Introduction to Horticulture

 

 

Chapter 2

Introduction to
Horticulture

Learning Objectives
1. Identify the basic vegetative parts
of plants and their main functions

2. Identify the basic reproductive parts
of plants and their main functions

3. Identify different types of leaves

or buds by shape, margin,
position and/or venation

4. Differentiate types of inflorescences

in plants and fruit formation

5. Explain the basic steps to seed
formation and germination

Introduction to Horticulture

H

orticulture is the science, business
and art of growing and promoting
plants. To gain a working knowledge

of horticulture, it is necessary to understand
the structure and function of plants and the
environmental factors that affect plant growth.
Botany is the study of plants. Understand-
ing plant function and physiology is critical
for successful horticulture. In the greatly
diversified kingdom of plants, all flowering
plants have certain structures and functions
in common. Higher order flowering plants are
divided into two groups: monocots and dicots.
Although monocots and dicots are similar in
many ways, differences with respect to the
number of seed leaves, the number of flower
parts, the leaf vein pattern and the root struc-
ture exist. In addition, physiological differenc-
es exist that result in different responses when
plants are subjected to natural or synthetic
factors. Corn, grass, lilies and orchids are
examples of monocots. Sunflowers, pea plants
and roses are examples of dicots.
Classification of Plant
Growth Habit
The number of growing seasons that are
required to complete a life cycle classifies
plants as annual, biennial or perennial. An-
nuals pass through their entire life cycle from
seed germination to seed production in one
growing season and then die. Annuals mature
quickly producing an abundance of flowers and
are normally planted after the last frost in the
spring and generally live until the killing frost
in fall. Most annuals require little effort to es-
tablish in the garden and are rarely troubled by
disease. Generally, annuals bloom more than
biennials and perennials with the goal of set-
ting seeds to perpetuate its species. Deadhead-
ing – continually removing the faded flowers
– is a method that will prevent seed formation
and signal the plant to continue blooming.

Some annuals are characterized as cool-sea-

son plants and others as warm season plants.
Cool-season annuals perform their best in
the cool weather conditions of spring and fall
and tend to slack off during the summer heat.
Sweet Alyssum is one example of a cool-season

annual. On the other hand, warm season
annuals flower best in the hot summer sun.
Zinnia, cosmos and marigolds are examples of
warm season annuals.

In the South, some annuals are winter har-
dy and will provide winter color and interest
in the garden. A hardy annual refers to a plant
that can tolerate cold temperatures. Annuals
viable through our Tennessee winters include
Pansies, Violas, Dianthus chinensis, Snapdrag-
ons, and Ornamental Cabbage and Kale.

Biennials are plants that take two years to
complete their life cycle. They germinate from
seeds and produce vegetative structures and
food-storage organs in the first season. During
the first winter, a hardy evergreen rosette of
basal leaves persists. During the second season,
flowers, fruit and seed develop to complete
the life cycle. The plant then dies. The bloom
period of the biennial is usually much shorter
than the annual, typically two to three weeks.
Some biennials can be unpredictable and
behave as short-lived perennials, blooming for
two or even three years, consecutively, before
dying. Other biennials may complete the cycle
of growth from seed germination to seed
production in only one growing season. This
situation occurs when drought, variations in
temperature or other climatic conditions cause
the plant to pass through the equivalent of two
growing seasons physiologically, in a single
growing season. This phenomenon is referred
to as bolting. Foxgloves, hollyhocks, carrots
and onions are examples of biennial plants.
Perennial plants live for many years, and

after reaching maturity, typically produce
flowers and seeds each year. Perennials are
classified as herbaceous if the top dies back to
the ground each winter and new stems grow
from the roots each spring. They are classified
as woody if the top persists, as in shrubs or
trees. Some perennials are referred to as tender
perennials. This means that they are plants
that are technically classified as perennials, but
may not be able to withstand winter conditions
in Tennessee. Blue salvia, heliotrope and zonal
geraniums are examples of tender perennials.
Most perennials bloom for only a few weeks,
but some will have a second flowering, or will
continue to have scattered flowering, through-

