Plant Root Flashcards
Main Functions of Roots
- Anchoring the plant firmly to a substrate (soil)
- Absorption of water and minerals
- Production of hormones.
• shoot growth and development depend on the hormones
Hormones
cytokinin and gibberellin
Additional Functions
• Carbohydrate storage
• Protection
• Vegetative reproduction
• Parasitic roots
Three types of root system:
- TapRoot
- FibrousRoot
- Adventitious
• single prominent root (tap root) that is much larger than all the rest and numerous small roots (lateral roots or branch roots) coming out of it
• Derived from the radicle (embryonic root)
• Anchorage
• Common in dicot and gymnosperm
• perennial and woody plants: roots undergo secondary growth
Tap Root
(Taproot)
Nicotiana tabacum
Tobacco
(Taproot)
Raphanus sativus
Radish
(Taproot)
carrots, beets, radish, turnips
Fleshy taproot
(taproot)
sweet potatoes, cassava
Swollen lateral roots
taproot is about the carrot same size as the laterals
Sunflower
• mass of many similarly sized, delicate and hair-like roots
• no prominent enlarged primary root
• Produced after death of radicle
• Derived from the root primordia found at the end of radicle
• Common in monocots and some dicots
• Absorption
Fibrous Root
• Do not arise from pre-existing root
• Present in monocot and dicot
Adventitious Root
arise from main trunk stem
brace root
arise from the lateral branches of the main stem.
prop root
• Secondary growth = ↑ quantity of healthy, functional wood (xylem) in the roots = ↑ no. of leaves and fine absorptive roots
Dicot Roots
• No secondary growth = stem of an older plant is not wider than young plant, no ↑ conducting capacity, no more leaves or roots than young plant
• Stolons or rhizomes = ↑ size
• horizontal shoots branch and then produce adventitious roots
Monocot Roots
the growing portion protected by root cap; push through the soil
Root tip
Thick layer of cells that protects root apical meristem (root tip); constantly being worn away and renewed
Root cap
complex polysaccharide secreted by dictyosomes of root cap for lubrication
Mucigel (Slime)
• found behind the root cap
and root apical meristem
• few mm long
• where cells undergo division
and expansion
Zone of elongation
• single celled extension of
epidermal cells which
increase absorptive area
• no line of demarcation in the
epidermal cells
• transitory(diewithin4-5
days)
• form only in a part of the
root that is not elongating
Root hairs
• Cells are meristematic (cell division with transverse walls & forming files of cells that are pushed forward)
• As cells are pushed forward, they develop dense starch grains and their endoplasmic reticulum becomes displaced to the forward end of the cell = detects gravity because the starch grains settle to the lower side of the cell
Root Cap
• more orderly than the shoot because it experiences no disruptions
Root Apical Meristem
mitotically inactive central region
• cells are more resistant to various types of harmful agents such as radiation and toxic chemicals
• Serves as reserve of healthy cells
Quiescent center
• Becomes active when root apical meristem or root cap is damaged and forms new apical meristem
• Once the new meristem is established, its central cells become inactive, forming a
Quiescent Center
• the region where the cells expand greatly; some meristematic, but mostly enlarging
Zone of Elongation
outermost; forms epidermis
(dermal region)
Protoderm
at center; forms primary xylem/ phloem then metaxylem/ metaphloem (Stele/ vascular region)
Provascular tissue
between protoderm and provascular tissue; Parenchyma cells that form root cortex
ground tissue
• Root hairs grow outward
• zone of elongation merges gradually with the
Zone of Maturation
-transfer of minerals from the epidermis to the vascular tissue
Cortex
diffusion through the walls and intercellular spaces
apoplastic transport
absorption into the cytoplasm of a cortical cell and then transfer from cell to cell through plasmodesmata
• Minerals do not have free access to the vascular tissues because the innermost layer of cortical cells differentiates into a cylinder called the endodermis
symplastic transport
• Controls the passage of minerals across the vascular tissues
• Consist of thick-walled cells (encrusted with suberin and lignin); diffusion is inhibited
Endodermis
• bands of lignin and suberin on the radial walls (top, bottom and side walls) causing the cell walls to be water proof
• Impermeable
• minerals can cross the endodermis only if the endodermal protoplasts absorb them, then secrete them into the vascular tissues
Casparian strip
Internal anatomy of young dicot root shows three general sections:
Dermal region
Cortex
Stele or vascular cylinder
epidermis with root hairs
Dermal region
outer collenchyma
middle parenchyma
inner endodermis
Cortex
xylem forms a solid mass in the center, surrounded by strands of phloem; no pith
(Vascular)
Dicot
strands of xylem and phloem are distributed in ground tissue
(Vascular)
Monocot
inner wide cells
Metaxylem
outer narrow cells
protoxylem
Dicot: (Xylem)
Two to four or more groups of protoxylem may be present (larger roots = ↑ no.)
