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Botany Reference Guide CII
582754793 (6 May 2017)
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Topic 1
Stems & Roots
Botany Reference Guide CII
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1.1 Stems
Stems of ‘typical’ dicotyledon plants have many features to recognise and
name. These features need to be identified in order to:

correctly prune trees and shrubs

properly select propagation material

identify plants, especially deciduous plants in winter.
Functions of stems
The functions of stems include:

supporting leaves, in a way that maximises light for photosynthesis

supporting flowers, in a way that facilitates pollination

supporting fruit, in a way that ensures seed dispersal

providing a pathway for transport of substances between leaves and
roots and roots and leaves

storage of food substances

photosynthesis in green (herbaceous) stems

a means of propagation.
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External features of a woody stem
1 - Terminal or apical bud
This is a bud located at the apex of a stem
or shoot. The growing point is either
enclosed within bud scales which are
modified leaves, or protected by newly
developing leaves.
4 - Node
The node is the region of the stem where
the leaf is attached and the axillary bud is
found in the junction of the leaf and the
stem. In some species there may be two
or more leaves attached at each node,
and consequently, the same number of
axillary buds as there are leaves.
5 - Internode
The region or portion of the stem between
two successive nodes is called the
internode.
2 - Axillary or lateral bud
This bud is located in the axil of the leaf
stem or leaf base and the stem.
6 - Leaf scar
This is the mark left on the stem after the
leaf falls either as a result of ageing and
senescence, or in winter if the plant is
deciduous. It is where the leaf stalk or
petiole joined the stem.
7 - Terminal bud scar
This scar is the mark left on the stem
where the terminal bud from a previous
season was in a dormant condition. The
terminal bud scar is made up of a number
of closely spaced scale scars where the
dormant bud was protected by bud scales
or overlapping immature leaves.
By counting the number of terminal bud
scars back down a stem we can tell the
age of the wood, which is important for
pruning and propagation of some plants.
8 - Lenticels
Lenticels are small pores in the bark of
the stem which allow for diffusion of gases
into and out of the stem. Even in winter
the living cells of a dormant plant have a
low rate of respiration continuing and this
requires oxygen and releases carbon
dioxide. Lenticels are found in many
young stems and are often a prominent
feature of deciduous species (such as
Betula species – silver birch and some
Prunus species).
3 - Branchlet
A branchlet is a side stem arising from a
lateral bud.
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Seasonal growth
The effects of seasonal growth can be observed in stems, especially woody
deciduous stems. When growth recommences in spring, the new shoot and
new leaves that form are classed as current season’s growth. This growth
continues through a stage of rapid growth lengthwise and then this growth
slows and a certain amount of thickening and ‘hardening’ occurs during the
summer months.
When autumn arrives, the leaves fall. Once the leaves have been shed, this
growth that in spring was classed as current season’s wood is now classed
as previous season’s wood or one-year old-wood.
Also notice how the wood is aged in terms of years of lengthwise growth,
which can be summarised as follows:
a) Current season’s growth
is the length of stem produced since the beginning of the growing
season. It is the length of stem between the terminal bud and the first
terminal bud scar and remains as current season’s growth while the
leaves are held on the plant.
b) Previous season’s growth (or one-year-old wood)
This refers to that section of stem which was produced in the
previous growing season.
c) Two year old wood
This is the section of stem produced two growing season’s ago.
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Stem firmness
Another characteristic of all stems which must be considered and which is
exceptionally important for plant propagation, is the ‘firmness’ of the stem
tissue.
The degree of ‘firmness’ allows you to decide if the wood is soft-tip (or
softwood), semi-hardwood or hardwood.
Soft - tip wood
Soft-tip wood is current season’s growth while it is still undergoing
longitudinal growth.
Semi-hardwood wood
This is the same current season’s growth which has stopped longitudinal
growth and is starting to thicken. You will often see the initial stages of bark
formation on this type of wood.
Hardwood wood
Hardwood wood is growth which has completed a full year’s growth. If the
wood is from a deciduous plant it is called hardwood; if the wood if from an
evergreen species it is called evergreen hardwood.
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Modified stems
We’ve looked at ‘typical’ aerial stems from woody dicotyledonous plants.
Many plant stems have evolved for special functions other than the usual
ones of support, conduction, propagation and production of new growth.



