Download BACKGROUND INFORMATION Angiosperms is the name given to

BACKGROUND INFORMATION
Angiosperms is the name given to seed-bearing plants that produce flowers. The flower
contains the reproductive organs of the plant. If a flower contains both male and female
reproductive structures, it is called a perfect flower. If it contains only one, either male
or female, it is referred to as an imperfect flower. Some plants produce separate
flowers; some flowers have only male reproductive structures and other flowers have
only the female reproductive structures, all on the same plant. This plant is called an
imperfect plant. Two examples of this are American sweet corn and yellow summer
squash. Plants such as Holly and Gingko trees have separate male and female plants.
The male Holly plant produces flowers that contain only the male reproductive
structures. They do not produce berries, since only female flower structures mature into
fruits, but are necessary to provide the pollen to fertilize the female Holly shrub.
There are two groups associated with flower structure: a. essential organs, which are
necessary for reproduction – ex. stamens (male) and pistils or carpels (female); b.
accessory organs, which surround and protect the flower and attract pollinators – ex.
sepals, the outer set of whorls (circles), protects the flower, and petals, the inner set of
whorls, attract pollinators. If a flower is missing any one of these parts it is called
incomplete. An example of an incomplete flower is the rue anemone, which has no
petals, (the sepals look like petals). The poinsetta is also incomplete. It lacks petals;
the bright red or colored “petals” are specialized leaves which are called bracts.
Poinsettia
Rue Anemone
If the pollen from a flower is transferred to the egg cells in the same flower, selffertilization occurs. Wind, insects, water, animals, and humans can also carry pollen to
flowers of other plants of the same species and cross-fertilization will occur, which
increases genetic variability.
A flower’s male reproductive organs are the stamens, which are made up of an
enlarged top, the anther, and a slender stalk, the filament. The anther is composed of
four pollen sacs (observe in microscope lab) found in the tissue called sporogenous
tissue (observe in microscope lab). In this tissue, diploid microspores undergo meiosis
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to produce four haploid microspores. Each microspore divides by mitosis to produce
two cells, i.e., a generative cell and a tube cell. Together these make up a pollen grain.
The female reproductive structure of the flower is the carpel. A flower may have one or
more carpels, either single or fused. A single carpel or a group of fused carpels is also
called a pistil. The pistil is made up of the stigma, style and ovary.
The stigma, at the top, becomes moist and sticky when mature so that pollen can more
easily adhere. The style is the middle part. Depending on the plant species, it may be
long and slender, short or even missing. At the bottom of the pistil, the enlarged
structure is the ovary. Within the ovary are one or more compartments, called locules,
containing one or more ovules. Through meiosis, four haploid megaspores are formed
in the ovules. Usually only one survives. Its nucleus then divides three times by mitosis,
forming the embryo sac, the egg cell and two cells flanking the egg cell.
For the egg to be fertilized, pollination must occur. If compatible pollen reaches the
stigma of a flower, a pollen tube will begin to grow down the style to the ovary. Within
the pollen tube the generative cell divides by mitosis forming two sperm nuclei. After
the two sperm nuclei enter the ovules, one sperm fuses with the egg to fertilize it. The
other sperm fuses with the two haploid polar bodies to form the endocarp, which helps
to nourish the fertilized egg, called the embryo. The embryo is a diploid zygote, formed
from the fusing of the haploid sperm nuclei (pollen) and the haploid egg cell in the ovule.
As the wall of the ovule thickens, it encloses and protects the embryo and forms the
seed. In some plants, the ovary wall thickens and develops into a fruit. A fruit is
defined as a seed-bearing structure that develops from the ovary of a flowering plant. In
some plants such as apples and pears, the ovary walls become fleshy and contain
stored sugars and starches. Many “vegetables” such as squash, cucumbers, tomatoes
and eggplant are really fruits. In other plants, such as walnut and hazelnuts, the ovary
walls become dry and hard and the seed remains attached or fused to the ovary wall.
The receptacle is the part of the fruit we like to eat. Fruits are classified into three major
groups based on the number of ovaries and the number of flowers involved in their
formation. Within these three groups there are many further subdivisions that depend
on varying characteristics. The three major types of fruit are: a. simple; b. aggregate;
c.multiple.
A simple fruit develops from a single matured ovary in a single flower; examples of
simple fruits are apples and pears.
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An aggregate fruit consists of a number of matured ovaries formed in a single flower
and arranged over the surface of a single receptacle. Individual ovaries are called
fruitlets. Some examples of aggregate fruits are blackberries, raspberries and
strawberries.
Multiple fruits consist of the matured ovaries of many flowers more or less united into a
mass. Most are also accessory fruits, which means that they have some other flower
part united with the ovary. Some example of multiple fruits are the Osage orange (mock
orange), pineapple, mulberry, fig and breadfruit.
There are two classes or angiosperms, the monocots and dicots. Monocots have one
cotyledon. Cotyledon are the first leaves produced by plants. They are not “true”
leaves and are referred to as “seed” leaves. They are found in the seed or plant
embryo. They access stored food and nutrients for a growing plant until its true leaves
can grow and begin the process of photosynthesis. There are typical physical
characteristics that characterize a plant as either a monocot or dicot. The sepals and
petals of a monocot usually occur in multiples of three and the veins in the leaves run
parallel. The sepals and petals in dicots usually occur in multiples of four or five and
their leaves are net-veined.
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