External Parts of the Plant
ROOTS
Function
1. Anchor the plant in the soil
2. Absorb water and minerals
3. Store excess food for future needs
Development of the root system
The root system develops from the radicle (which emerges from the
germinating seed) by a process of growth from the within the root tip (this
area is known as the apical meristem). The cells subsequently elongate,
pushing the root tip through the soil. A root cap is formed ahead of the
meristem to protect it. The root cap cells are continually being rubbed off
but are quickly replaced.
Branching of the roots (lateral roots) develop from an area deep within the
root (the pericycle) in the slightly older root sections some distance from
the tip. They tend to grow at right angles to it to better explore other
regions of soil. The lateral roots then develop their own branch roots.
Terms relating to roots
Primary
The system that develops directly from the radicle.
Plants may have a taproot system, a fibrous root system or a combination
of both.
Secondary
Lateral roots that originate from deep within the root and grow out through
the cells of the root into the soil. This occurs in both fibrous and tap root
systems.
Taproot
Develop directly from the radicle. Rapidly-growing roots with little branching
with the aim of providing anchorage and accessing deep moisture and
minerals. Tap roots are also able to store larger quantities of food than
fibrous roots. Characteristic of conifers in temperate regions, and
vegetable biennials such as carrots, and other herbaceous perennials and
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weeds. Has survival value in areas experiencing periods of surface soil
dryness.
Fibrous
Multiple thin branching roots close to the surface. The shallow structure
allows roots to quickly obtain water before it evaporates. The mat-like root
system can help reduce soil erosion. Grass is very effective in this regard.
Depend on moisture and nutrients close to the surface. Food storage
capacity is less than in tap roots. Develops from the lower part of stem in
grasses following death of the radicle
Adventitious roots
A root arising in an unexpected position. This term is used to describe the
roots produced along the stem of Hedera helix (ivy) to help it climb, and
roots produced by leaf or stem cuttings. Stolons and rhizomes (modified
stems) also produce adventitious roots.
External structure of root tip; role of root cap and root hairs
The root tip pushes down through the earth and as such is subject to
abrasion from soil particles. To prevent damage to the root tip, it is
protected by a structure called the root cap. Root cap cells are readily
rubbed off but are quickly replaced from within. When the cells are ruptured
they form a slimy coat (mucigel), lubricating the root tip as it moves through
the soil and so aiding penetration of the soil
The root hairs absorb water and soil nutrients. They increase by several
100-fold the root's absorptive surface area, thus increasing the rate of
water and mineral uptake. Older root hairs die off as new root hairs are
produced, so the number of root hairs remains constant.
Root hairs are easily broken when a root is dug from the soil or moved
from one pot to another. However they are readily produced and new ones
will replace the damaged or dead ones. It is, therefore, understandable that
repotting must be done with care, because the root hair cells are pulled off
for the most part. This is why planting-out leaves the plant withered for
some time.
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Adaptations of the root
Tubers
Note that some tubers, such as potatoes are stem tubers. (i.e. they are
modified stems, not roots)
Some plants, such as Dahlia, develop root tubers, a food storage organ
that also possesses adventitious buds, which produce stems.
Aerial roots
Roots which are above ground. These are found in various tropical genera
of humid regions, such as Monstera deliciosa and combine climbing and
water absorbing capability. In Taxodium distichum, the Swamp Cypress,
roots extend above the water line to absorb atmospheric oxygen.
Epiphytic orchids possess aerial roots to collect water vapour from the
atmosphere as well as rain.
Aerial roots also arise on plants such as corn (known as prop roots) and
tropical trees (known as buttress roots) to stabilise the plant.
Roots may also be used to store water. A plant which has developed
tissues for the storage of water is known as a succulent. Agave is a genus
of succulent that stores water in the roots as do some members of the
Pumpkin family.
STEM
Functions
1.
2.
3.
4.
5.
Support
Transport of water and nutrients
Storage of food
Protection
Manufacture of sugars (photosynthesis)
Development of the stem
The stem develops from the plumule (the stem of a germinating seed) and
continues growing as a single stem. The stem's growing tip, found in the
apical bud, makes the stem grow longer (by cell division and elongation)
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and is responsible for arranging the leaves on the stem and provides for
development of branches.
When the stem is being formed it divides into nodes and clear stem
sections between nodes called internodes Leaves are produced at the
node. Here the stem is slightly thicker and a discernible ridge may be
noticed in older stems.
When the stem grows, the internodes stretch and spread the leaves apart.
In seedlings, a lack of light leads to an elongation of the internodes,
leading to weak, spindly growth.