Aerial stems
o
erect
o
modified (cladode, thorn)
Supported stems
o
twining
o
climbing
o
scrambling
Prostrate stems
o


stolon (or runner)
Underground stems
o
basal plate of bulbs
o
corms
o
rhizomes
o
tubers
Aquatic stems
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1.2 Buds
Buds may enclose developing leaves and/or flowers. They consist of
meristematic tissues (that is, actively dividing cells) and undeveloped,
immature new shoot and/or flower organs.
Figure 1 - Leaf and flower buds
a. Sycamore, Acer
pseudoplatanus
e. Poppy, Papaver sp.
f.
Primrose Primula vulgaris
b. Hazel, Corylus avellana
g. Foxglove, Digitalis purpurea.
c. Ash, Fraxinus excelsior
d. Oak, Quercus robur
The bud is generally protected by small modified leaves known as bud
scales. Such a dormant structure with its protective scales is referred to as a
covered or protected bud.
Camellia species have well defined covered buds as do Fraxinus (Ash) and
Ulmus (Elm) as well as many other species.
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Figure 2 - A covered or protected vegetative bud
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Figure 3 - A covered or protected flower bud
If the protective scales are not present, the unprotected dormant apex is
called a naked bud. The young leaves afford some protection to the growing
tip, but will expand into normal leaves. With a covered bud the scales drop
when the bud opens and do not become normal leaves. A good example of
this is lettuce, a longitudinal section of which is shown below.
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Types of buds
Buds may be actively growing or dormant. There are three basic types:

vegetative which produce only leaves and stems

floral which produce only flowers

mixed which produce both vegetative organs and floral organs.
Vegetative buds
These buds consist mainly of immature leaves. In comparison with other
types of buds, vegetative buds are normally smaller in size, pointed and
darker in colour. Vegetative buds eventually develop into shoots and are
found as either terminal or lateral buds. The lateral vegetative buds of stone
fruit are often associated in groups with flower buds.
Flower buds
A flower bud consists mainly of a single immature flower. Flower buds can
be found in either the terminal or lateral position.
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Mixed buds
These buds are found on pome fruit trees (such as apples and pears). They
may be seen on the tips of laterals and spurs—spurs are short shoots found
as short lateral growth.
Mixed buds consist of immature flower and vegetative materials. They are
plump, being larger in size than vegetative buds.
Mixed buds produce flowers and shoots or leaves
Epicormic buds
The bud system in many Eucalyptus spp. and some other Australian native
species has some unusual features. In the axil of each leaf there are always
originally two buds. One is called a naked bud, in that it has no bud scales
and is evident always in the leaf axil. It is the source during the growing
season of the lateral branches. At the same time there is a second bud
concealed just below the bark - this bud is dormant and covered by petiole
and axillary tissue.
This bud does not develop unless the naked bud is aborted due to the
stresses of drought, severe seasons or fire. The concealed or epicormic
buds produce new growth in the season following the stress. They sprout to
form leafy shoots called epicormic shoots all over the trunk and main
branches. Epicormic buds can also be found in lignotubers which occur in
many Eucalyptus species. You may have noticed this type of regrowth after
a bush fire or severe storms. The morphology of the leaves of epicormic
growth is juvenile or similar to seedling leaves.
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Figure 4 - An epicormic bud
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1.3 Roots
The most neglected part of a plant is often the root system. Out of sight
means out of mind. Humans seem to think that they can mess as much as
they like with the conditions around a plant’s root system and then wonder
why plants fail to thrive.
The type of root system an established plant has often determines how well
it responds to transplanting and how well it withstands dry periods.
Very common situations affecting plant root systems include:

excavations for trenches or swimming pools

changes of soil level, terracing

compacted, denatured or contaminated soils,

areas of paving

lack of water, air or nutrients in the soil

large plants, inappropriately placed, without sufficient space.
The maintenance of a healthy root system is of vital importance to any
person who is growing plants, whether in agriculture, horticulture or
conservation and land management as it means optimal plant growth and
health.
Types of root systems
The main types of roots are:

Tap roots

Fibrous roots

Adventitious roots.
While these absorb water and nutrients and anchor the plant, there are
modifications which are adaptations for different functions, such as storage,
climbing or additional support for very large trees.
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Tap root system
This is a root system with one
primary root (main root) and a
secondary root system (minor
roots). It is the type of root system
formed in dicotyledonous and
coniferous plants which have been
grown from seed.
Although dicotyledons have
taproots in their early stages of
growth, these do not persist forever,
and large trees have a radiating
system of woody roots confined to
the topsoil (where the air is). They
do not usually have a big woody
taproot plunging to the centre of the
earth!
Fibrous root system
This is a root system with no single primary root. It is
the type of root system typical of and found in
monocotyledonous plants which have been grown from
seed.
Adventitious roots
Adventitious roots are those roots arising from above ground portions of a stem or from stolons, rhizomes,
corms and soon. They do not arise directly from a seed
as do tap and fibrous root systems. Adventitious roots
are also found occurring naturally on plants such as
Hedera spp. (ivy), Philodendron and many grasses such
as kikuyu and couch.
Adventitious roots which arise on dicotyledonous plants
propagated vegetatively (ie by cuttings or layering) and will give rise to the
appearance of a fibrous root system. This does not mean that these plants
are no longer classified as a dicotyledon.
Root structure - external
Primary roots have soft herbaceous tissue with root hairs at a particular
distance back from the tip, and a protective root cap shielding the delicate
growing tip as it pushes through the soil particles, actively growing out into
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soil between particles and absorbing water and nutrients. If you have ever
eaten bean sprouts then you know what these primary roots are like.
When dicotyledons become woody there is an increase in diameter of roots,
lateral roots are produced, which also become woody, and the external
epidermis of the primary root is replaced by a cork layer which is impervious
to water. The tips of these woody roots taper to thin soft primary growth
which is able to absorb water and nutrients. The woody parts of roots
function for anchorage, transport and storage; they are not active in water
uptake.
So in large woody trees and shrubs, the woody roots provide anchorage and
storage, while the herbaceous root tips and feeder roots take up water and
nutrients.
Development of root hairs
In young roots the epidermis is specialised as an absorbing tissue and
usually bears root hairs which are tubular extensions of epidermal cells. This
region is known as the root hair zone and occurs in the zone of
differentiation region.
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The root hair and the epidermal cell from which it grows constitutes a single
cell. In most plants the life of any one root hair is short and may function
anything from a few days to a few weeks depending upon the species. New
hairs are constantly forming at the anterior end of the root hair zone, whilst
those at the posterior end are dying. Thus, as the root advances through
soil, fresh and actively growing root hairs are constantly coming into contact
with soil particles.
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Root modifications
The tap, fibrous and adventitious root systems are the basic types found in
flowering plants. However, well as these there are some modifications that
are commonly seen:


Adventitious roots: these arise from stems or leaves. Several types of
adventitious roots commonly seen are:
o
prop or stilt roots - arising from the above ground stem and
growing down into the soil to give the plant extra support. For
example, Monstera, Moreton Bay figs, Pandanus, Maize.
o
climbing roots - arising from aerial stems, allowing the stem to
cling to, and climb, walls, tree trunks etc. For example, Ivy,
Parthenocissus sp., kikuyu and couch grasses.
o
root systems on plants propagated by stem or leaf cuttings.
Storage roots -large swollen taproots modified to store large amounts
of carbohydrates. For example, carrots, turnips, beetroots. Dahlia spp.
have tuberous storage roots, which are not taproots, but arise each
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season at the base of the plant.

Parasitic roots - which penetrate the stem of the host plant absorbing
its water and nutrients. For example, mistletoes, dodder.

Buttress roots - wide flanges of stem and root tissue which develop at
the base of the trunk of many rainforest species to help support the
tree.

Breathing roots -found in a number of different forms on mangrove
species which grow in tidal flats where the soil is regularly inundated
by salt water. Also found in some freshwater lake edge species.
storage
prop
aerial
parasitic
Epiphytic roots
Roots of plants which grow attached to
other plants
Examples: Zygocactus, Bromeliads &
Epiphytic orchid
Parasitic
Roots of parasitic plants grow into other plants to take their food supplies.
Mistletoe
Example: Dodder
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Buttress roots
Formed by the asymmetrical growth
of the base of the main roots and
gives additional support to the plant.
Examples: Moreton Bay Fig
Contractile roots
Roots which are capable of
contracting c pulling plant down
through the soil; usually last only 2
to 3 months. May be found at the
top of some tap roots, and on many
bulbous, cormous roots.
Examples: Dandelion & Gladiolus
Stilt
A cluster of roots which develop from the first
one or two nodes above the ground – they grow
downwards into the soil giving additional
support to the stems.
Example: Corn
Prop
Formed from the main branches of a
tree – they grow straight downwards
to the soil which they enter and give
absorbing water and nutrients.
Examples: Ficus bergalensis &
Pandanas
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