Triarch
Tetrarch
Pentarch
In monocot: (xylem)
many xylem poles
Polyarch
In monocot: (xylem)
strands of xylem and phloem surrounding a parenchymatous pith
Siphonostele
Vascular Tissue – most dicots
(Phloem)
found on the outer side
Protophloem
Vascular Tissue – most dicots
(Phloem)
found on the inner side
Metaphloem
• Found between the vascular tissue and the endodermis
• parenchyma cells that constitute an irregular region
• Where growth of lateral roots are initiated
Pericycle
• Initiated by cell divisions in the pericycle
• Localized cells remains mitotically active creating a small root primordium that become a root apical meristem and pushes outward
• As it pushes outward, the new lateral root destroys the cells of the cortex and epidermis that lie in its path, ultimately breaking the endodermis.
• Then, formation of a root cap, first protoxylem and protophloem elements connected to the vascular tissues of the parent root.
Origin Development of Lateral Roots
Old dicot root shows two distinct regions:
- Regionofsecondaryvascular tissues which include the
- Region of periderm layers
• provide long-term storage for carbohydrates that accumulate during summer photosynthesis
Storage Roots
(Specialized roots)
carrots, ube, radish, turnips
fleshy taproots
• Adventitious roots from stems
• Pandanus (screwpine), Ficus (Banyan trees)
Prop roots
(specialized roots)
Pandanus
screwpine
Ficus
Banyan trees
in corn plant – for additional support and absorption
Brace roots (specialized root)
Velamen in aerial roots of orchids
Water retention
dead cells, white; acts as waterproof barrier
Velamen
Chlorophyllous roots
Photosynthesis
Contractile roots of bulbs like onions, gladiolus, garlic
Movement
due to change in shape of cortical cells
Contraction
for absorption
Parasitic (haustorial roots)
diffuse root system i.e Tristerix
Haustorium
tall, plate-like, expanded roots for great support
Buttresses
breathing roots of mangrove for aeration
Pneumatophores
nitrogen fixation, formed by mitosis of cortical cells
Root nodules
bacteria, fill host cells with bacteroids w/c convert N2 to nitrogenous compds
Rhizobium
Other types and root modifications
Protection
Spiny roots of tugue
• Birds deposit seeds on the branch of a host tree
• When the seed germinates, the roots cling to the bark of the host tree, grow, hugging the host tree until it reach the soil.
Strangler Figs
association between a soil fungus (fungal hyphae) and roots
Mycorrhizae
hyphae penetrate between the outermost root cortex cells but never invade the cells
ectomycorrhizal relationship
hyphae penetrate the root cortex as far as the endodermis; but cannot pass the Casparian strip
endomycorrhizal
small structure formed by fungi; filled with P
Arbuscule
Fleshy taproots (i.e. carrots, beets, radishes)
Carbohydrate storage
in Crysophila and Mauritia, roots grow out of the trunk and harden into sharp spines
protection
roots spread horizontally and produce shoot buds (i.e. willows, sorrel)
Vegetative reproduction
modified roots attack other plants and draw water and nutrients out of them (i.e. mistletoe, dodder)
Parasitic roots
