WHMF121 Session Seven - · PDF filethe axillary bud (in the leaf axil) ... proliferation oOn some stems, like ... o A well known tuber is the potato

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WHMF121

Session Seven

Plant Morphology

Stems


Todays Topic

o Plant stems:

• Stem development

• Vascular bundles

• Modified stems

• Function of stems


Stemso When a seed

germinates the

radicle grows

downwards

and develops

into the roots,

and the

plumule grows

upwards and

develops into

the stem. We often may think of stems as fragile structures, however, think

upon the fact that the trunk of this magnificent redwood is a stem,

that would take an entire football team hand to hand to encircle.

This specimen is circa 180m high and almost 2000 years old.

(http://en.wikipedia.org/wiki/Plant_stem)

http://en.wikipedia.org/wiki/Plant_stem


Dicotyledon Stems

o Development:

• Epigeous or Epigeal - e.g. garden beans video

• The cotyledons may be carried above the ground during

germination as the stem grows

• Hypogeous or Hypogeal - e.g. pea video

• The cotyledons remain underground within the seed coat

and the stem grows up from them. (Refer to Session 3 - Germination)

In both cases the cotyledons provide nourishment for the

plant until it grows foliage leaves that can photosynthesise.


Monocotyledon Stems

o Development:

• Epigeous or Epigeal - e.g. onion Allium cepa.

• The cotyledon elongates, carrying the seed coat up

with it and the foliage leaves develop near the base.

• Hypogeous or Hypogeal - e.g. corn Zea mays

• The seed coat and the first foliage leaves emerge

directly from the seed surrounded by the coleoptile.

• Video of corn developing a stemhttps://www.youtube.com/watch?v=rp-jYOQBLtk

Permission given from Neil Bromhall

https://www.youtube.com/watch?v=rp-jYOQBLtk

https://www.youtube.com/user/neilbromhall


Vascular Bundles

Monocots:

o Vascular bundles are present and scattered throughout

stem, although concentrated towards the outside.

Dicots:

o Vascular bundles differ as a they form in a ring towards to

outer part of the stem - often none in the centre.

o This is true to some extent of almost all monocots. Monocots

rarely produce secondary growth and are therefore seldom

woody, with Palms and Bamboo being the exceptions.


Stems

Roystonea regia (Royal palm)

(Aracaceae family)Phyllostachys bambusoides

(Bamboo) (Poaceae family)(http://en.wikipedia.org/wiki/Roystonea_regia)

(http://en.wikipedia.org/wiki/File:Poaceae_-

_Phyllostachis_bambusoides.JPG)

http://en.wikipedia.org/wiki/Roystonea_regia

http://en.wikipedia.org/wiki/File:Poaceae_-_Phyllostachis_bambusoides.JPG


Stems are needed for

transporting plant requirements

Phloem

• Pushes the sugars and other molecules created by the plant through photosynthesis down to roots

• Phloem is always alive

Xylem

• Pulls moisture up from the roots

• This tissue dies after one year and then develops anew (rings in the tree trunk)

• Becomes the wood


Stem Cross-Section of a Dicot

(http://en.wikipedia.org/wiki/Plant_stem)

Flax stem cross-

section, showing

locations of underlying

tissues

Ep = epidermis

C = cortex

BF = bast fibres

P = phloem

X = xylem

Pi = pith

http://en.wikipedia.org/wiki/Plant_stem


(https://www.flickr.com/photos/blueridgekitties/4427414696)

Zea mays stem

(left) and root

(right) cross

sections

Observe the

difference in the

arrangement of

the vascular

bundles

https://www.flickr.com/photos/blueridgekitties/4427414696


Stems

o The terms stems and shoots are sometimes used interchangeably but it must be remembered that the stem is the axis. (This will be especially relevant when looking at flowers).

oStems have nodes.

o The shoot is the stem from which any buds, new stems, leaves and flowers emerge from.

oAll flowering plants have the same basic stem structure, whether they are dicots or monocots.

(Mauseth, 2014)


Stems

o Stems grow in a series of nodes and

internodes

o At the nodes, the stem tends to

widen a bit and have the appearance

of a joint

o Leaves grow at the nodes of a stem

o The angle between the stem and the

leaf is called the leaf axil

(http://en.wikipedia.org/wiki/File:Pla

nt_nodes_c.jpg)

http://en.wikipedia.org/wiki/File:Plant_nodes_c.jpg


Stems

o Lateral stems or branches

may also grow at a node,

from the axillary bud (in

the leaf axil)

o If anything were to

happen to the leaf, the

axillary bud would start to

develop

Texas A&M Bioinformatics Working Group 2010

(http://botany.csdl.tamu.edu/FLORA/tfplab/axil.jpg)

http://botany.csdl.tamu.edu/FLORA/tfplab/axil.jpg


Stems

oNodes are a point of

proliferation

oOn some stems, like

rhizomes and stolons,

roots as well as stems

grow at nodes giving rise

to a new plant

o Internodes are the areas

of stem between the

nodes

(http://en.wikipedia.org/wiki/File:Plant_nodes_c.jpg)

http://en.wikipedia.org/wiki/File:Plant_nodes_c.jpg


Stems

Node of a Monocot

(http://en.wikipedia.org/wiki/Arundo_donax)

http://en.wikipedia.org/wiki/Arundo_donax


Stems

o Like roots, stems do most of their growing near the tip.

