1 of 66

Vegetative Shoots�Pt.1: Stems

​

​

Stem Development

Stem Structure

Specialized Stems

​

Rainbow eucalyptus

Bristlecone pine

2 of 66

Plant Organs

  • Roots
  • Shoots: Stems
    • Functions
      • Transport of water and minerals (from roots)
      • Support of leaves and other stems
  • Shoots: Leaves
  • Shoots: Reproductive

3 of 66

Cotyledon

(Seed leaf)

Cotyledon

(Seed leaf)

Germination

Radicle

​

​

​

​

x

​

p

Epi-

cotyl

p

x

Hypocotyl

Radish

Celeriac

Turnip

​

​

​

​

Hypocoytl

How many cotyledons?

🡪 2 cotyledons or Di-cotyledon

or Dicot (for short)

Radicle

“True root”

Hypocotyl

Epicotyl

“True stem”

Above

Below

cotyledons

cotyledons

4 of 66

Primary Stem Growth

Shoot Apical Meristem (SAM) creates 3 meristems

    • Procambium: creates vascular tissues
    • Ground meristem: creates ground tissues
    • Protoderm: creates epidermis

Axillary

bud

Stem

SAM

Axillary

bud

Stem

Young

leaf

Older

leaf

Leaf (stalk)

Axil of

the leaf

5 of 66

Leaf

Leaf

Leaf

Leaf

leaf

Apical

meristem

Leaf

Leaf

Leaf

Leaf

Axillary

buds

Node = location on stem where leaves are attached

​

Internode = in between nodes

node

Internode

Node

Internode

Node

Internode

Node

Shoot Morphology

Alternate arrangement

  • 1 leaf per node

Opposite arrangement

  • 2 leaves per node

Leaf

Leaf

Leaf

Leaf

Whorled arrangement

  • 3+ leaves per node

1

2

3

1

2

1

1

1

Stems, Leaves, and Buds

Summer 2022

6 of 66

Axillary

bud

Stems, Leaves, and Buds

Fall 2022

Terminal

bud

Unopened bud

Leaf scar

Unopened bud

Leaf scar

Leaf scar

Unopened bud

7 of 66

Stems, Leaves, and Buds

Winter 2022-2023

Terminal

bud

Node

Node

Internode

Leaf scar

Unopened bud

Leaf scar

Can you tell if this plant is alternate, opposite, or whorled?

Unopened bud

8 of 66

Stems, Leaves and Buds

Spring 2023

Apical

meristems

Bud scale

scars

No bud

Leaf

Leaf

Branch (stem)

Unopened bud

Leaf scar

Leaf scar

9 of 66

←Leaf scar

Branch (stem)

←Unopened bud

←Leaf scar

Apical

meristems

No

bud

Leaf

leaf

Axillary

buds

Leaf

Stems, Leaves and Buds

Summer 2023

Bud scale

scars

10 of 66

  • Cuticle/Epidermis: protects tissues
  • No pith or cortex
  • Vascular Bundles
    • 1º xylem and 1º phloem

Monocot Stem Anatomy

Monocots stems have a vascular pattern called an atactostele

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

phloem

xylem

sclerenchyma

11 of 66

  • Cuticle/Epidermis: protects tissues
  • Pith/Cortex: parenchyma cells that store materials
  • Cambium (in woody species): creates 2º tissues
  • Vascular Cylinder
    • 1º xylem and 1º phloem

Eudicot Stem Anatomy

Eudicot stems have a vascular pattern called an eustele (ring of xylem & phloem)

pith

What group had a eustele last session?

12 of 66

Secondary Stem Growth

Epidermis

Cortex / Pith

1º xylem

2º xylem

Vascular cambium

2º phloem

1º phloem

Vascular Cambium creates

2º xylem and 2º phloem

13 of 66

Secondary Stem Growth

1º xylem and 1º phloem are pushed further apart

Cork cambium creates bark

Bark

Cortex / Pith

1º xylem

2º xylem

Vascular cambium

2º phloem

1º phloem

1º xylem

1º phloem

2º xylem

2º phloem

14 of 66

Secondary Stem Growth

1º phloem is crushed

Bark replaces cortex

Bark

Cortex / Pith

1º xylem

2º xylem

Vascular cambium

2º phloem

1º phloem

1º xylem

2º xylem

2º phloem

15 of 66

pith

cortex

1º xylem

1º phloem

sclerenchyma

Outside

stem

epidermis

What is happening at the cellular level during secondary growth?

