ROOT
Function and features of root
Root is the lower part of a plant, usually
underground, by which the plant is
anchored and through which water and
mineral nutrients enter the plant.
It is the first structure emerging from
seed germination.
It usually functions as storage organ (e.g.
starch).
Root synthesizes hormones (cytokinins
and
gibberellins)
and
secondary
metabolites (e.g. nicotine).
Root development
dicotyledones
Taproots
monocotyledones
Fibrous roots
Root development
Organography
Root cap
•The root cap is the tissue that covers the apex of a
root. It is composed of parenchymatous cells producing
mucilaginous substances (mucigel, formed by
exocytosis of pectins coming from Golgi vesicles).
•Mucigel protects apical meristem from damage as it is
pushed through the soil.
•Root cap is also capable of perceiving the gravity force
and of orienting the downward growth.
Root elongation happens only in the tip
Root hairs
•Root hairs are outgrowths from single epidermal cells in the
piliferous layer of a root.
•They are thin-walled (no secondary wall) and increase the total
surface area available for the absorption of water and nutrients.
Transversal sections of roots of
Dicotyledones (left side) and
Monocotyledones (right side)
Anatomy
•Exodermis is the layer of cells immediately below the
epidermis of root. It is composed of Casparian strips
(suberin thickenings of secondary walls) that avoid the
discharge of water and provide protection against attacks of
pathogens.
•Endodermis is the innermost layer of the cortex
surrounding the stele, which plays an important role in
controlling the transport of substances within the plant.
Transversal section of root of Dicotyledones
Transversal section of root of monocotyledones
Casparian strip
•Casparian strip is a band of waterproof,
corky tissue (made of suberin) that is
found on the side and walls (primary) of
endodermal cells. The strip prevents
water from entering the pericycle except
through the cytoplasm of endodermal
cells.
•Sometimes endodermis can present
lignin playing supporting functions.
•In monocotyledones, when observed
through
transversal
sections,
endodermis appears as U-shaped
(pharmacognostic character).
Casparian stripe between
two endodermal cells
Casparian strip as a selective barrier
•Endodermis
stops
apoplastic
movement (pathway of water
through the cell walls continuum),
allowing substances to enter or exit
from stele only through symplastic
movement (continuum of cytoplasm
of different cells being connected by
plasmodesmata).
In
do
that,
substances undergo filtration at
membrane level.
Root apex
Primary thickening
Development of
root of
dicotyledones
origin of vascular
cambium
Beginning of secondary
thickening
Secondary thickening
with development of
periderm
primary thickening of willow root
•Periderm is waterproof so
that only the young parts close
to the root tip are able to
absorb water and mineral
nutrients.
Secondary thickening of willow root
Being underground, root is not influenced by seasonal
variations so that it does not show growth rings
(unlike stem).
•Lateral roots originate from pericycle, a layer of cells that lies
between endodermis and phloem. It consists mainly of parenchyma
whose cells are capable of secreting enzymes favouring the growth of
lateral roots.
• Pericycle also contributes to the formation of vascular cambium
and phellogen or cork cambium.
Prop roots of Zea mays
Pneumatophores (aeriferous roots) of
mangrove
Pharmaceutical importance
of root
•From the root of many plants, active
compouns are obtained and used as
plant drugs. Main examples are altea,
gentian, ipecac, licorice, Poligala,
Ratana, valerian, ginseng, devil's
claw.