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Offical TMGInstructorCopy Figure 1. USDA Hardiness Zone Map

out the season. Many perennials spread,
forming larger clumps, and need to be dug up
and divided every few years or they will lose
their vigor. When growing perennials, a good
rule to teach gardeners is: The first year they
sleep, the second year they creep and the third
year they leap. Therefore, patience is usually
required for the full beauty of a perennial to be
reached. Hostas, roses and dahlias are exam-
ples of perennials.
Structure or Form
The basic size, structure and form of plants
can be used to classify or group them in broad
descriptive terms. Plants that are fibrous,
rigid or hard are classified as woody. Plants
with succulent or tender stems are considered
herbaceous plants.

Woody plants can be further classified as
vines, shrubs or trees, referring to their growth
habit. A vine is a plant that develops long
trailing stems that grow along the ground, or
must be supported by another plant or struc-
ture. Some twining vines circle the support
clockwise (hops or honeysuckle), while others
circle counter-clockwise (pole beans or Dutch-
man’s pipe vine). Aerial roots, such as in Eng-
lish ivy or poison ivy, support climbing vines

Fruit or Vegetable?

When a tomato is on the plant, it is considered a fruit; when it is
on the table, it is considered a vegetable, based on its nutritional
value. Tomatoes contain seeds but are used or consumed in pasta
sauces or salads during the main course; therefore, they are con-
sidered to be a vegetable.

by slender tendrils that encircle the supporting
object. Tendrils may also have adhesive tips
that support vine growth.

Shrubs are perennial woody plants that may
have one or several main stems and are usually
less than 12 feet tall at maturity. Trees are
perennial woody plants with one, or some-
times several, main trunk(s) that are usually
more than 12 feet tall at maturity. Trees may
be further defined as the shape of their canopy
(oval, vase, weeping and columnar)
Leaf Retention
Perennial plants (including woody plants)
usually fall into one of two categories: decidu-
ous or evergreen. Deciduous plants usually
lose their leaves to prepare for dormancy.
Evergreen plants retain their leaves seemingly
year round; however, they cast off older leaves
and grow newer leaves during the growing
season. Evergreen plants are further divided
into broadleaf (azalea, holly) or needle-leaved
plants (pines, junipers).
Climatic Adaptation
Herbaceous plants are classified according to
minimum temperature requirements or hardi-
ness. Tropical plants tolerate temperatures
from 32 degrees F, or 0 degrees C. Subtropical
plants, plants typically native to Tennessee,
tolerate short day exposures to slightly below
freezing and night temperatures to around
freezing. Most native plants in Tennessee are
subtropical. Temperate plants are well adapted
to prolonged subfreezing temperatures and can
endure temperatures well below freezing.
Vegetables, fruits and flowers that can
tolerate cool weather are considered cool-
season crops. Those that do not tolerate colder
temperature should be grown as warm season
crops. Cool-season crops typically grow best
in daytime temperature ranges of 55 to 75 de-
grees F (13 to 24 degrees C) and warm season
crops grow best at daytime temperature ranges
of 65 to 95 degrees F (18 to 35 degrees C).
Uses
Plants are often categorized by their use or by
the part of the plant consumed. Ornamental
plants are cultivated for aesthetic beauty or for
environmental enhancement values. Horticul-
tural plants grown for edible parts are referred
to as fruits, nuts, herbs or vegetables. Botani-
cally, a fruit is any part of the plant structure

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Offical TMGInstructorCopy that contains a seed. However, from a use
perspective, a fruit is consumed for its dessert
qualities; a fruit is called a vegetable or herb
when it is consumed during the main portion
of the meal.

Botanical or Scientific
Classification

Binomial nomenclature is the scientific
system of giving a double name to each plant
or animal. The first name (genus) is followed
by a descriptive or species name. Modern
plant classification, or taxonomy, is based on

Rules of Nomenclature

Nomenclature governs how plants are named, which then determines how they are written and pronounced.