16 of 66

Vascular cambium

forms between

primary xylem & phloem

epidermis

17 of 66

epidermis

18 of 66

Vascular cambium

divides (mitosis)

epidermis

19 of 66

One cell

develops into xylem

epidermis

One cell remains

as cambium cell

20 of 66

Secondary

xylem cell

(wood)

Vascular Cambium

epidermis

21 of 66

epidermis

22 of 66

One cell develops

into phloem

One cell remains

as cambium cell

epidermis

23 of 66

Vascular cambium

Secondary phloem

epidermis

24 of 66

epidermis

25 of 66

epidermis

26 of 66

epidermis

Vascular cambium

27 of 66

epidermis

28 of 66

epidermis

29 of 66

Cork

cambium

forms

epidermis

Vascular cambium

30 of 66

Cork

cambium

(also called the

Phellogen)

epidermis

Cork cambium

31 of 66

Cork

cambium

divides

epidermis

32 of 66

One cell

remains cork cambium

epidermis

One cell becomes phellem

Vascular cambium

Cork cambium

33 of 66

epidermis

34 of 66

One cell

remains

cork cambium

Periderm

epidermis

One cell becomes phelloderm

Cork cambium

35 of 66

epidermis

Vascular cambium

36 of 66

epidermis

Cork cambium

37 of 66

epidermis

38 of 66

Bark visible

Cork cambium

Vascular cambium

39 of 66

Bark visible

40 of 66

Bark visible

Cork cambium

41 of 66

Cork cambium

Phelloderm

Phellem

Periderm

1º phloem

1º xylem

sclerenchyma

2º xylem

(wood)

​

2º phloem

Vascular cambium

Cork cambium

Tissue Check

42 of 66

It’s Winter

Growth ceases…

Let’s go to

Spring

Vascular cambium

Cork cambium

43 of 66

Trees start to

grow with the

added rainfall

Vascular cambium

Cork cambium

44 of 66

These wood cells

tend to be large

These cells are

called

Early Wood

Vascular cambium

Cork cambium

45 of 66

Now, let’s go to

Summer

What does this

mean for cell size?

(compared to spring)

Vascular cambium

Cork cambium

46 of 66

Vascular cambium

Cork cambium

47 of 66

Now, let’s go to

Autumn

What impact will this

have on plant energy

and cell size?

Vascular cambium

Cork cambium

48 of 66

Vascular cambium

Cork cambium

49 of 66

These cells are

called Late Wood

Vascular cambium

Cork cambium

50 of 66

Dicot Stems

51 of 66

Bristlecone pine�(Pinus longaeva)

  • Oldest individual trees on Earth
  • Some are nearly 5,000 years old
  • Grow in western USA in dry, mountainous regions
  • Growth rings are very small
    • <1 mm each

1 mm

52 of 66

Monocot vs. Eudicot

  • Fibrous roots
  • Primary growth only in roots & stems
  • Non-hierarchical growth
  • Eustele in roots
  • Atactostele in stem
  • Tap root
  • Primary growth, and possibly secondary growth in roots & stems
  • Hierarchical organization
  • Protostele in roots
  • Eustele in stem

​

53 of 66

Specialized Stems

Above ground

  • Thorns
  • Stem tendrils
  • Cladophylls
  • Water storage
  • Stolons or runners

​

Underground

  • Rhizomes
  • Food storage
  • Corms

​

Stems that have functions other than transport and support

54 of 66

Thorns

  • Stems modified to be sharp and pointed
  • Function to protect against larger herbivores

e.g. Hawthorn (Craetagus), Firethorn

(Pyracantha), Citrus

​

Don’t confuse with…

  • Spines (sharp modified leaves)
  • Prickles (sharp modified hairs)

