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chapter 7 (other types of roots and root modification (prop roots (the…
chapter 7
other types of roots and root modification
storage roots
provide long term storage for carbohydrates
prop roots
the stem of a monocot can become wider, with more vascular bundles
make contact with soil, transport water and nutrients to the cells
act like stabilizers
buttress roots- plate-like, upper side grows more rapidly so the trees can't be blown over by the wind
aerial roots of orchids
many roots are epiphytic(lives by being attached to the branches of trees)
their roots spred along the surface of the bark and often dangle freely in the air
the root epidermis (velamen) are composed of several layers of large dead cells
contractile roots
mycorrhizae
root nodules and nitrogen fixation
haustorial roots of parasitic flowering plants
roots of strangler figs
internal structure of roots
root cap
cells are meristemic
root apical meristem
their nuclei do not take thymidine and do not become radioactive
the mitotically inactive central region is known as the quiescent center
zone of maturation/ root hair zone
root hairs only function for several days then die and degenerate
minerals do not have free access to the vascular tissues because of the innermost layer of the cortical cells called the endodermis
the bands of altered calls called casparian strips
between the vascular tissue and the endodoermis are the parenchyma cells that make a region called the pericycle
zone of elongation
mature portions of the root
the powerful absorption of water and water pressure called root pressure
passage cells- thought to once represent passageways for the absorption of minerals, it is now thought they were slow to develop
origin and development of lateral roots
lateral roots are initiated by cell divisions in the pericycle
cells are more densely cytoplasmic with smaller vacuoles and resume mitotic activity
differs than the bud formation in shoots
external structure of roots
organization of root systems
most seed plants have a single prominent taproot and small lateral roots or branch roots comint out
taproot develops from the embryonic root called the radicle
most monocots and some eudicots have a mass of many similarly sized roots constituting in a fibrous root system
adventitious roots are roots that don't develop from the taproot
increase the absorptive and transport capacities of the root system
most monocots cannot undergo secondary growth after their stem is formed
structure of individual roots
the root tip is the region here growth in length occurs
the zone of elongation is just behind the root cap and root apical meristem
cells undergo division and expansion
the root of the apical meristem is protected by a thick layer of cells called the root cap
dictyosomes of the root cap cells secrete a complex polysaccharide called mucigel which lubricates passage of the roots
behind the zone of elongation is the root hair zone
many epidermal cells extend out as narrow tricomes
root hairs form only in a part of the root that is not elongating or they would be cut off
expands the roots surface area