▪ Botanical Latin follows most, but not all, rules of Latin
pronunciation. Words are pronounced in the English
method, which is easier on the modern ear than classical
Latin.

▪ Plant names consist of two words: the genus name and

▪ The genus name is a noun in the nominative singular

the species name.

case.

▪ The genus name is always capitalized, for example: Acer
▪ The species name is an adjective describing or modify-
ing the noun. It agrees in case and number. The spe-
cies name is usually not capitalized, for example: Acer
palmatum

▪ The species name may be capitalized if it is derived:
▫ Directly from a person’s name
▫ From a vernacular name
▫ From the name of a genus
▫ For example: Picea Breweriana
▪ The species name may be followed by a word indicating
another level of scientific classification, for example, Acer
palmatum dissectum
▫ Subspecies: A significant geographic race of a spe-

▫ Variety: A minor race not deserving subspecies

cies: ssp. or susp.

status: var. or v.

▫ Form: A seedling variation, used when a percentage
of seedlings exhibit a distinctive characteristic: f.

▪ Botanic names are always either italicized or underlined.

This is simply the general rule for writing foreign words in
English text.

▪ The species may be followed by a cultivar name. Cultivar
is a contraction of the words, cultivated variety. Cultivars
are most often propagated by vegetative means rather
than by seed. They are defined by published descrip-
tions that indicate the particular features of the plant that
makes it unique. Cultivar names are:
▫ Always surrounded by single quotes
▫ Always capitalized
▫ Never in italics
▫ For example: Acer palmatum dissectum ‘Bloodgood’

The Importance of Cultivars:

Being given a cultivar name indicates that the plant is a
product of:
▫ Human selection
▫ Genetic mutation
▫ Breeding or hybridization
▫ For example: Camellia japonica ‘Kumasaka.’ This plant
is dark pink, incomplete double flower, blooming late
season and cold hardy.

▫ Another example: Camellia japonica ‘Chojo Haku.’ This
plant is white, single, fall-blooming and not especially
cold hardy.

Hybrids

▪ Hybrid plants result from a sexual cross between two dif-
ferent species or between clearly defined varieties within
a species, for example: Lagerstroemia farriei x Lagerstro-
emia indica

▪ The immediate result of a sexual cross is one or more

seeds. Plants that result from the germination of these
seeds are the progeny of the cross. This group of plants
may be called a grex and given a grex name , or they may
not, for example: Clematis lanuginosa x Clematis vitacella
= Clematis x jackmanii. Jackmanii is the grex name for this
group of seedlings. From this group of seedlings, selec-
tions may be made for superior color or growth habit,
etc. For example, Clematis x jackmanii ‘Niobe’ has a dark
red flower. Lagerstroemia farriei x Lagerstroemia indica has
no grex name. Selections from this cross are written this
way: Lagerstroemia x ‘Natchez’

▪ It is seldom possible to cross plants from different genera,
for example: Canus x Felis or Buxus x Juniperus or bi-gener-
ic Hybrids. However, sometimes with diligent effort, it is
possible to hybridize closely related genera. For example:
Cupressus macrocarpa x Chamaecyparis nootkatensis = X
Cupressocyparis leylandii, Leyland Cypress, and bi-generic
Hybrids. In this case, the plant gets a genus name made
up of parts of the two parent genera and the name is
preceded by a capital X

▫ X Cupressocyparis leylandii
▫ X Mahoberberis
▫ X Chitalpa taskentensis

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Offical TMGInstructorCopy a system of binomial nomenclature developed
by the Swedish physician, Carl von Linne
(Linnaeus). Prior to Linnaeus, people tried to
base classification on the leaf shape, plant size,