Spine

(=leaf)

Thorn

(=stem)

Berbis

spine

Rosa prickles

bud

Craetagus thorn

leaf scar

Pyracantha thorn

leaf

Leaf

scar

No

Leaf

scar

55 of 66

Stem Tendrils

  • Specialized stems used for climbing and support
    • e.g. Grape (Vitis), Virginia creeper (Parthenocissus), Passionflower (Passiflora)
    • Tendril is side shoot that appears from axillary bud

FYI - Don’t confuse with leaf tendrils (next week)

​

Vitis tendrils

Parthencissus tendrils

leaf

Stem

Tendril

Leaf

Tendril

Bud

56 of 66

Cladophylls or Cladodes or Phylloclades

  • Stems that appear to be leaf-like
  • Stems are photosynthetic

e.g. Prickly-pear cacti (Opuntia), Butcher’s

broom (Ruscus), Christmas cactus,

Ribbon plant (Homalcladium), Umbrella

pine (Sciadopitys)

​

Homalocladium

Opuntia

Ruscus

Sciadopitys

Cladophylls

(stems)

​

​

Leaves

(stunted)

Phyllocladus

57 of 66

Water Storage Stems

  • Above-ground stems with storage parenchyma cells with large vacuoles
  • Also called stem succulents
  • Found in arid / xeric environments
  • …or where water is limiting
  • e.g. Cacti, Baobab (Adansonia), Xmas Cactus (Schlumbergia)

​

​

Carnegiea

Echinocactus

Adansonia

Salicornia

Schlumbergera

Beaucarnea recurvata

Rhipsalis

58 of 66

Stolons & Rhizomes

  • Stems that creep horizontally (diagravitropic)
  • Stolons, “run” along the surface

e.g. Strawberry (Fragaria),

Hawkweed (Hieracium)

  • Rhizomes “creep” under soil

e.g. Herbaceous perennials

Fragaria

Davallia

Phyllostachys

59 of 66

Food Storage Stems

  • Modified rhizomes have parenchyma cells with amyloplasts
  • Called “stem tubers”

e.g. White potato (Solanum tuberosum),

Cyclamens (Cyclamen)

Solanum tuberosum

Cyclamen

60 of 66

Corms

  • Compressed underground shoot for storage
  • Functions as underground bud
  • Don’t confuse with bulbs (modified leaves)

e.g. Gladiolus, Crocus, Taro (Colocasia)

  • Pseudobulbs in orchids are corm-like, but aboveground

Colocasia

Orchid pseudobulb

Crocus

Gladiolus

61 of 66

Stem Lab

  • Dicot stem (x.s.): 
    • Epidermis, Cortex, Pith, Vascular cylinder (Xylem, Phloem)

​

  • Monocot stem (x.s.): 
    • Epidermis, Vascular bundles (Xylem, Phloem)

​

  • Tilia (Linden) 3-year old stem (x.s.): 
    • Periderm, Cork cambium, Secondary phloem, Vascular cambium, Secondary xylem (wood), Primary xylem, Pith

​

62 of 66

phloem

xylem

sclerenchyma

​

​

xylem

phloem

sclerenchyma

x100

x40

x400

x40

Dicot Stem (xs)

Monocot Stem (xs)

63 of 66

Periderm

Cork cambium

Secondary phloem

Secondary xylem (wood)

Vascular cambium

Pith

Year 1

Year 2

Year 3

Primary xylem

Primary xylem

x40

Woody Stem (xs)

64 of 66

Add’l Slides

65 of 66

Stems, Leaves, and Buds

Spring 2022

Terminal

bud

Node

Node

Internode

Unopened bud

Leaf scar

Leaf scar

66 of 66

Monocot & Dicot Stems

  • Cuticle/Epidermis: protects tissues
  • Pith/Cortex: parenchyma cells that store materials
  • Vascular Cylinder
    • 1º xylem and 1º phloem
  • Cambium: creates secondary tissues

Eudicot

Monocot