Angiosperms

Angiosperms

The 250,000 known species of flowering

plants are called angiosperms because

their ovules, unlike those of

gymnosperms, are enclosed within

diploid tissues at the time of pollination.

The name angiosperm derives

from the Greek words angeion, “vessel,”

and sperma, “seed.” The “vessel”

in this instance refers to the carpel,

which is a modified leaf that encapsulates

seeds. The carpel develops into

the fruit, a unique angiosperm feature.

While some gymnosperms, including

yew, have fleshlike tissue around their

seeds, it is of a different origin and not

a true fruit.

The origins of the angiosperms puzzled even Darwin (his “abominable mystery”). Recently, consensus

has been growing on the most basal, living angiosperm—

Amborella trichopoda. Amborella, with small, cream-colored

flowers, is even more primitive than magnolias or water

lilies, other candidates for basal ancestors. This small

shrub, found only on the island of New Caledonia in the

South Pacific, is the last remaining species of the earliest

extant lineage of the angiosperms, arising about 135 million

years ago. While Amborella is not the original angiosperm,

it is sufficiently close that much may be learned

from studying its reproductive biology that will help us

understand the early radiation of the angiosperms.

Monocots and Dicots

There are two classes of angiosperms, phylum Anthophyta:

the Monocotyledonae, or monocots (about 65,000

species) and the Dicotyledonae, or dicots (about 175,000

species). Dicots are the more primitive of the two classes,

with monocots apparently having derived from early dicots.

Included in the dicots are the great majority of familiar angiosperms—

almost all kinds of trees and shrubs, snapdragons,

mints, peas, sunflowers, and other plants. Monocots

include the lilies, grasses, cattails, palms, agaves, yuccas,

pondweeds, orchids, and irises.

Monocots and dicots differ from each other in a number

of features, some are listed in figure 37.15. Monocots and

dicots differ fundamentally in other ways. For example,

about a sixth of all dicot species are annuals (plants that

complete their entire growth cycle within a year); there are,

however, very few annual monocots. Underground swollen

storage organs, such as bulbs, occur much more frequently

in monocots than they do in dicots. There are many species

of woody dicots (mostly trees or shrubs), but no monocots

have true wood; however, a few monocots, such as palms

and bamboos, produce extra bundles of conducting tissues

that give them a woody texture. Endosperm, which is usually

present in mature monocot seeds, is largely absent in

mature dicot seeds. Other specific differences will be presented

The Structure of Flowers

Flowers are considered to be modified stems bearing

modified leaves. Regardless of their size and shape, they

all share certain features. Each flower originates as a primordium that develops into a bud at the end of a stalk called a pedicel. The pedicel expands slightly at the tip into a base, the receptacle, to which the remaining flower parts are attached. The other flower parts typically are attached in circles called whorls. The outermost whorl is composed of sepals. In most flowers there are three to five sepals, which are green and somewhat leaflike; they often function in protecting the immature flower and in some species may drop off as the flower opens. The next whorl consists of petals that are often colored and attract pollinators such as insects and birds. The petals, which commonly number three to five, may be separate, fused together, or missing altogether in windpollinated flowers.

The third whorl consists of stamens, collectively called

the androecium, a term derived from the Greek words andros,

“male,” and oikos, “house.” Each stamen consists of a

pollen-bearing anther and a stalk called a filament, which

may be missing in some flowers. The gynoecium, consisting

of one or more carpels, is at the center of the flower.

The term gynoecium derives from the Greek words gynos,

which means “female,” and oikos, or “house.” The first

carpel is believed to have been formed from a leaflike structure

with ovules along its margins. The edges of the blade

then rolled inward and fused together, forming a carpel.

Chapter 37 Evolutionary History of Plants 749

Nonvascular plants

Seedless vascular plants

Gymnosperms

Angiosperms

MONOCOTS

1. Seed with one cotyledon (“seed leaf”).

2. Leaves with parallel veins.

3. Lateral meristems rarely occur.

4. Flower parts mostly in threes or

multiples of three.

DICOTS

1. Seed with two cotyledons (“seed

leaves”).

2. Leaves with a network of veins.

3. Lateral meristems (cambia)

present.

4. Flower parts mostly in fours or fives

or multiples of four or five.

Comparison of monocots and dicots.

Primitive flowers can have several to many separate carpels,

but in most flowers, two to several carpels are fused together.