Table 1. Meaning of Specific Epithets

Meaning

Pressed against; lying flat against, as the hairs on
the stems of some plants

Angustifolius

Narrow leaved

Epithets

Adpressus

Arboreus

Aristosus

Asperimus

Asiaticus

Atrpupureus

Aureus

Australis

Avium

Azureus

Tree-like

Bearded

Very rough

Asian

Dark purple

Golden yellow

Of the birds

Sky blue

Of the Southern Hemisphere

Figure 2A. Principal Parts of a Vascular Plant

flower color, etc. None of these systems proved
workable. Linnaeus’ revolutionary approach
was to base classification on the flowers and/or
reproductive parts of a plant and to give plants
a genus and species name. This has proven
to be the best system, since flowers are the
plant part least influenced by environmental
changes. For this reason, knowledge of the
flower and its parts is essential for anyone who
is interested in plant identification. Although
plants are very diverse in nature, they are
grouped together by functions and characteris-
tics that they have in common.
Plant Structure
Plants cells are independent units with special-
ized purposes and structures to help carry out
the processes essential to life. Plant parts can
be divided into two very basic groups: vegeta-
tive and sexual reproductive parts. The vegeta-
tive parts include: meristems, roots, stems,
shoots, leaves, leaves and buds. The vegeta-
tive parts are not directly involved in sexual
reproduction. Sexual reproductive parts are
those involved in the production of seed. They
include: flowers, flower buds, seeds and fruits.
Vegetative
Meristems

Meristems, or growing points, are plant tis-
sue in which cells divide to reproduce, grow
and develop new tissue. The most common
meristems are terminal (apical) and lateral.
Found in shoot tips, root tips and buds, apical
meristems are responsible for the increase in
length of these plant parts. The increase in
stem and root diameter or thickness is due to
the lateral meristems, called the cambium. In
many grasses, the meristem is responsible for
shoot growth and is found near the base of
the plant. Therefore, mowing turfgrass at the
proper height does not injure or remove the
growing point of the plants.

Meristematic areas, which are normally just

a few cells deep, may produce shoots (vegeta-
tive growth) or flowers (reproductive growth),
depending on when and where the meristems
are active. All active meristems receive prior-
ity for food materials and minerals available
within the plant. For this reason, they are
often referred to as sinks.

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Offical TMGInstructorCopy Figure 2B.

Vascular System of a
Monocotyledon and a
Dicotyledonous Stem:
Xylem Cambium,
Phloem and Pith

Figure 3. Portion of a Dicot Root

Root Cap, Root Hairs and Emergence of New Lateral Roots.
Taprooted plants have a longer primary root that anchors the plant
deeper into the ground.

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Roots

The functions of the root system are absorp-
tion, anchorage, conduction and storage.
In land plants, large amounts of water are
absorbed through millions of thin-walled
root hairs that are in close contact with the
soil particles. Each root hair is formed from a
single epidermal cell near the growing tip of
the root. Each one lasts only a short time and
many are killed during transplanting. Anchor-
age is gained by the extensiveness of the root
system, both spread and depth. Plants are
more resistant to being blown over by the wind
and are less susceptible to damage by cultiva-
tion when soil conditions allow roots to grow
into the soil.

A primary root originates at the lower end

of the embryo of a seedling plant. A taproot
is formed when the primary root continues to
elongate downward into the soil and becomes
the central and most important feature of the
root system with a somewhat limited amount
of secondary branching. Some trees, especially
nut trees like pecans, have a long taproot with
very few lateral or fibrous roots. As a result,
these trees are difficult to transplant. Ad-
ditionally, because of the long taproot, these
trees must be planted only in deep, well-
drained soil.

A lateral or secondary root is a side or
branch root that arises from another root.
A fibrous root is one that remains small in
diameter because of very little cambial activity.
One factor that causes shrubs and dwarf trees
to remain smaller than a standard or large size
tree is the inactivity of the cambium tissue in
the roots.

A fibrous root system is one in which the
primary root ceases to elongate and numerous
lateral roots develop. The lateral roots branch
repeatedly and form the feeding root system of
the plant. Most nursery plants have this type
of root system.

