Plant Development II: Primary and Secondary Growth Flashcards
Determinant Growth
growth to a specific body size + shape then stop growing (ANIMALS)
Indeterminant Growth
continue adding new organs (leaves, stems, roots) as long as the organism has access to the necessary resources (PLANTS)
- plants are able to continue growing indefinitely due to specialized tissues - MERISTEMS
Meristems
regions of continuous cell division and growth; these tissue cells are essentially stem cells that are undifferentiated and continue to produce cells that quickly differentiate + become permanent tissues (vascular, ground, dermal)
- 3 types of meristems, based on their location in the plant: apical, lateral, intercalary
- contribute to PRIMARY and SECONDARY growth
Apical Mertistem
tips of stems and roots which enable a plant to extend in length
Lateral Meristem
facilitate growth in thickness/width - also called CAMBIA
Intercalary (BASAL) Meristem
occur only in some monocots, at the bases of leaf blades and nodes
- allows the monocot leaf blade to increase in length from the leaf base
Primary Growth
taller/longer; controlled by root apical and shoot apical meristems
Secondary Growth
wider; controlled by the 2 lateral meristems - vascular cambium and cork cambium
- not all plants exhibit secondary growth
Root Growth
root growth begins with seed germination. when the plant embryo emerges from the seed the radicle of the embryo forms the root system
- root cap
- three zones of the root tip: cellular division, cellular elongation, and cellular maturation
Root Cap
structure exclusive to roots that protects the tip of the root
- continuously replaced because it is quickly damaged by the soil as the root pushed through the soil
- the cell division zone is right being the root cap
Zone of Cellular Division
location where new cells are being generated by miotic cell division
- driven by the activity of the apical meristem located immediately behind/under the root cap
Zone of Cellular Elongation
location where new cells are growing/increasing in length to add length to the root
- requires water uptake which stretches cells and increases their size
Zone of Cellular Maturation
location where newly elongated cells complete their differentiation into the dermal, vascular, or ground tissue
- driven by changes in gene expression
Lateral Roots
another form of primary growth; roots present in some plants that branch from a main tap root:
- these originate from meristematic tissue in the PERICYCLE - a layer of parenchyma cells that surround the vascular cylinder in the root’s center
- once emerged, these roots display their own primary growth by continually adding length to the lateral root
- lateral roots are NOT the same as secondary growth
Primary Growth in Shoots
primary growth is a result of rapidly dividing cells in the apical meristems at the shoot tip
- subsequent cell elongation then leads to primary growth
- APICAL DOMINANCE
Apical Dominance
a phenomenon where primary growth occurs primarily at the apical bud instead of the axillary buds
- apical bud prevents the growth of axillary buds that form on the sides of stems and branches
- when the apical bud is removed, axillary buds start to grow
Secondary Growth in Shoots + Roots
secondary growth is controlled by the lateral meristems in shoots + roots
- lateral meristems include the VASCULAR + CORK CAMBIUM (the second one is usually in woody plants)
- Herbaceous (non-woody) plants mostly undergo primary growth and little to none secondary growth
- noticeable in wooly plants, takes place in some dicots and is rare in monocots
Vascular Cambium
contributes to a vast majority of secondary growth, referred to as “VC”:
- located between the primary (1) xylem + phloem within the vascular bundle; vascular bundles are arranged in a ring near the periphery of dicot stems, with the xylem on the interior and phloem on the exterior of the ring
- VC cells divide and form secondary (2) X + P that appear between the 1 X + P on either side of the cambium. 2 X inside the ring and 2 P on the outside
- the cells of 2 X contain lignin
- the xylem with the pith form WOOD in a woody stem
Cork Cambium
the outermost lateral meristem, referred to as “CC”:
- CC produces cork cells, which contain a water repellant waxy substance
- the phloem and cork cells form the BARK, which protects the plant from physical damage and water loss
- CC also produces a layer of PHELLODERM CELLS, which grow inward from the cambium.
- the CC, cork cells, and phelloderm collectively make up the PERIDERM, which substitutes for the epidermis in mature woody-stemmed plants
Components of Woody Stems
a new layer of xylem and phloem are added to the vascular cambium each year during growing season, creating new wood and bark each year
- Wood
- Bark
Wood
composed of xylem and pith; is everything from the vascular cambium toward the INTERIOR of the woody stem
Bark
composed of phloem and cork, is everything from the vascular cambium toward the EXTERIOR of the woody stem
Growing Woody Stem Diameter
over time, the wood gets larger in diameter, while bark remains the same depth
- new xylem layers are added each year towards the ring’s interior. these interior rings fill with resin and become nonfunctional for carrying water and provide structural support as the tree grows wider
- new phloem layer is added to the ring’s exterior every year. the CC is also adding new cork layers. the exterior most phloem layer becomes crushed against the CC and are broken down with the bark
Mature Tree Structure
- many interior layers of older, nonfunctional xylem deep within the stem (WOOD)
- a small amount of older phloem (BARK)
