THE SEED
-
Upload
aracelis-torres -
Category
Documents
-
view
213 -
download
0
description
Transcript of THE SEED
AGRONOMY ENGINEERING
ENGLISH I
LCDA. ARACELIS TORRES
INTRODUCTION
Reproductive structure in plants that consists of a plant embryo, usually
accompanied by a supply of food (endosperm, which is produced during
fertilization) and enclosed in a protective coat. Seed embryos contain one
or more cotyledons. In typical flowering plants, seed production follows
pollination and fertilization. As seeds mature, the ovary that enclosed the
ovules develops into a fruit containing the seeds. Most seeds are small,
weighing less than a gram; the smallest contain no food reserve. At the
opposite extreme, the seed of the double coconut palm may weigh up to
about 60 lb. (27 kg). Seeds are highly adapted to transportation by
animals, wind, and water. When circumstances are favorable, water and
oxygen penetrate the seed coat, and the new plant begins to grow (see
germination). The longevity of seeds varies widely: some remain viable
for only about a week; others have been known to germinate after
hundreds or even thousands of years.
DEFINITION
A seed is a small embryonic plant enclosed in a covering called the seed
coat, usually with some stored food. It is the product of the ripened ovule
of gymnosperm and angiosperm plants which occurs after fertilization
and some growth within the mother plant. The formation of the seed
completes the process of reproduction in seed plants (started with the
development of flowers and pollination), with the embryo developed
from the zygote and the seed coat from the integuments of the ovule.
Seeds have been an important development in the reproduction and
spread of flowering plants, relative to more primitive plants such as
mosses, ferns and liverworts, which do not have seeds and use other
means to propagate themselves. This can be seen by the success of seed
plants (both gymnosperms and angiosperms) in dominating biological
niches on land, from forests to grasslands both in hot and cold climates.
The term "seed" also has a general meaning that antedates the above —
anything that can be sown, e.g. "seed" potatoes, "seeds" of corn or
sunflower "seeds". In the case of sunflower and corn "seeds", what is
sown is the seed enclosed in a shell or husk, whereas the potato is a tuber.
Ontogeny of the Seed-Coat
The bitegmic, anatropous ovule develops into an exalbuminous, partially
pachychalazal and endotegmic seed. In the mature seed-coat the
extensive chalazae with associated tanniniferous hypostases sensu lato
manifests externally as a characteristic brown patch. The walls of the
cells of the hypostases are impregnated with callose and lipidic
substances, which most probably represent cutin. Ultimately the outer
integument and outer parts of the inner integument are more or less
squashed. However, the cell walls of the inner epidermis of the inner
integument show distinct secondary thickening and lignifications. The
pachychalazal seed with undifferentiated seed-coat characterizes not only
a number of the genera of the tribe Anacardieae, but also occurs in
Heeria of the tribe Rhoeae. A number of genera of the tribe Spondiadeae
have a partially pachychalazal seed. The seed-coat of the latter shows
varying degrees of traces of an exo-, meso- and/or endotestal
lignifications. The seed of certain genera of the Rhoeae is partially
pachychalazal and endotegmic, or probably only endotegmic.
Origin
The seed is a reproductive unit complex, characteristic of higher vascular
plants, which is formed from the egg plant, usually after fertilization. It is
found in flowering plants (angiosperms) and gymnosperms. In
angiosperms the eggs develop within an ovary, while in gymnosperms,
the structure that contains them is very different, as not a real flower, but
the structure of the seeds of these plants is basically similar to that of
flowering plants.
Function
Seeds serve several functions for the plants that produce them. Key
among these functions is nourishment of the embryo, dispersal to a new
location, and dormancy during unfavorable conditions. Seeds
fundamentally are a means of reproduction and most seeds are the
product of sexual reproduction which produces a remixing of genetic
material and phenotype variability that natural selection acts on.
