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Classifying or grouping plants into categories is an important aspect of ornamental horticulture. Classifications allow people to compare or contrast various characteristics (features) of plants and to organize how plants will be used in the landscape. Plants can be classified in various ways—by their anatomy, use, life cycle (time between generations), and temperature tolerance.
The anatomy (or morphology or form) of
plants can be used to classify plants. Land plants (terrestrial
plants or terraphytes) all have anatomical characteristics in
common—such as, leaves, stems, and roots. But, there also
are differences in how the various plant parts are arranged and
function. Plants are classified most often on reproductive
(flower) characteristics as opposed to vegetative (leaf, stem)
characteristics. This has become customary since reproductive
parts (petals, sepals, stamens, pistils) remain relatively
unchanged over diverse environments, whereas vegetative parts
tend to change depending on the environment in which they grow.
The two basic groups in which terrestrial plants are placed are
the angiosperms and gymnosperms.
angio = case, vessel or capsule + sperm
= seed
This is the group or class of plants characterized by conspicuous flowers and fruits that enclose seeds.
gymno = naked + sperm = seed
This is the group or class of plants characterized by inconspicuous flowers and fruits with exposed seeds. Conifers (pine trees) are good examples and show the typical gymnosperm fruit (pine cone).
Of course, plants are used for many different purposes—for example for food, to enhance or improve our environment, or for aesthetic reasons.
One could classify edible plants as
vegetables, fruits, and herbs.
One also could classify plants according
to how they affect our living and working environments, to
provide:
shade - temperature effects
a windbreak - wind velocity effect
a screen - for privacy
Plants also could be classified
according to aesthetic characteristics:
flowering plants, trees and shrubs
container-grown plants
pot chrysanthemums
lilies
cut flowers
foliage plants
The life cycle or generation time of plants is defined as that length of time required for a plant to go from seed to seed. While humans measure this with "time," plants respond to changes in the environment in which they grow (e.g. temperature, light, and daylength).
These are plants requiring one year (growing season)or less to go from seed to seed. Once seeds are produced, the plant producing them dies. Generally, annuals complete their life cycle in one growing season ("time" from the last spring frost to the first fall frost)
These are plants requiring two years (growing seasons) to go from seed to seed. The plant producing the seeds dies.
Perennials produce seeds each year and the plant producing the seeds remains intact for many years. Some perennials are 'herbaceous' which means they die back to their roots sometime after producing new seeds, but re-appear the next season. Other perennials are 'woody' in nature with above-ground stems (and often leaves) remaining and increasing in number through several growing seasons.
Temperature plays a major role in the
development and evolution of plants. Plants have developed
various ways of adapting to the environments in which they grow.
For example, temperate, sub-tropical, and tropical areas of the
world are characterized by certain minimum and maximum
temperatures.
Being able to know and name the various anatomical parts of plants is an important skill for the ornamental horticulturist. All the parts of plants and the myriad variations have specific names or terms associated with them. The most prominent and visible parts of plants are above ground; however, the below-ground parts of plants perform important functions.
The flower (or 'inflorescence' if many
flowers are attached to one another) is the reproductive part of
the plant and may have male and/or female reproductive organs. A
female flower has a pistil made up of a stigma, style and ovary
or many ovules. A male flower has a stamen made up of an anther
and supporting filament. Male flowers may have many individual
stamens. Pollen is produced in the anther. Individual flowers are
supported by pedicels and if they are arranged into an
inflorescence, they are supported by a peduncle. The pistil and
stamen are usually surrounded by one or more petals and one or
more sepals. Petals are usually brightly colored and arranged to
attract pollinating insects, birds or other animals. Sepals are
usually green. All the petals taken together are the corolla and
all the sepals are the calyx. Petal comes from the Latin word
"petalum" meaning "leaf" and corolla comes
from the Latin "corona" meaning "garland".
Sepal comes from the New Latin word "sepalum" meaning
"covering petal" and calyx comes from the Latin
"calix" meaning "cup".
Here
is a 'perfect' flower, meaning it has both
male and female flower parts. Click on the picture to practice
identifying the parts of a flower.
Plants form many different kinds and shapes of fruit including berries (grape, orange), hips (rose) and drupes (cherry, peach). Fruits of angiosperms are usually fleshy enclosing one or more seeds of the plant. In the examples below, you need not memorize the different parts of the fruits, but see if you can identify where the flower parts went as the fruits formed!
Meristems, in general, are the most
rapidly growing parts of the plant. The cells in the meristem are
rapidly dividing, forming news cells that will develop into
specific tissues and ultimately into organs. The shoot apical
meristem is the origin of new leaves, stem tissue and flowers.
