Can you imagine how confusing life would be if we had no names
and every time you wanted to refer to a person, place, or thing
you had to describe it and its relationship to all others of its kind?
We are fortunate that our predecessors came up with the idea of
names and that knowledge about plants has been successfully
passed down from generation to generation.
is a term used to describe the process of grouping
related plants together and naming the groups. These groups
have been historically based on morphological and anatomical
features and, more recently, on genetics. Members of a species
have characteristics that set them apart from all other populations
of plants, and they naturally breed with each other. Closely
related species with similar characteristics are grouped into a
genus. The genera are grouped into families and the families are
grouped into classes. All the land plant classes collectively form
two divisions, the bryophytes and the tracheophytes.
Bryophytes are called nonvascular plants because they have a
very poorly developed vascular system
. Since the vascular system
is involved in the transport of water throughout the plant, bryophytes
are generally—but not always—found in moist places and
when water is scarce. There are approximately 24,000
species of bryophytes, most of which are mosses. These plants are
not widely cultivated commercially but may be of great interest
to botanists and naturalists. Mosses may sometimes be found
growing on moist surfaces in greenhouses.
Tracheophytes have a well-developed vascular system that
enables them to grow taller than bryophytes and to survive temporary
shortages of water. They are divided into seed plants and
seedless plants. The seedless plants reproduce by spores
are found in wet habitats. Ferns are seedless tracheophytes that
are sometimes grown in gardens or as houseplants.
Tracheophytes that produce seeds are divided into two groups,
the gymnosperms and the angiosperms. Gymnosperms are
plants that produce seeds that are not enclosed in an ovary
, such as the cycads, gingko, conifers, and gnetinae. The cycads have
palm or fern-like leaves and produce cones. Gingko trees have
leaves and stinky, fleshy fruits. Conifers such as
the Pinus spp.
(pines) and Picea spp.
(spruce) have needle-like evergreen
leaves and produce seeds in cones, whereas Juniperus
(junipers) may have scale-like leaves and produce their seeds
in berries. The gnetinae, which include Ephedra spp.
, have xylem
vessels similar to those found in mosses, horsetails, and ferns. Of
all the gymnosperms, the conifers are the most widely cultivated.
Gingko trees are also often used for landscaping, especially along
roads and in public places.
Angiosperms are seed plants that produce flowers. The seeds
are enclosed in a mature ovary, which forms a fruit. There are
more than 250,000 species of angiosperms; this group is the
most widely cultivated group of plants on Earth. Angiosperms
are divided into two classes, the monocots and the dicots, based
and anatomy. Monocots have seeds with a single cotyledon
; dicot seeds have two cotyledons. These traits are the
basis for these groups’ names: monocot
is short for “monocotyledon”
- means “single”) and dicot
is short for “dicotyledon”
- means “two”). The parts of monocot flowers (sepals, petals,
ovary) usually occur in multiples of three, whereas
the parts of dicot flowers are found in multiples of four or
five. Additionally, monocot leaves have parallel venation. Dicot
leaves have net venation because of the arrangement of their
vascular system. Figures 2.1 and 2.2 show some of the differences
between monocot and dicot plant parts.
The vascular system is a transportation network of connected
cells that form tunnels in the plant that extend from the roots
through the stem to the leaves, flowers, and fruits. Xylem transports
water and minerals upwards from the roots and distributes it
throughout the plant. Phloem
transports the sugars created by photosynthesis
from the leaves to other parts of the plant. These tunnels
are bundled together and can be seen as the veins on a leaf.
|Figure 2.1 (TOP) The lily is an example of a monocot with flower parts typically found in multiples of three. There are three outer sepals, which in this example look identical to the three inner petals. The stigma has three lobes and there are six stamens. Florists will often remove the anthers from the stamens because they shed large amounts of pollen. (BOTTOM) The veins of leaves of monocots, such as this lily leaf, run parallel to each other.
Figure 2.2 (TOP) The hibiscus flower is a dicot with five petals, five small green sepals, a five-lobed stigma, and many yellow stamens fused
to the style. (BOTTOM) This hybrid camellia is a dicot with a rose-like flower. The veins of leaves of dicots form a net-like pattern, known as net venation.
The monocot and dicot classes are subdivided further into
families. Members of botanical families share similar characteristics.
For example, plants in the Lamiaceae family have characteristically
square stems, and many have trichome hairs
hairs can make the leaves or stems feel rough, and they help
reduce airflow across the surface of the leaf. Glandular trichome
hair secretes essential oils
, which can make these surfaces
sticky or fragrant. Many medicinal and culinary herbs, such
as mint, sage, lavender, and thyme, are grown for their ability to
produce essential oils and resins.
Plants in the family Solanaceae produce mildly to highly
and have similar flowers. The introduction
of tomato and potato crops from the Americas to Europe took
some time because of the reputation of their highly toxic relatives,
such as the deadly nightshade.
Plants in the family Leguminosae
also have similar flowers
and produce fruits called legumes. The pea flower and pea pods
are examples of Leguminosae flowers and fruits. Many of these
plants also have important relationships with soil bacteria that
are capable of nitrogen fixation
. The bacteria form nodules
roots of the plants.
Seed catalogs, plant encyclopedias, and garden centers often
group plants in ways other than by family—for example, by life
cycle, edibility, cultural requirements, or geography. Life cycle
can be annual, biennial, or perennial. Annual plants flower and
set seed in the first year and then die. They generally bloom all
summer but need to be replanted every year; sometimes they will
self-sow if the flowers have been allowed to go to seed. Biennials
flower and produce seed in the second year and then die. Perennials
take two or more years to flower and set seed, and they live
for many years. They generally bloom every year after the first
flower. Perennials usually bloom for a short period each season
and then die back until the following spring. They can be shortlived
(four to five years) or long-lived (many thousands of years)
like the bristlecone and Sequoia pine trees.
Deciduous plants lose their leaves in the fall, whereas evergreen
plants retain their leaves year round. Some deciduous
plants have brilliant displays of colorful foliage in the fall prior to
leaf drop. This is due to the carotinoid
pigments, which are colored
in shades of red and yellow. The green chlorophyll pigments are
more numerous when the plant is growing, so during this time
carotinoid pigments are masked by the green. When the plant
gets ready to go dormant, it stops producing chlorophyll and the
red and yellow pigments become visible.
Cultural requirements include the amount of sun and water
required by plants, the preferred temperature and soil type, and
nutrient needs. Xeric
periods of drought; they are adapted to hot, dry summers and
cool, moist winters. Shade plants, as you may have guessed, like to
be shaded from the summer sun. Vegetables are often separated
into warm-season and cool-season crops based on the temperatures
at which their seeds prefer to germinate. Grasses are also
classified as warm or cool season. Annual flowers are sometimes
labeled as frost tolerant or frost sensitive.
Native plants are those that are believed to have originated in
the region where they are to be cultivated. Generally, the cultivation
of native plants requires less work because they are already
adapted to the soil and climate. Plants may also be grouped
according to the ecosystem where they are found growing wild,
such as alpine, tropical, desert, or riparian. Alpine plants are
found at high altitudes, tropical plants in rain forests, and riparian
plants by rivers and streams.