Primary and Secondary Growth

Question 1

root primary growth

Answer 1

root apical meristem (RAM)
- composed of apical meristem in centre of root tip

subdivided into 3 meristematic areas:
- protoderm
- ground meristem
- procambium

Question 2

protoderm

Answer 2

gives rise to epidermis

Question 3

ground meristem

Answer 3

gives rise to cortex (parenchyma, exodermis, endodermis)

Question 4

procambium

Answer 4

gives rise to primary xylem, primary phloem, and pericycle (vascular cylinder)

Question 5

region of maturation

Answer 5

cells mature (differentiate) into distinctive cell types

Question 6

region of elongation

Answer 6

cells become several times original length; not differentiated (yet)

Question 7

pericycle

Answer 7

single cell layer, located directly below endodermis
- gives rise to lateral roots and part of vascular cambium

Question 8

lateral root development: pericycle

Answer 8

• pericycle displaces cortex and epidermis as lateral root grows outward, from vascular cylinder to epidermis
• lateral root eventually breaks through epidermis
• vascular cylinder (xylem and phloem) of lateral root stays connected to that of parent root

Question 9

secondary root growth

Answer 9

• no secondary root growth in fibrous monocot root systems
• as in stems, secondary growth occurs where primary growth has been completed
• dicot and conifer taproots have secondary growth to:

-increase conductivity
-replace old vascular tissue
-provide protection (periderm=bark)
-form specialized storage roots by anomalous secondary growth
-provide support in specialized roots

Question 10

2 types of lateral meristems

Answer 10

present in both dicot and conifer stems and roots

vascular cambium and cork cambium

Question 11

secondary root growth: vascular cambium

Answer 11

• forms from dedifferentiated parenchyma between primary xylem and phloem and pericycle
• initiated by auxin
• forms continuous ring of meristematic cells
  as in stems, secondary xylem = wood and less secondary phloem is produced

Question 12

secondary root growth in dicot and conifer: cork cambium

Answer 12

• forms from dedifferentiated parenchyma cortex, then secondary phloem

Question 13

secondary root growth in dicto and conifer: annual growth rings

Answer 13

roots also have growth rings but harder to distinguish as secondary xylem sizes do not vary by growing seasons

Question 14

anomalous secondary thickening: food storage

Answer 14

• storage cells of root vegetables like beets, carrots, radishes are a combination of root and stem, and store simple carbohydrates
• storage root expands by anomalous secondary thickening of xylem, phloem, and parenchyma
• beets have growth rings, unlike in stem where only secondary xylem, rings are comprised of repeating secondary xylem, secondary phloem, and storage parenchyma

Question 15

root periderm scars

Answer 15

• many food storage tap root shave root periderms scares resembling lenticels but NOT for gas exchange
• when lateral root breaks through periderm, wound response creates scarring