Such fusion can be seen in an orange sliced in half;

each segment represents one carpel. A carpel has three

major regions  The ovary is the swollen base,

which contains from one to hundreds of ovules; the ovary

later develops into a fruit. The tip of the carpel is called a

stigma. Most stigmas are sticky or feathery, causing pollen

grains that land on them to adhere. Typically there is a neck

or stalk called a style connecting the stigma and the ovary;

in some flowers, the style may be very short or even missing.

Many flowers have nectar-secreting glands called nectaries,

often located toward the base of the ovary. Nectar is a

fluid containing sugars, amino acids, and other molecules

used to attract insects, birds, and other animals to flowers.

The Angiosperm Life Cycle

While a flower bud is developing, a single megaspore

mother cell in the ovule undergoes meiosis, producing four

megaspores . In most flowering plants, three

of the megaspores soon disappear while the nucleus of the

remaining one divides mitotically, and the cell slowly expands

until it becomes many times its original size. While

this expansion is occurring, each of the daughter nuclei divide

twice, resulting in eight haploid nuclei arranged in two

groups of four. At the same time, two layers of the ovule,

the integuments, differentiate and become the seed coat of

a seed. The integuments, as they develop, leave a small gap

or pore at one end—the micropyle (see figure 37.16). One

nucleus from each group of four migrates toward the center,

where they function as polar nuclei. Polar nuclei may

fuse together, forming a single diploid nucleus, or they may

form a single cell with two haploid nuclei. Cell walls also

form around the remaining nuclei. In the group closest to

the micropyle, one cell functions as the egg; the other two

nuclei are called synergids. At the other end, the three

cells are now called antipodals; they have no apparent

function and eventually break down and disappear. The

large sac with eight nuclei in seven cells is called an

embryo sac; it constitutes the female gametophyte. Although

it is completely dependent on the sporophyte for

nutrition, it is a multicellular, haploid individual.

While the female gametophyte is developing, a similar

but less complex process takes place in the anthers. Most

anthers have patches of tissue (usually four) that eventually

become chambers lined with nutritive cells. The

tissue in each patch is composed of many diploid microspore

mother cells that undergo meiosis more or less

simultaneously, each producing four microspores. The

four microspores at first remain together as a quartet or

tetrad, and the nucleus of each microspore divides once;

in most species the microspores of each quartet then separate.

At the same time, a two-layered wall develops

around each microspore. As the anther matures, the wall

between adjacent pairs of chambers breaks down, leaving

two larger sacs. At this point, the binucleate microspores

have become pollen grains. The outer pollen grain wall

layer often becomes beautifully sculptured, and it contains

chemicals that may react with others in a stigma to

signal whether or not development of the male gametophyte

should proceed to completion. The pollen grain

has areas called apertures, through which a pollen tube

may later emerge.

Pollination is simply the mechanical transfer of pollen

from its source (an anther) to a receptive area (the stigma of

a flowering plant). Most pollination takes place between

flowers of different plants and is brought about by insects,

wind, water, gravity, bats, and other animals. In as many as a

quarter of all angiosperms, however, a pollen grain may be

deposited directly on the stigma of its own flower, and selpollination occurs. Pollination may or may not be followed

by fertilization, depending on the genetic compatibility of

the pollen grain and the flower on whose stigma it has

landed. (In some species, complex, genetically controlled

mechanisms prevent self-fertilization to enhance genetic diversity in the progeny.) If the stigma is receptive, the pollen

grain’s dense cytoplasm absorbs substances from the stigma

and bulges through an aperture. The bulge develops into a

pollen tube that responds to chemical and mechanical stimuli

that guide it to the embryo sac. It follows a diffusion gradient

of the chemicals and grows down through the style and

into the micropyle. The pollen tube usually takes several

hours to two days to reach the micropyle, but in a few instances,it may take up to a year. One of the pollen grain’s two cells, the generative cell, lags behind. Its nucleus divides,

Diagram of an angiosperm flower.