o New leaves and branches start to develop in minute

form in the bud - the tip of a stem or shoot.

o As the tip grows the leaves develop and mature.

o Two aspects to stem growth:

• Development of structures at the nodes

• Elongation of the internodes.

o At this growing tip the nodes are developing directly

one after another.


Stems

o The stem tip has an inhibiting effect on lateral stem growth.

o This is called apical dominance

• Cutting off the tops of plants makes them grow bushier.

o The inhibiting effect is decreased the further the growing tip

is from a node.

o Therefore as the main stem grows,

lower nodes are able to develop

lateral stems or branches.

(https://en.wikipedia.org/

wiki/Apical_dominance)

https://en.wikipedia.org/wiki/Apical_dominance


A hedge is an excellent

example of apical

dominance. As the plant

stems grow up towards the

light, trimming them promotes

lateral growth and makes the

plant bushier. Hedges are

often used as decorative

ornamental features in a

garden, but also as wind

breaks for farmers crops.

Stems

(https://au.pinterest.com/pin/455426581045626523/)

https://au.pinterest.com/pin/455426581045626523/


Stems

o Most plants depend on light for nourishment

o One of the functions of stems is to hold the leaves up to the light

o Like roots, stems have a mechanism for directing their growth

o Root growth is directed by gravity (geotropism)

o Stem growth is directed by light (phototropism)

(http://jjmbio4.blogspot.com.au/)

http://jjmbio4.blogspot.com.au/


Stems

oThe tips of the stems contain the hormone auxin which regulates plant growth by causing cells to grow longer.

oAuxin is involved in phototropism.

oIf a stem begins to grow away from the light, light will fall unevenly on the stem• Auxin migrates to the darker side of the stem, so the

cells will lengthen at a faster rate than those on the light side and the stem tip will eventually point upwards.

Permission granted by CSIRO to show video – link?


Types of Modified StemsThe modified stems fall into categories:

Aerial stems

- Tendrils

- Thorns

- Cladodes

Sub-aerial stems

- Stolon

- Runners

Underground stems

- Bulbs

- Corms

- Rhizomes

- Tubers


Stolon

• Stolons are modified lateral stems that grow across the ground and can

send out roots and new shoots.

• Many grasses and other plants like potatoes, violets and strawberries have

stolon.

• Stolons enable plants to perform vegetative or asexual reproduction, i.e.

offspring are formed without the production and union of sex cells.

• Offspring from asexual reproduction are genetically identical to the parent

plant.

• A common example is Fragaria spp. (strawberry).


Rhizomeso Rhizome comes from the Greek meaning mass of

roots

o Rhizomes are modified underground stems.

o Like stolons they are very common in grasses

and occur in many other plants.

o They vary in size and appearance.

o Some plants have thick, fleshy rhizomes like

Zingiber officinale (ginger) but others like Urtica

spp (nettle) and Symphytum officinale (comfrey)

do not.


Rhizomes

o Each node can send out roots

and stems and develop into a

whole new plant.

o Rhizomes may look like roots

since they grow underground

o Roots - do not have nodes with

leaves or axillary buds and

lateral stems.

o Rhizomes do have these

structures though they often

look different to those of a

typical stem.

(http://commons.wikimedia.org/wiki/Zingiber_officinale#mediaviewe

r/File:Zingiber_officinale_fresh_rhizome.JPG)

http://commons.wikimedia.org/wiki/Zingiber_officinale#mediaviewer/File:Zingiber_officinale_fresh_rhizome.JPG


Tubers

o Tubers are underground stems

especially adapted for the storage of

starch.

o A well known tuber is the potato (Solanum tuberosum).

It grows from the end of either a stolon or a rhizome of

the potato plant.

o The “eyes” of potatoes are the axils of tiny, scale-like

leaves and contain axillary buds from which lateral

stems may grow.

o Root tubers also exist but they do not have leaves or

buds since roots do not have nodes.


Bulbs

o Bulbs consist of a small central stem surrounded by modified, fleshy leaves.

o The leaves are scaly above and thickened at the base where food is stored.

o They are often found just under the earth and adventitious roots grow from the bottom of the stem.

o Bulbs serve as storage and propagative organs.

o After the growing season the foliage leaves wither and

die.