The term apical meristem comes from the Latin "apic"
meaning "apex" and the Greek "meristos"
meaning "divided".
Axillary buds are formed in the axils (i.e., upper angles) between stems and leaves. When these buds begin to grow they eventually form the plant's branches. Axillary buds contain an axillary meristem that had its origins in the apical meristem.
Leaves make up the vast majority of the
above-ground volume and surface area of most plants. They are
usually green in color due to chlorophyll-containing cells;
however, at various times of the year, as changes take place in
the leaf's chlorophyll content, they can "turn" yellow,
orange, red, violet, or brown. A leaf is attached to the stem by
its petiole.
A 'leaf' is defined by the location of the axillary bud at its connection to the stem. To determine where the stem ends and the leaf begins, one must identify where the axillary bud is. If there is no axillary bud present in the junction between the stem and petiole, the leaf is known as a leaflet and is part of a compound leaf. Below is a compound leaf composed of many leaflets. You would not find an axillary bud at the base of any of the leaflets. The diagram above is of a simple leaf since the blade is not divided into separate leaflets.
Along the stem, each location with an
axillary bud is a node. The stem tissue between nodes is the
internode. Distance between nodes is a characteristic unique to
each plant and can be influenced by the environment,
horticultural management, and growing conditions of the plant.
Water from the soil and sugars formed through photosynthesis in the leaves must move throughout the plant. The vascular (conducting) tissue is comprised of specialized cells for the movement of water and nutrients. Water (and dissolved mineral nutrients) moves throughout the plant through the xylem (from the Greek "xulon" meaning "wood"). It is actually an organized system of small-diameter, dead cells resulting in a series of capillary tubes where water moves under adhesive and cohesive forces. Sugars and other organic compounds move throughout the plant via the phloem (from the Greek "phloios" meaning "bark"), a complex network of specialized living cells. The development of a vascular system is what separates land plants from many ferns and aquatic plants and was an important evolutionary step to insure that plants would successfully "colonize" the land masses on Earth.
The root system originates
from an apical meristem (root apical meristem) similar to the
shoot apical meristem. The major differences are that the root
apical meristem responds positively to gravitropic forces (it
grows downward), whereas the shoot apical meristem responds
negatively to gravity (and grows upward). The root apical
meristem does not form axillary buds in the axils of branches. It
is protected from the harsh soil environment by a covering root
cap. Many root cells develop specialized cell walls called root
hairs. This increases the surface area of the root thus
increasing the uptake of water and soil-borne nutrients.
Root systems can have many branches (fibrous roots) or have few, if any, branches (e.g., tap roots in seedlings).
Roots that form on parts of the plant where they are not expected (e.g., on a stem or leaf) are called adventitious roots. Examples of these include brace roots of corn and aerial roots on branches of English Ivy.
Stolons, or runners, are modified stems
that grow horizontally at or just below ground level. Plants with
stolons include strawberry and bermuda grass.
Rhizomes are modified, thickened stems
that grow horizontally below ground. They root at nodes. Plants
with rhizomes include iris, canna, and banana.
Tubers are enlarged, fleshy, underground
stems, often with prominent axillary buds. Potatoes are tubers
and the "eyes" are the axillary buds.
Bulbs are highly compressed,
underground stems with fleshy leaves called scales. Plants with
bulbs include Easter lilies, tulips, onions, and daffodils.
Corms are short, solid, vertical, enlarged,
underground stems containing undeveloped leaf and flower buds.
Gladiolus, crocus, and freesia are plants that grow from corms.
Flowers (and inflorescences) function in
reproduction of the plant. Plants may have flowers that are all
male or all female (called 'dioecious' meaning two houses or
dwellings) or they may have flowers of both sexes on a single
plant (monoecious).
Leaves are made up of cells that contain the green pigment, chlorophyll. Because of this pigment, leaves are the primary location where photosynthesis takes place. Leaves also have specialized surface cells that permit carbon dioxide in (for photosynthesis) and water vapor out. The evaporation of water from leaves is their primary method of temperature regulation (evaporating water cools the leaf) and is known as transpiration.
Below is a diagram of the Process of Photosynthesis. Get familiar with the basic functions and players.
Stems support
the plant, provide a system (xylem) for the transport of water
and nutrients, contain growing points (an apical meristem and
axillary buds), and develop protective tissues (bark).
Roots serve several functions. They
anchor the plant, take up water from the soil, absorb nutrients
from the soil, and can serve as storage organs.
Congratulations! That completes this chapter. You may want to take a break, then try the Practice test and then check the Answers. Practice identifying features of plants around your home, on campus, and in town. For ideas, look at the 'In Your Neighborhood' Activities.