Types of buds
The bud at the top of the stem is known as the apical bud, the buds in the
leaf axils are know as axillary buds. A large percentage of axillary buds
tend to remain dormant unless the apical bud is damaged say by frost or
disease or removed. In horticulture, pinching-out of the apical bud
stimulates the axillary buds to produce a bushier plant.
Buds found in unexpected places, such as the root, are known as
adventitious buds. An example is a sucker of blackberry and raspberry
which is an upright shoot arising from a horizontal root.
Adaptations of the stem
Hairs
Hairs might have a variety of functions. In the scrambling weed 'cleaver'
(goosegrass) Galium aparine, the hairs on the stem stick to other plants.
The feeling of ‘stickiness’ in many plants is actually a covering of fine hairs.
In some cacti, hairiness might function as a reflective surface to reduce
absorption of sunlight, or as a method of trapping moisture to maintain a
humid microclimate around the leaf.
Thorns
Thorns may have either a protective function say from animals or a
climbing function where they act as supportive hooks for long branches.
Thorns are modified stems branches growing from axillary buds,
terminating in sharp, hard points e.g. Crataegus monogyna (hawthorn),
Pyracantha atalantoides
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Note that spines – as in cactus spines, are modified leaves, and that rose
‘thorns’ are technically prickles, which are woody extensions of the outer
stem.
Corms
A short, swollen, vertical, underground stem evolved to store food and
produce flower and leaf buds.
For example Crocosmia ‘Lucifer’, Crocus vernus
Tubers
The tuber itself is the swollen tip of a rhizome, and stores food to provide
energy for the development of the plant, and possesses axillary buds and
internodes.
For example Solanum tuberosum (potato)
Stolon
A horizontal stem growing overground with the capacity to produce
adventitious roots from the node, which root, with the subsequent
development of a plantlet at the node.
For example Fragaria xananassa (strawberry), Ajuga reptans
Rhizomes
Examples: Iris germanica, Elymus repens (couch grass). An elongated
horizontal stem modified for food and water storage, fully or partially
underground, producing adventitious roots, flower stems and leaves.
LEAF
Function
1. Production of sugars (photosynthesis)
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Structure of the Leaf
The leaf of many garden plants comprises a stalk, known as the petiole
and a blade, the lamina. The main central vein, really an extension of the
leaf stalk, is known as the midrib.
Some plants, for example Streptocarpus, have a very defined midrib with
lateral veins branching off it. However other plants have what is known as
a reticulate or net-like veinal system.
Veins composed of water and food conducting vessels (xylem and phloem)
traverse the petiole and form a network throughout the lamina. The lamina
is supported by the veins which carry water, mineral salts and food to and
from the leaf.
The petiole improves the photosynthetic efficiency of the leaf by extending
the lamina away from the stem so reducing shading form other leaves and
allows the lamina to move in response to air currents which brings in more
carbon dioxide for manufacturing sugars (photosynthesis).
Variations in the shape, size form and colour of leaves
There are many terms used to describe different leaf shapes;
lanceolate
ovate
elliptic
cordate
saggitate
pinnate
bipinnate
palmate
compound palmate
The following describe leaf margin shapes
entire
lobed
serrate
Leaf colour
Leaves are usually green because of the pigment, chlorophyll contained
within. However, the leaf contains other colourings: anthocyanins,
xanthophylls, carotenes, which are masked by the chlorophyll, but which
become apparent in deciduous trees in the autumn, when chlorophyll is
withdrawn prior to leaf drop.
Many species have forms or cultivars in which the leaves are coloured
other than green. In this instance the dominance of the colour is obscuring
the chlorophyll, which nonetheless is present.
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Although unusual in species, there are many cultivars with variegated
leaves. Variegation may be marginal, medial or dispersed. In variegated
leaves, photosynthesis is restricted to the green parts of the leaf. e.g.
Euonymus 'Emerald 'n' Gold'
Size
Large leaves tend to be characteristic of marsh plants, because water loss
in these large leaves is not a problem as they are living in a moist
environment.
Large leaves also tend be characteristic of forest plants, and small leaves
more characteristic of plants inhabiting sunny open places where water
loss can be very high. Cacti for example, have their leaves reduced to the
size of spines to reduce water loss.
It is generally true that the larger the leaf of a rhododendron, the deeper in
a forest it naturally grows.
The alpine Geranium cinereum has tiny leaves, whereas the wood-dwelling
G. sylvestris has much larger leaves.
Leaf adaptations
Climate
Many leaf adaptions have survival value in particular climates.
The leaves of cacti have been reduced to spines. Whiteness caused by
colouration of cells, reflects light keeping the leaf cool. Hairs may trap
moisture and reduce water loss. Leaves may have a thick shiny cuticle to
reflect light and reduce water loss.
