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Lecture 8

BIOL120 Lecture Notes - Lecture 8: Meristem, Haustorium, Phloem


Department
Biology
Course Code
BIOL120
Professor
Simon Chuong
Lecture
8

Page:
of 6
LECTURE 8 – ROOTS
Plant Root Systems
1. Key functions: anchor, water and mineral absorption
2. Additional functions: produce hormones and secondary metabolites regulate plant
development, reproduction, storage
3. Two types of root systems
a. Taproot
b. Fibrous root systems
Taproot Systems – typical of Dicots and Gymnosperms (develop directly from embryonic
root)
Taproots penetrate deeply although conifers generally do not
Small plants can have significant taproot systems
oEx. Legumes, dandelions
Taproots can enhance survival in harsh environments (drough tolerance)
oThey have found a way to live in the desert
Fibrous Root Systems – typical of monocots and seedless vascular plants (embryonic root
dies). Root system arises from lower part of stem: therefore adventitious
Each adventitious root forms lateral roots
There larger mass can obtain water much more efficiently
Root system is typically shallower
Better at water acquisition
Often found in annuals (plants that have only a one year growing season)
Stabilize the soil – less erosion
Root systems can vary in depth and length
Typically 50-90% occurs in the top 30cm of soil
Potato root systems: 0.9m
Wheat, oats, barley: 0.9-1.8m
Can have significant radius around plant
Length of root systems can be far greater than shoot system (corn: 457 meters root
to 2.5 meters)
Root Structure – grows from tip and matures in a retrograde manner
1. A supply of dividing cells are found at the tip of the root
2. This region is known as the ROOT APICAL MERISTEM (“fountain of youth”)
3. The root apical meristem consists of a group of ‘initials’ or dividing cells
4. These cells reside within the quiescent centre
5. The QUIESCENT CENTRE is made up of cells or initials that divide relatively
slowly
6. When each initial divides, one daughter cell remains in the quiescent centre while
the other becomes a ‘derivative’
7. A derivative is primed to undergo further differentiation and growth
Derivatives give rise to the primary tissue (going from outside to inside):
1. Protoderm (meaning before skin)
2. Ground meristem: produces ground tissue
3. Procambium: source of primary vascular tissue
Derivatives divide much more quickly than initials
1. ZONE OF CELL DIVISION – consists of root apical meristem and three primary
tissues
2. ZONE OF ELONGATION – division stops and cells elongate. Its aregion where
most of root growth occurs.
3. ZONE OF MATURATION – where the cells begin to specialize. You see the root
hair formation and maturation of conducting tissues. Lateral roots form
immediately behind this region.
The Root Cap
Consists of cells produced at leading edge of growth
Cells contain amyloplasts (starch grains) that likely function as gravity sensors
Sacrificed to protect growing tip (meristem)
Produce a polysaccharide known as mucigel (biological grease) that facilitates
movement through the soil
Water and Mineral Absorption – function mainly served by root hairs.
Root hairs are produced in the zone of maturation
New root hairs are continuously produced with older hairs dying as the root
continues to grow
The region of the root containing root hairs may be well below ground
Ground and Vascular tissues
Division, growth and differentiation can be
traced linearly through three overlapping
regions or zones
Cell elongation
Elongation
Maturation – where we find the root
hairs growing
Form cylinders that run the length of the root
Stele: central cylinder of tissue surrounded by the cortex
Protostele: contains the vascular tissue
The arrangement of vascular tissues varies
Dicot and Conifer Vascular Tissues
Monocot Vascular Tissues
Vascular tissues are made up of two complex
tissues – xylem and phloem:
Xylem: water-conducting tissue (root to
shoot)
Phloem: food-conducting tissue (shoot to
root)
Dicots and conifers have solid spokes or lobes of
xylem with phloem intercalating
A core of undifferentiated tissue (pith) is
surrounded by alternating rings of xylem
and phloem.
These pith cells arise from procambium and
not ground meristem.
NOTE: Seed plants have two important cell
layers surrounding the stele – the pericycle
and the endodermis