Bulbs

o The energy from them is drawn

down into the bulb and stored

through the winter to be used

when the plant grows again.

o Plants in the Amaryllidaceae

family and related families

often have bulbs e.g. Allium

cepa (onion) and Allium

sativum (garlic).

(http://en.wikipedia.org/wiki/Onion)

http://en.wikipedia.org/wiki/Onion


Corms

o Corms are like bulbs except the

storage organ is the fleshy stem

which is surrounded by thin scales,

the bases of last season’s harvest.

o Like bulbs, corms are found just

under the ground and have

adventitious roots growing from the

base of the stem.

(http://commons.wikimedia.org/wiki/File:Corm.jpg)

http://commons.wikimedia.org/wiki/File:Corm.jpg


Corms

• Colchicum autumnale – Autumn

Crocus

• Once a corm has flowered it

never flowers again and starts to

develop new corms for next

season as the old corm starts to

die off

(http://commons.wikimedia.org/wiki/File:Colchicum_autumn

ale_ENBLA05.jpeg)

http://commons.wikimedia.org/wiki/File:Colchicum_autumnale_ENBLA05.jpeg


Spot the Difference?

Corm• Solid mass of stem tissue

Bulb• The inner leaves are fleshy

while the outer ones are dry


Cladodeso In most plants photosynthesis is performed by the leaves,

but in some the stem has evolved to be the photosynthetic organ.

o These stems are green because they contain chlorophyll and are called cladodes.

o Equisteum arvense (horsetail) and Ephedra sinica (ma huang) are medicinal cladodes

o .

o Casuarina spp. (she-oaks) and Opuntia spp. (prickly pear)

are also plants with cladodes.


Cladodes

Aerial modifications

o The leaves of Opuntia spp.

have evolved to be thorns or

spines

o Ephedra sinica & Equisteum

arvense - the leaves are like

scales at the nodes of the

stem.

Equisteum arvense

(Photograph with permission - Wendy Williams)


Cladodes

(http://en.wikipedia.org/wiki/File:Prickly_Pear_Closeup.jpg)

Opuntia spp. (Prickly Pear)

http://en.wikipedia.org/wiki/File:Prickly_Pear_Closeup.jpg


Functions of Stems

• 1 - Support

• Stems hold up the aerial parts of the

plant and orient the leaves towards

light.

• Tendrils are sometimes modified

stems. Passiflora spp. (passionfruit

vines) have tendrils at the ends of

lateral stems that twist around

whatever they encounter and hold the

plant up.

Many plants employ tendrils to

provide added support.

Of the herbal medicines we

use, Passiflora incarnata is an

example of one that utilises

tendrils.(http://en.wikipedia.org/wiki/File:Vine.jpg)

http://en.wikipedia.org/wiki/File:Vine.jpg


Functions of Stems2 - Conduction

o Like roots, stems contain vascular tissues that transport

fluids around the plant body.

o Remember the two kinds of vascular tissue, xylem and

phloem, they run next to each other all through the plant.

o When water is absorbed into the roots it is passed to the

xylem and drawn up through the roots and stems by

evaporation of moisture from the leaves.

o This fluid reaches the leaves and is enriched by the

glucose produced during photosynthesis.

o This enriched sap is then carried around the plant by the

phloem.


Observing Conduction with Food Dye

(Image Credit: ECNH Lecturer, Tanya Quod, 2013)


Stems

A cross section of Apium graveoluns (Celery) Apiaceae(http://en.wikipedia.org/wiki/File:Celery_cross_section.jpg)

http://en.wikipedia.org/wiki/File:Celery_cross_section.jpg


Functions of Stemso The xylem typically lies close to the axis with phloem

away from the axis.

o The xylem is closer to the centre of the stem or root while

the phloem is closer to the exterior.

o In a leaf, the xylem will usually be the upper side, with the

phloem on the lower side.

o This is why aphids are typically found on the underside of

a leaf rather than on the top - the sugars manufactured by

the plant are transported by the phloem, which is closer to

the lower surface.

o The position of vascular bundles relative to each other

may vary considerably


Aphids Under Leaf

(http://commons.wikimedia.org/wiki/File:Aphi

ds_and_live_young_under_Sycamore_leaf.J

PG)

(http://en.wikipedia.org/wiki/File:Illinoia_pepperi_-_blueberry_aphid.jpg)

http://commons.wikimedia.org/wiki/File:Aphids_and_live_young_under_Sycamore_leaf.JPG

http://en.wikipedia.org/wiki/File:Illinoia_pepperi_-_blueberry_aphid.jpg


Tree Trunk Cross Section

(http://commons.wikimedia.org/wiki/File:Tree_Trunk_Cross_Section_-_Kolkata_2011-06-04_3688.JPG)

