Plant Growth AP Biology Unit 5
Plant Growth • Plant growth occurs at specific tissues called meristems • Apical Meristem • Lateral Meristems
Plant Growth • Plants can go through two different types of growth • Primary Growth- growth in length • Secondary Growth- growth in width
Primary Growth- in Roots • In roots, primary growth occurs in 3 different zones • Zone of Cell Division (contains root apical meristem) • Zone of Elongation • Zone of Maturation
Zone of Cell Division • Lots of mitosis occurring in the cells here • Contains the newest cells (newly divided)
Zone of Elongation • Cells get longer to push the root tip into the soil • Cells start developing their specialized functions
Zone of Maturation • Cells become fully specialized • Considered mature cells
Root Cap • Root cap protects the root apical meristem as root pushes through the soil • Like a helmet for the root cells
Primary Growth- Stems • Growth occurs at the shoot apical meristems • Shoot apical meristems are located at the tips of buds • Plants grow from the top, not the bottom of the stem
Secondary Growth • Growth in width of stems and roots • Degree of secondary growth varies– some plants barely have any, others have extensive secondary growth. • Results in the formation of wood and bark • Results from activity in the lateral meristems • Vascular Cambium • Cork Cambium
Vascular Cambium • Divides into • Secondary Xylem • On the side closer to the center of the stem/root • Secondary Phloem • On the side closer to the outside of the stem/root • As the vascular cambium divides, it continues to push older cells farther away.
Cork Cambium • As the secondary phloem grows, it breaks the outer layer of the stems and root (epidermis) • Secondary phloem on the very outside becomes cork cambium • Cork cambium grows into cork • Cork cells contain suberin to protect from water loss, physical damage and harmful substances.
Question… • Where is the oldest part of a tree? • The very center – oldest xylem that has been pushed all the way in • Outer layers keep flaking off
Types of wood • Heartwood • Older secondary xylem • Cells get clogged stop conducting water • Darker in color • Sapwood • Newer secondary xylem • Still actively conducting water (and minerals) • Lighter in color
Annual Rings in Wood • The age of a tree can be determined by looking at the number of rings that it has. • Wood = secondary xylem • What forms the rings? • Spring: water most available, water transport cells are large and have thin walls • Summer: less water available, water transport cells have thicker walls and are darker