways. • Animals grow until they reach a genetically fixed adult size. After that, growth stops. • Plants continue growing throughout their entire lives , but only in certain regions.

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    Study Guide Plant growth occurs mainly in meristematic tissues , which leads to: • Two types of tissues: primary and secondary • Two body forms: primary plant body and secondary plant body • Two types of meristems: apical and lateral 1.6 Apical Meristems and Primary Growth Apical meristems are regions of active cell division found at the tips of roots and stems . Their main function is to: • Increase the length of the plant • Produce primary tissues As these new cells grow: • Their vacuoles enlarge • Stems and roots increase in girth (thickness) up to a limit Once the maximum girth is reached (based on cell wall elasticity), the plant can still grow longer , but not thicker. Plants that have only primary tissues — such as most herbaceous plants — remain relatively small. 1.7 Lateral Meristems and Secondary Growth Woody plants can grow very large because they have lateral meristems . Lateral meristems: • Develop around the outer edges of roots and stems • Produce secondary tissues • Add strength and protection These tissues form the secondary plant body , allowing trees and shrubs to grow thick, strong, and tall over many years.

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