Tree Music https://www.youtube.com/watch?v=ZYLaPVi_I2U&list=PLWyhhrElFdw9xRShweZLOwgywyJ4fGkZI&index=1

http://commons.wikimedia.org/wiki/File:Tree_Trunk_Cross_Section_-_Kolkata_2011-06-04_3688.JPG

https://www.youtube.com/watch?v=ZYLaPVi_I2U&list=PLWyhhrElFdw9xRShweZLOwgywyJ4fGkZI&index=1


Functions of Stems

3. Storage

o In many plants energy produced by the leaves during

warmer months is stored over winter in modified

stems (e.g. bulbs, corns, tubers, rhizomes).

o This allows the plant to become dormant over the

colder months.

o When spring returns this energy is used to grow new

leaves and roots.


Functions of Stems

o Storage in these stems is different

to storage in roots because the

energy is stored to allow dormancy

than simply to be used later.

o Other storage stems include fleshy

or bulbous stems like celery, and

succulent stems that can store

water.

(Image Credit: ECNH Lecturer,

Tanya Quod, 2013)


The Boab…prime storage area!

Adansonia gregorii

(http://en.wikipedia.org/wiki/File:Derby_boab,_Western_Australia.jpg)

http://en.wikipedia.org/wiki/File:Derby_boab,_Western_Australia.jpg


Functions of Stems4. Propagation

o Most stem modifications seem to have evolved for this

purpose.

o Rhizomes and stolons can grow new plants at the

nodes, and bulbs and divide over winter so more

plants are produced in spring.

o Tubers store a lot of starch and can grow new roots

and shoots from the eyes.


Propagation

Propagation by stem cuttings


Tutorial Session

o Practical: Check on your monocotyledon and

dicotyledon seeds germinating in the cottonwool

and notice the changes since last week

o Participate in the class exercises (following slides)

regarding stems


Practical Session

o Draw a germinating plant. Label cotyledons,

roots, stem.

o Draw a typical stem and label the nodes,

internodes, leaf axil and axillary bud.

o Draw a cross section of a celery stalk. Label

xylem.


Practical Session

• Draw a stolon.

• Label main stem, stolon, nodes and internodes.• Refer to p. 123 of text book (Capon)


Practical Session

(Image Credit: ECNH Lecturer, Tanya Quod, 2013)

• Draw Zingiber off.

(Ginger) Rhizome

• Label the nodes,

internodes, axillary

buds and scale like

leaves


Practical Session

• Draw Solanum

tuberosum L. (Potato)

tuber

• Label the attachment

scar, node, leaf, leaf

axil, axillary bud

(http://commons.wikimedia.org/wiki/Category:Potato_sprouts#mediaviewer/Fil

e:Potato_sprouts.jpg)

http://commons.wikimedia.org/wiki/Category:Potato_sprouts#mediaviewer/File:Potato_sprouts.jpg


Practical Session

• Cut Allium cepa (onion) in half

lengthwise and draw bulb

• Label roots, stem and leaves

(http://commons.wikimedia.org/wiki/File:Red_onion_cut.jpg)

http://commons.wikimedia.org/wiki/File:Red_onion_cut.jpg


Next Week

o Preparation:

• Read through the slides for session 9 on leaves

• Read the section in your textbook on leaves

• Bring some bunches of leaves if you have access to them so we

can look more closely at them and draw some diagrams


Suggested Readings

Capon, B. (2010). Botany for gardeners (pp. 41-50). Portland, OR:

Timber Press.

Clarke, I., & Lee, H. (1987). Name that flower: The identification

of flowering plants (p. 41). Carlton, Vic: Melbourne University

Press.

Mauseth, J. (2014). Botany: An introduction to plant biology (5th

ed.). MASS: Jones & Bartlett Publishers.

Tan, E. (2013). Botany of the flowering plants (4th ed.). (pp. 23-30).

Preston, Vic: Northern Melbourne Institute of TAFE.


References

Clarke, I., & Lee, H. (1987). Name that flower: The identification of flowering

plants. Carlton, Vic: Melbourne University Press.

Tan, E. (2004). Herbal preparations laboratory manual. Victoria, Australia:

Northern Melbourne Institute of TAFE.

Tan, E. (2013). Botany of the flowering plants (4th ed.). Preston, Vic: Northern

Melbourne Institute of TAFE.

Wohlmuth, H. (1992). An introduction to botany and plant identification (2nd

ed.). Lismore, NSW: MacPlatypus Productions.


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Documents

WHMF121 Session Seven - · PDF filethe axillary bud (in the leaf axil) ... proliferation oOn some stems, like ... o A well known tuber is the potato