(a) The main structures of the flower are labeled. (b) Details of an ovule. The ovary as it matures will become a fruit; as the ovule’s outer layers (integuments) mature,

they will become a seed coat.in the pollen grain or in the pollen tube, producing two sperm cells. Unlike sperm in mosses, ferns, and some gymnosperms,the sperm of flowering plants have no flagella. At this point, the pollen grain with its tube and sperm has become

a mature male gametophyte.As the pollen tube enters the embryo sac, it destroys a synergid in the process and then discharges its contents.Both sperm are functional, and an event called doublefertilization, unique to angiosperms, follows. One sperm unites with the egg and forms a zygote, which develops into an embryo sporophyte plant. The other sperm and the two polar nuclei unite, forming a triploid primary endosperm nucleus. The primary endosperm nucleus begins dividing rapidly and repeatedly, becoming triploid endosperm tissue that may soon consist of thousands of cells. Endosperm tissue can become an extensive part of the seed in grasses such as corn In most flowering plants, it provides nutrients for theembryo that develops from the zygote; in many species,

such as peas and beans, it disappears completely by the

time the seed is mature. Following double fertilization, the

integuments harden and become the seed coat of a seed.

The haploid cells remaining in the embryo sac (antipodals,

synergid, tube nucleus) degenerate. There is some evidence

for a type of double fertilization in gymnosperms

believed to be closely related to the angiosperms. Further

studies of this and of fertilization in Amborella, the most

basal, extant angiosperm, may provide clues to the evolution

of this double fertilization event.

Angiosperms are characterized by ovules that at

pollination are enclosed within an ovary at the base of a

carpel—a structure unique to the phylum; a fruit

develops from the ovary. Evolutionary innovations

including flowers to attract pollinators, fruits to protect

and aid in embryo dispersal, and double fertilization

providing additional nutrients for the embryo all have

contributed to the widespread success of this phylum.

Life cycle of a typical angiosperm. As in pines, external water is no longer required for fertilization. In most species of angiosperms,

animals carry pollen to the carpel. The outer wall of the carpel forms the fruit that entices animals to disperse the seed

in the pollen grain or in the pollen tube, producing two
sperm cells. Unlike sperm in mosses, ferns, and some gymnosperms,
the sperm of flowering plants have no flagella. At
this point, the pollen grain with its tube and sperm has become
a mature male gametophyte.
As the pollen tube enters the embryo sac, it destroys a
synergid in the process and then discharges its contents.
Both sperm are functional, and an event called double
fertilization, unique to angiosperms, follows. One sperm
unites with the egg and forms a zygote, which develops
into an embryo sporophyte plant. The other sperm and
the two polar nuclei unite, forming a triploid primary endosperm
nucleus. The primary endosperm nucleus begins
dividing rapidly and repeatedly, becoming triploid
endosperm tissue that may soon consist of thousands of
cells. Endosperm tissue can become an extensive part of
the seed in grasses such as corn (see figure 40.7).
In most flowering plants, it provides nutrients for the
embryo that develops from the zygote; in many species,
such as peas and beans, it disappears completely by the
time the seed is mature. Following double fertilization, the
integuments harden and become the seed coat of a seed.
The haploid cells remaining in the embryo sac (antipodals,
synergid, tube nucleus) degenerate. There is some evidence
for a type of double fertilization in gymnosperms
believed to be closely related to the angiosperms. Further
studies of this and of fertilization in Amborella, the most
basal, extant angiosperm, may provide clues to the evolution
of this double fertilization event.
Angiosperms are characterized by ovules that at
pollination are enclosed within an ovary at the base of a
carpel—a structure unique to the phylum; a fruit
develops from the ovary. Evolutionary innovations
including flowers to attract pollinators, fruits to protect
and aid in embryo dispersal, and double fertilization
providing additional nutrients for the embryo all have

contributed to the widespread success of this phylum.

References

• Bakker,Robert.(1978).dinosaur feeding behaviour and orgin of flowering plant.Macimillan london
• Stevens.P.F.(2011).Angiosperm phylogeny Website (at missouri botanical Garden)
• Dutta, A.C. 1999. Botany for Degree Students. Oxford     University Press, Calcuta.
• Pandey, SN; Trivedi, PS and Misra, SP. 1996. A text book of botany. Vol I. Vikas Publishing House PVT ltd.
• Pandey, SN; Trivedi, PS and Misra, SP. 1998. A text book of botany. 11th revised Edition Vol II. Vikas Publishing House PVT ltd.
• Jensen, WA & Salisbury, FB. Botany: An ecological Approach. Wadsworth Publishing Company inc., California.
• Gorge B. Johnson 2000. The living world. Second edition
• Murray W. Nabors 2004. Introduction to Botany. Pearson Benjamin Cummings. University of Mississippi
• Berg, LR. 1997. Introductory Botany: Plants, People and the Environment. St Pettersburg Junior college.