If plants that normally develop a taproot are

undercut so that the taproot is severed early
in the plant’s life, the root will lose its tap-
root characteristic and develop a fibrous root
system. This is done commercially in nurseries
so that trees, which naturally have taproots,
will develop a compact, fibrous root system.
This allows a high rate of transplanting success
in the field.

The quantity and distribution of plant roots

are very important to plant health because

Offical TMGInstructorCopy these factors have a major influence on the
absorption of moisture and nutrients by roots.
The depth and spread of the roots are depen-
dent on the inherent growth characteristics of
the plant and the texture and structure of the
soil. Roots will penetrate much deeper in a
loose, well-drained soil than in a heavy, poorly
drained one. A dense compacted layer in the
soil will restrict or terminate root growth.

During early development, a seedling plant

absorbs nutrients and moisture from the soil
that is within a few inches of the location of
the seed from which the plant grew. Therefore,
the early growth of most horticultural crops
that are seeded in rows benefit from band
application of fertilizer several inches on each
side and slightly below the location of the
seeds.

As plants become well-established, the root

system develops laterally and usually extends
somewhat beyond the spread of the branches.
For most cultivated crops, roots meet and over-
lap between the rows. The greatest concentra-
tion of fibrous roots occurs in the top foot of
soil, but significant numbers of laterals may
grow downward from these roots to provide an
effective absorption system several feet deep.
Internally, there are three major parts of a
root, the meristem, the zone of elongation and
the maturation zone. The meristem is at the
tip and manufactures new cells; it is an area
of cell division and growth. Behind it is the
zone of elongation. In this area, cells increase
in size through food and water absorption.
These cells, by increasing in size, push the root
through the soil. The third is the maturation
zone where cells undergo changes to become
specific tissues such as epidermis, cortex, or
vascular tissue. The epidermis is the outermost
layer of cells surrounding the root. These cells
are responsible for the absorption of water and
minerals dissolved in water. Cortex cells are
involved in the movement of water from the
epidermis and in food storage. Vascular tissue
is located in the center of the root and con-
ducts food and water.

Externally, there are two areas of impor-
tance: root hairs and the root cap. Root hairs
are found along the main root and perform
much of the actual work of water nutrient
absorption. The root cap is the outmost tip of
the root and consists of cells that are sloughed
off as the root grows through the soil. The

meristem, the area of cell division, is protected
behind the root cap.
Stems and shoots

The primary functions of the stems and shoots
are support, conduction and storage. Shoot
refers to the tissue of developing stems and
leaves (leafy shoots) and stems and flowers
(flowering shoots). The stem holds the leaves
up so they receive more light, which is impor-
tant in the manufacture of plant food. Some
plants have a single stem, often called a trunk.
Other plants have many stems arising from the
crown.

Stems may be long, with great distances
between leaves and buds (branches of trees,
runners or strawberries) or compressed with
short distances between buds or leaves (fruit
spurs, crowns of strawberry plants, dandeli-
ons). Stems can be above the ground like most
stems with which we are familiar, or below the
ground (potatoes, tulip bulbs). All stems must
have buds or leaves present to be classified as
stem tissue.

The area of the stem where leaves are
located is called the node. Nodes are areas of
great cellular activity and growth. It is at the
node that buds develop into leaves or flowers.
The area between nodes is called an internode.
The length of an internode depends on many
factors, including fertility, amount of daylight
and competition from other plants. If fertil-
ity is decreased, the internode length will be
shortened. If light is low, the internode will
elongate, causing a spindly stem. This situation
is known as stretch or etiolation. Addition-
ally, if there is competition from surrounding
stems or developing fruit, the internode length
will be shortened. Growth produced early in
the season has the greatest internode length.
Internode length decreases as the growing
season nears its end. Vigorously growing plants
tend to have greater internode length than less
vigorous plants.

