plant biology lecture 8- secondary growth in stems

what is an annual plant

plants that complete their growth in a single year

• plant dies after seed production

• no secondary growth

what is a biennial plant

plants that take 2 years to complete life cycle

• usually goes dormant over winter

• resumes vegetative growth after dormancy

• reproduced then dies

• aka ‘winter annual’ plants

what is a perennial plant

plants the live for more than 2 years

• reproduce then go dormant over winter

• resume vegetative growth afterwards

• some plants can die but some can live for thousands of years

where does secondary growth come from

lateral meristems

where in the plant stem is this diagram

close to the apical meristem

• the primary meristematic tissues are identified

where in the plant stem is this diagram

further down the stem

• vascular bundles are differentiated

• procambium is seen

what is anticlinal division

plane of division is perpendicular (90 degrees) to the surface of the plant tissue

what is periclinal division

plane of division runs parallel to the surface it’s closest to

whats another name for vascular bundles

fasicles

how does transition from primary to secondary growth begin

1. molecular signals come from the procambium in each vascular bundle

2. signal tells parenchyma cells to become meristematic (begin to actively divide)

3. procambium turns into vascular cambium (a lateral meristem)

4. vascular cambium divides and creates secondary xylem/phloem

5. occurs in a ring around the stem

6. primary phloem/xylem gets pushed out and sometimes crushed

what types of initials does the vascular cambium have

fusiform initials and ray initials

what are fusiform initials

• Much longer than wide

• Producing xylem and phloem cells that will transport material along the plant’s vertical axis

what are ray initals

• Square-like shape and more even

• Produce vascular rays (xylem and phloem) that will transport materials across the width of a plant stem

what does the periderm consist of

phellogen, phelloderm and phellum

what is phellogen

cork cambium. produced phelloderm to the inside and phellum to the outside of cells

what is the purpose of the phellum

to protect the stem

produced secondary wall that is suberised and lignified and tightly packed together

what is phelloderm made of

parenchyma cells (secondary cortex)

what are lenticels

region of periderm with lots of intercellular space

• allows for gas exchange

• near stomata

• seen at small woody dots along a stem

what happens to the apical meristem at the end of a growth season

will produce scales instead of phytomers to protect the apical meristem during end of season

what is wood

the secondary xylem that vascular cambium produces

how do trees produce early wood

• vascular cambium in trees from from dormant to active after a season, and produce secondary xylem/phloem

• spring has favourable conditions (water) so vascular cambium produces larger cells (early wood)

how do trees produce late wood

when season progresses, growing conditions become less favourable, and vascular cambium produced late wood

what is heartwood

the darker wood in the middle of a tree trunk. made of dead xylem cells filled with resins and tannins for structural integrity

• not conductive

what is sapwood

conductive lighter outer wood. sap oozes out of tissues

where can vascular cambium and phloem be found on a tree

in a thin later on the outside of the wood

what are vascular rays

• made of xylem/phloem parenchyma (thin primary cell walls)

• run across the trunk

• creates weak points + cracks along the wood. represents where xylem rays were produced