DOUBLE FERTILIZATION
Double fertilization is a complex fertilization mechanism that has
evolved in flowering plants (angiosperms). This process involves the
joining of a female gametophyte (mega gametophyte, also called the
embryo sac) with two male gametes (sperm). It begins when a pollen
grain adheres to the stigma of the carpel, the female reproductive
structure of a flower. The pollen grain then takes in moisture and begins
to germinate, forming a pollen tube that extends down toward
the ovary through the style. The tip of the pollen tube then enters the
ovary and penetrates through the micropyle opening in the ovule. The
pollen tube proceeds to release the two sperm in the mega gametophyte.
STRUCTURE OF EXTERNAL AND INTERNALOF THE SEED
OF MONOCOTYLEDONS AND DICOTYLEDONS
A seed consists of three parts: a Dormant Embryo, a Storage Tissue, and
a Seed Coat. Not every seed that has evolved on this planet has precisely
the same structure. In some seeds, the endosperm is retained as the
storage tissue. In other seeds, the endosperm is more or less used up to
put storage chemicals into the embryo itself (commonly in the
cotyledons). Below is a diagram of two hypothetical seeds. The upper
seed shows a dicot that lacks endosperm; its storage material (blue) is
held in the cotyledons, the lower seed shows a monocot that has a well-
developed endosperm (also blue). Both of these examples qualify as true
seeds because they possess all three parts needed to make a true seed.
Below is a photomicrograph of a longitudinal section of a Capsella seed.
The seed coat is the three-layered outer covering. The outermost and
innermost layers of the seed coat pick up a red dye used in the
preparation of this sample. That dye is preferentially held by parts of the
specimen that have waxy or brittle biomolecules. Cutin, sabering, lignin
are three components of plant cells that pick up the red dye. This leads us
to the idea that the integument has a waterproofing (cutin/sobering)
function and also possibly a mechanical strengthening (lignin) function
that could impede herbivore.
Looking toward the inside of the seed coat, there is a bent embryo (dark
looking) surrounded by some loose and light
colored cells of the remaining endosperm?
Indeed the paucity of endosperm leads us to the
conclusion that much of the storage material
has been moved to the embryo itself...perhaps
much of that in the cotyledons. However, this
seed is clearly somewhere between the two
extremes diagrammed above.
The embryo has an axis (on the left) with a
downward-pointing radicle including a root
apex. The radicle ultimately penetrates the seed
coat in seed germination, branches profusely,
and becomes the primary root system. Toward the top of the embryonic
axis, the radicle becomes hypocotyl...transitioning from root-type
anatomy into stem-type anatomy as we shall see later. In seeds with
hypogenous germination, this hypocotyl is too short and does not grow
above the soil. In species with epigeous germination, the hypocotyl
elongates rapidly, lifting the cotyledon(s) out of the soil and into the air.
The axis has two appendages, the cotyledons; as there are two
cotyledons, Capsella is a dicotyledonous plant. There are two classes of
flowering plants, the dicots and the monocots, and they are distinguished
(in part) on the basis of the number of cotyledons found on their dormant
embryos. The part of the axis where the cotyledons attach is called a
node because this zone of the axis does not elongate. At the extreme top
of the axis is a shoot apex; after germination this shoot apex will
elongate, and make appendages to produce the collection of stems and
leaves that constitute the plant shoot.
MORPHOLOGICAL DIFFERENCES BETWEEN SEEDS OF
ANGIOSPERMSPES AND GYMNOSPERMS
Gymnosperms
A gymnosperm (Greek for "naked seed") is a vascular plant that produces
seeds that are not protected by fruit but are hidden in a woody cone. They
do not have flowers, but most retain their leaves year round.
Gymnosperms include over 600 species in four divisions: Conifers,
Cycads, Ginkgoes, and Gnetophytes. Most of today's gymnosperms
belong to the conifer division. Gymnosperms are found in most of the
world's regions and take most of the credit for timber and paper products.
Angiosperms
Angiosperms are vascular plants that produce flowers and fruit with one
or more seeds. Angiosperms make up two classes: monocotyledonous
and dicotyledonous plants. Angiosperms make up more than 80% of all
plant species, ranging from roses to palm trees.
