Arrangement of tissues in plant organs (stems, leaves, roots)

- arrangement of tissues is related to organ function

Stems:

functions

a) support and positioning of leaves

to maximize photosynthesis

b) transport

- from roots to leaves, leaves to roots, etc.

shoot apical meristem multiple functions

a) increase stem length

b) form leaf primordia

c) form bud primordia

nodes

internodes

bud primordia develop into axillary buds

primary meristems:

protoderm

procambium

ground meristem

-primary tissues:

epidermis at the surface

xylem and phloem in strands,usually arranged in a circle

ground tissue

pith

cortex

pith rays

- in monocots:

- vascular bundles “scattered”

- ground tissue = ground parenchyma

no pith and cortex

secondary growth in stems:

- occurs in some dicots

- requires lateral meristems

vascular cambium

cork cambium

- process begins when lateral meristems develop

- vascular cambium develops as a single layer of cells;

- in ring passing through and between vascular bundles

- vascular cambium cells divide, producing new cells

- secondary xylem cells

- secondary phloem cells

cork cambium develops as a ring within the cortex

- cells divide, producing cork cells at stem surface

resulting stem structure:

- periderm at surface

- cortex

- continuous layer of secondary phloem

- vascular cambium

- continuous layer of secondary xylem

- pith

lateral meristems continue to function;

new layers of periderm, secondary phloem, and secondary xylem are formed

– adds to stem width

wood

bark

hardwood

softwood

growth rings

early wood

late wood

transition makes growth rings visible

sapwood

heartwood

Leaves:

major function = photosynthesis

- structure designed to optimize photosynthesis

attached to stems at nodes

generally consist of

1) blade

2) petiole

sessile leaves

have no petiole

- may also have leaf sheath

leaf blade may be undivided (simple leaf)

or

the leaf blade may be divided into leaf segments (compound leaf)

palmately compound leaf

pinnately compound leaf

leaf phyllotaxy = arrangement of leaves on the stem

- can influence the ability of leaves to absorb light

spiral phyllotaxy –

opposite phyllotaxy –

whorled phyllotaxy -

Leaf Anatomy:

each leaf consists of:

epidermis

mesophyll

veins

epidermis - upper epidermis and a lower epidermis, covered with cuticle

- single layer of epidermal cells,

some guard cells,

possibly some trichomes

the mesophyll is photosynthetic ground tissue

- must absorb light energy and CO2

- light energy absorbed as it passes through leaf

- CO2 enters at stomate, diffuses to mesophyll cells

in dicots, leaf mesophyll in two layers:

palisade mesophyll

- specialized for light absorption

spongy mesophyll

- allows gas diffusion within leaf

palisade mesophyll

spongy mesophyll

monocots,

often have no distinct palisade mesophyll

vascular tissue - in veins

- each vein contains some xylem and some phloem

veins – a few large veins and many smaller veins

large veins create “venation pattern”

e.g., netted venation

- typical of dicots

parallel venation

- typical of monocots

in each vein,

xylem on upper side;

phloem on lower side

each vein surrounded by a bundle sheath

veins required to

1) bring in water and minerals

2) collect metabolites for transport in phloem

Roots:

usually underground

functions:

- anchorage

- absorption of water and minerals from the soil

- food storage

- other

two common root systems:

1) a taproot system

- one major root (= the taproot) with many lateral roots (root branches)

- the taproot originates from the radicle (in the embryo)

- seen in most dicots

- advantages

2) fibrous root system

- many adventitious roots

, plus many lateral roots

- often spreads horizontally; does not grow deeply into soil

- common in monocots

advantages:

for either type of root system,

continued new growth is essential to function

1) to reach new sources of water & minerals

2) to balance increases in shoot size

3) compensates for the loss of absorptive ability in older roots

- most absorption occurs at root tips(where root hairs are)

Root growth:

- occurs at root tip

- every root tip has a

1) root cap

2) region of cell division

3) a region of elongation

4) a region of maturation

in dicots, the mature root includes:

epidermis

cortex

vascular tissue, in central cylinder

in monocots,

- pith is present in the center of the vascular tissue

pericycle

- located between the endodermis and the phloem

- responsible for lateral root formation

in some dicots, the roots may undergo secondary growth

(form 2° xylem, 2° phloem, periderm)