Photosynthetically manufactured food
moves out of the leaves through stem tis-
sues called phloem. Materials can move up or
down in the phloem. Soil water and mineral
solutions are conducted upward from roots to
the leaves through inner tissues called xylem.
Together, the xylem and phloem form a con-
tinuous, pipe-work system (vascular system)
sending nutrients and water from every root
tip to every shoot and leaf tip in the plant.

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Offical TMGInstructorCopy Table 2. Specialized Stems and Roots

Above ground modified stems are crowns, stolons, runners or spurs

Belowground stems are bulbs, corms, rhizomes and tubers

Crowns are compressed stems having leaves and flowers on short internodes.

Spurs are short, stubby, side stems that arise from the main stem and are
common on fruit trees where they may bear fruit. If severe pruning is done
close to fruit bearing spurs, the spurs can revert to a non-fruiting long stem.

A stolon is a horizontal stem that is fleshy or semi-woody and lies along the
top of the ground. All stems have nodes and buds or leaves. The leaves on
strawberry runners are small but are easily located at the nodes. The nodes on
the runner are the points where roots begin to form.

African violets,
dandelions

Apples, pears, cherries

Strawberry runners,
spider plant

Rhizomes are similar to stolons, but grow underground rather than above
ground. Some rhizomes are compressed and fleshy such as Iris; they can also be
slender with elongated internodes such as bentgrass.

Bentgrass, Johnson
grass, iris

Bulbs are shortened, compressed underground stems surrounded by fleshy
scales (leaves) that envelop a central bud located at the tip of the stem.

Tulips, hyacinth,
snowdrop, narcissus

A corm is a solid, swollen stem whose scales have been reduced to dry, scale-
like leaves. Corms have shapes similar to bulbs, but do not contain fleshy scales.

Gladiolas

A tuber is an enlarged portion of an underground stem. The tuber, like any
other stem, has nodes that produce buds. The eyes of a potato are actually the
nodes on the stem. Each eye contains a cluster of buds.

Potato

Some plants produce a modified stem that is referred to as a tuberous stem.
The stem is shortened, flattened, enlarged and underground. Buds and shoots
arise from the top or crown and fibrous roots are found on the bottom of the
tuberous stem.

Tuberous roots are underground storage organ often confused with bulbs
and tubers. However, these are roots, not stems and have neither nodes nor
internodes.

Tuberous begonia and
cyclamen.

Dahlia, Sweet potatoes

Between the xylem and the phloem in some
plants is a tissue called the cambium layer. The
cambium layer forms new xylem and phloem
that may be required by the plant. The cambial
layer is responsible for the increased diam-
eter growth of stems and roots. In grasses, or
monocots (single- seed leaf) plants, the three
tissues are formed in bundles scattered in a
discontinuous vascular system throughout the
stem. In woody plants, the xylem and phloem
occur as concentric zones separated by the
cambium, which is a few cells wide. The two
types of cambium in woody plants are active
xylem (sapwood) and inactive xylem (heart-
wood). Sapwood is the younger, outermost
wood. It is not very thick. Heartwood is in the
inner part of the wood. It is usually thicker
than sapwood.

Herbaceous or non-woody plants and stems
have a thin outer protective tissue. This protec-
tive tissue is called an epidermis. Herbaceous
or succulent stems contain water-filled cells

Measuring Plant Health

Internode length is a great measuring
stick for plant health. Short internodes
may indicate slowed growth due to wa-
ter stress, heat or damage. Extremely
long internodes may indicate lack of
light.

(turgid cells) that provide adequate strength to
support the plant. In large, herbaceous plants,
a stiffening process called secondary growth
occurs as the plant becomes semi-woody
(sometimes referred to as a sub-shrub). Woody
stems, such as those in trees and shrubs, may
develop a thick or tough protective exterior
tissue called bark. In woody plants, specialized
tissues (fibers) begin to form as the stem elon-
gates and the stem becomes more or less rigid

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