Lecture 6: Root structure; medicines of good and evil Flashcards
Describe the 4 important zones of the root (tip) structure
- ) root cap
- ) zone of cell division
- ) zone of elongation
- ) zone of maturation
What is the complex morphology of roots?
long or short
horizontal or vertical
Describe the root cap
- composed of a lateral root cap and a columella ( this senses hydrotropism and gravitropism)
What is the function of the root cap?
- senses and transmits signals to the meristem and the elongation region of the root
- controls the direction of root movement throughout the soil
- protects the apical meristem
- assists in penetration by the release of mucigel
- strong root caps can force the soil particles apart
- remains in constant size as it grows
What is the purpose of the mucigel and what secretes it?
secreted by border cells
- reduces friction allowing the root cap to penetrate into the soil
does the root cap have to be regenerated?
yes
What the root apical meristem
- promeristem: region of small cells with dense cytoplasm and large nuclei
- acts as cell initials; the source of all root ground tissues
What does it mean by a root apical meristem being an ‘open type’
arise from one group of initials
- roots of lower plants (single apical cell)
What does it mean by a root apical meristem being a ‘closed type’
- roots of higher plants
- root cap, vascular cylinder and cortex have their own initials
What is the quiescent centre
- does not include initials of root cap
- able to repopulate bordering apical meristem regions
- these cells at extreme tip do not divide often
- these act as RESERVOIR of cells for growth when root is injured
- cell division is most frequent behind the quiescent centre
What is the elongation zone?
immediately behind the apical meristem
usually only a few mm long
responsible for all growth of the root
- where the cells are elongating
What does growth require
requires division and elongation
- an irreversible processes
What is the main function of the endodermis
separates the unregulated part of the root from the regulated part
- controls the amount of water uptake of the xylem
What is the rhizodermis of the root?
- hairs for water uptake
- elongated cells
- usually no cuticle ( so that it will be able to absorb water)
What is the root cortex comprised of?
- usually parenchyma; sometimes collenchyma, sclerenchyma
- cells often store starch
- lots of air space (enables the root to respire when starch is being released)
What is the root endodermis?
- innermost layer of cortex
- casparian strips in cell walls (made of suberin w/ lignin) making it hydrophobic
What are the two routes for water to reach the xylem?
apoplastic movement
symplastic movement
What does it mean by apoplastic movement?
throught the cell wall
What does it mean by symplastic movement
through cells via plasmodesmata
What does the casparian strip prevent
apoplastic movement
- forces the water to go via the symplastic route
What is the casparian strip
- band like portion of the primary wall and middle lamella
- acts as a BORDER control for selective uptake of ions and water
- water is forced to cross through cell membrane
- the suberin and lignin infiltrates the spaces in the wall usually occupied by water, thus imparting a hydrophobic property to the specific regions
When are casparian strips important?
in situations where there is too much water
- or after a long drought a plant is watered
What is the root pericycle
- single layer inside of endodermis (these cells divide perclinally; parallel to the surface of the meristem)
- origin of lateral roots
- lateral root eventually bursts through outer tissue of parent root
Why can’t the vascular root originate near the roots’ surface?
this is because its vascular system must be continuous with the vascular cylinder oat the centre of the established root
What is the stele
- comprised of vascular tissues (xylem and phloem)
What is the stele like in dicots?
star shaped arms
- phloem located between these arms
What is the stele like in monocots
a cylinder of vascular bundles
- central pith (parenchyma)
What are alkaloids; where are they produced?
- produced by roots and other organs to deter herbivores
> 12000 chemically diverse structure eg. morphine, nicotine, caffeine, cocaine, strychnine
What are alkaloids used for?
medicine
witchcraft
psychoactive drugs
What is Mandrake root (Mandragora officinarum)
- Asian/European plant with thickened bifurcated roots
- roots have human shape
What are the effects of mandrake roots?
- these contain alkaloids of atropine and scopolamine
- relax muscles
- dilate pupils
- lessen pain
- induce hallucinations
- increase heart rate & body temperature
What are the medicinal uses of atropine?
- antispasmodic for Parkinson’s disease
- epilepsy
- stomach cramps
- broncholidator for asthma
- heart stimulant for cardiac arrest
- antidote for various poisons
