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Chapter 7: Roots (Other Types of Roots/Modifications (Prop Roots:…
Chapter 7: Roots
Other Types of Roots/Modifications
Storage Roots:
provide long-term storage for carbohydrates
carbohydrates stored in root are used to produce a new shoot in the spring
accumulate during summer photosynthesis
Prop Roots:
adventitious root that arises from a stem or trunk, penetrates the soil, and helps support the plant
transport additional nutrients & water to stem
buttress roots:
plate like roots for stability
upper side grows more rapidly than other parts of the root
Aerial Roots of Orchids:
root epidermis (
velamen
) composed of many layers of dead cells
creates water proof barrier to retain water
Contractile Roots:
uppermost portion of roots slowly contract
stem pulled downward
caused by changes in shape of cortex cells
simultaneously shorten & expand radially, losing up to two thirds of height
Mycorrhizae:
relationship most roots of seed plants have with soil fungi
ectomycorhizal relationship:
fungal hyphae penetrate outermost root cortex cells
in woody forest plants
endomycorrhizal association:
hyphae penetrate the root cortex to the endodermis
invade cell forming structure called
arbuscule
which provides plant with phosphorus
Root Nodules:
nitrogen fixation:
chemical conversion of atmospheric nitrogen
bacteria in soil secrete substance to curl roots
bacterium attaches to convex side and uses
infection thread
to push into cell
bacterium sits in thread while it burrows to inner cortex
root nodule
formed by cortical cells
bacterium enters cytoplasm & proliferates
plant cells filled with bacterial cells capable of converting N2 into nitrogenous compounds for the plant
Internal Structure of Roots
root cap:
small/meristemic when first formed
develop dense starch grains which settle to lower side of cell, detecting gravity
as cells get closer to edge, starch grains digested & cell’s dictyosomes secrete
mucigel
middle lamella break down & releases cells
root apical meristem:
produces root tissue in meristem which extend into other regions
quiescent center:
mitotically inactive central region of root apical meristem
form new apical meristem if original is damaged
zone of elongation:
just behind apical meristem
some meristemic activity, mainly cells enlarging
zone of maturation:
root hairs grow outwards, increase absorption of water/minerals
merged gradually with zone of maturation
minerals transferred from epidermis to vascular tissue
2 Types of Transport
apoplastic transport:
diffusion through walls/intercellular space
symplastic transport:
absorbed into cytoplasm of cortical cell, transferred from cell to cell
endodermis:
innermost cortical cells differentiate into this
#hyphae penetrate to the endodermis in this relationship
have tangential walls, radial walls are covered in lignin & suberin
Mature Portions of Root:
layer of suberin applied over all radial surfaces & inner tangential face of cell
layer of lignin over same surfaces
another layer of suberin
forms watertight sheath around vascular tissues
prevents root pressure from forcing water to leak out into cortex
irregular process, some cells finish earlier than others
passage cells:
thought to represent passageways for mineral absorption
now suspected they are just slow to develop
External Structure of Roots
Organization of Root Systems
taproot
:
develops from embryonic root (aka
radicle
)
grows extensively after germination
most seed plants have single prominent taproot
has numerous
lateral
or
branch roots
can develop their own lateral rootd
fibrous root system
:
mass of many similarly sized roots
occurs in most monocots and some eudicots
radicle dies following germination
root primordial at base grow out in first stages of FRS
adventitious roots
:
do not rise of preexisting roots, not radicles
root primordia initiated in the stem tissue as plant ages
increase the absorptive and transport capacities of root system
Structure of Root Individuals
root tip:
region where growth in length occurs
root cap:
#lateral root emerges with root cap
thick layer of cells
protects root apical meristem
mucigel:
secreted by dictyosomes of roots cap
#process where mucigel is secreted
lubricates passage of cells through soil
zone of elongation:
behind root apical meristem & root cap
cells undergo division & expansion here
root hair zone:
region where many epidermal cells extend out as trichomes
Origin and Development of Lateral Roots
later roots initiated by cell divisions in the pericycle
cells become more densely cytoplasmic
resume mitotic activity in just a few cells
creates small root primordium
organizes into root apical meristem & pushes out
breaks through cortex, epidermis, and endodermis
emerges with root cap
protoxylem & protophloem elements begin to differentiate
establish connection to vascular tissue of parent root
have smaller vacuoles