Conifer leaves are reduced to needles, with a thick waxy coating and
sunken pores (stomata), which reduces water loss (transpiration) in
exposed windy situations. e.g. Pinus sylvestris
Bulbs
A bulb is a modified stem structure in which starch accumulates in the
thickened fleshy leaf bases attached to the stem. Bulbs of, for example, the
Narcissus (daffodil) are formed by food being passed down and stored in
the leaf bases, which swell.
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Tendrils
A slender coiling structure that attaches a climbing plant to a supporting
structure. They are often modified leaf parts.
e.g. terminal leaflets of Lathyrus odorata (sweet pea), and twining leaf
petioles of Clematis montana
Hairs
Long-haired leaves are referred to as pilose, giving rise to the species
taxon pilosa, and short haired leaves referred to as pubescent.
e.g. Stachys byzantina (lamb's ears)
In addition to reducing transpiration (by trapping water vapour), leaf hairs
may have a protective function, such as Urtica, stinging nettles, in which
the hair breaks, penetrates the skin and injects a toxin. Hairy plants are
more difficult for caterpillars to digest.
The native hairy potato of the Andes has been experimentally hybridised
with potato cultivars to develop resistence against potato cyst eelworm.
Bracts
A modified leaf below a flower or inflorescence. In some plants, for
example, the poinsettia Euphorbia pulcherrima, the large red bracts draw
the pollinator’s attention to the tiny yellow flowers.
Also found in Davidia involucrata (pocket handkerchief tree) where the
bracts attract pollinators at night.
FLOWER
Function
1. Sexual reproduction in plants
Main types of inflorescence found in plants.
Some plants produce a single flower per stem, or have multiples of
flowers on a single stem.
Names are given to the way in which multiple flowers are arranged
on a single stem.
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Spike
Raceme
Panicle
Umbel
Corymb
Composite
Group of flowers without stalks arising from main stem
Single flowers attached by short stalks of equal length to a main stem
A branched raceme, each branch having a smaller raceme of flowers
Stalks branch from a single node, are of same length to give an ‘umbrella’ form
Stalks of different lengths so that the flowers are all on the same level
Many small unstalked flowers grouped together to give the appearance of a single flower
Spike
e.g. Acanthus mollis
Raceme
e.g. Hyacinthus
orientalis
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Structure of a
Panicle
e.g. Phlox paniculata
Corymb
e.g. Viburnum tinus
Umbel
e.g. Agapanthus
africanus
Composite
e.g. Helianthus
annus
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A typical dicotyledonous flower
from Capon, B Botany for Gardeners Batsford (1990)
Role of the components of the flower
androecium
the collective name for the male parts of the flower,
comprising
anther
pollen-producing organ containing pollen sacs, supported by
the
filament
a stalk bearing the anther above the base of the flower which
ensures the correct position and mobility of the pollen sacs
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gynoecium
the collective name for the female parts of the flower
stigma
pollen-receiving organ, (may be sticky to retain insect
pollinated pollen or feathery to retain wind pollinated pollen
supported by the
style
a column-like structure which correctly positions the stigma
and guides and feeds to pollen tube as it grows down
towards the ovary. It connects the stigma to the
ovary
which contains unfertilised ovules. Once fertilised, the ovary
wall develops into the fruit and the
ovules
once fertilised, become the seeds
perianth
the collective name for the corolla and calyx
calyx
a collective name for the ring of
sepals
which are the green protective covering for the bud
corolla
a collective name for the ring of
petals
which attract insects if brightly coloured or may be small and
insignificant if wind pollinated. They surround and partially
protect the sexual organs. At the base of the petals are the
nectaries
producing a sugar-like liquid, sought by insects in insect
pollinated flowers
Terms relating to the locations of the male and female organs
Monoecious (Greek one household)
In a monoecious plant, the male and female organs are in separate
structures on the same plant. For example in Corylus avellana (Hazel)
the male organ is a catkin, and the female a tiny red flower.
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Dioecious (Greek two households)
In dioecious plants the male and female flowers are on separate plants.
Thus one can talk about a male plant and a female plant. Example Ilex
aquifolium, Gaultheria mucronata, Skimmia japonica
Hermaphrodite (possessing the sexual organs of both sexes)
In hermaphroditic plants both male and female organs are located in the
same flower
Tepals
Some plants, particularly monocotyledons (e.g. Tulipa) and early (in the
evolutionary sense) plants like the genus Magnolia, do not have a
differentiated calyx and corolla. In other words, they do not have sepals
and petals, but a combined component called a tepal.
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