Gymnosperm - 'naked seed'
The endosperm is (n haploid)
Reproduction is achieved through male and female cones
Mainly represented by tree species
Phylem coniferophyta
Phylem Gnetophytes
Phylem Cycadophyta
Phylem Ginkgophyta
Angiosperm - flowering plant
The endosperm is (3n triploid)
Male pollen and female ovules are produced
Fruits and flowers
Phylem anthophyta
Read more:
http://wiki.answers.com/Q/What_is_the_difference_between_angiosperm
s_and_gymnosperms#ixzz21Mg5yjuo
TYPES AND MODIFICATION OF THE SEEDS
We have seen the basic features that are common to most types of seeds.
However, different plant species have their own modifications. In stone
fruits, for example, the seed is surrounded by a hardened fruit layer,
known as the endocarp. More broadly, the seeds of angiosperm plants
differ from those produced by gymnosperm plants. ‘Angiosperm’
literally means ‘enclosed seed’, and gymnosperm ‘naked seed’.
Angiosperm seeds are enclosed by a hard or fleshy fruit, whereas
gymnosperm seeds are usually exposed to the outside world, for example
on the cone of a conifer.
Seeds also vary a lot in size. Orchid seeds look like tiny dust particles,
whereas other seeds are much larger, such as the giant (over 20kg) seeds
produced by the coco of palm tree. As a rule, larger seeds are produced in
smaller quantities, as more energy goes into their production.
ADAPTATION OF THE SEEDS TO SPREAD OR DISPERSION
For plant species to survive, it's critical that its seeds are spread over the
landscape. Otherwise, the seeds would drop in a clump at the base of the
plant. To grow, the seeds would then have to compete with the mature
plant and all the other seeds for water, sunlight and nutrients. To avoid
this fate, plants have developed seed adaptations.
Wind
Some plants depend on the wind to spread their
seeds. In these cases, the seeds are often
attached to fine, fluffy threads that catch the
breeze, enabling them to float to a new patch of
land. Examples of plants with these seed
adaptation are dandelions and cattails. Some
tree seeds, such as those from maples and ashes,
also depend on the wind. These seeds will often have a propeller shape.
Water
Plants also use water to transport their seeds to new
locations. Coconuts -- which are a seed -- are an
example. The thick shells keep the salt water out
and the buoyant coconuts can float on the sea for
hundreds of miles without being damaged.
Animals
Animals are another way seeds are dispersed.
Plants that use this method have seeds that attract
animals, such as berries. After the seeds have
passed through an animal's digestive tract, they
can take root at the site of the animal dropping
containing the seed. Other seeds are adapted to
stick to animal fur. Such seeds, often known to
humans as burrs, eventually fall off and take root.
Germination
Some seeds have extra adaptations that help
control when the seeds are germinated no matter
how the seeds were dispersed. For example,
some seeds will not germinate -- or begin to
grow -- until after a fire. Those seeds depend on
the heat to crack the shell or open the pine cone,
allowing the seeds to drop to the ground. Other
seeds depend on the wind to scratch their casings, allowing water to seep
into the seed and begin the germination process.
Read more: What Are Three Seed Adaptations? | eHow.com
http://www.ehow.com/info_8494069_three-seed
adaptations.html#ixzz21MtpS4pH
http://www.ehow.com/info_8494069_three-seed-
adaptations.html#ixzz21MuQOkyY
http://www.ehow.com/info_8494069_three-seed-
adaptations.html#ixzz21MvZQlni
http://www.exploringnature.org/db/detail.php?dbID=45&detID=2792
IMPORTANT AGRICULTURAL SEED LEVEL
Seed production is an essential
process in agriculture. Thanks to it,
peasants have been domesticated plant
species we consume today, creating a
huge variety within each
species to be adapted to the different
environmental conditions and cultural
needs. This process has continued in
some regions for at least ten
thousand years. From the twentieth century, there is
a current tech industrial production and seed varieties designed to
create work with chemicals and mass
production conditions.
At the beginning of XXI century, this
trend has shifted in many regions of
the planet to the traditional production of
seeds. The industry argues that the industrial
seeds are more productive and free
of pests. Peasant movements like
Via Campesina argue that such
production depends on the application of
agrochemicals and overall energy subsidy on
oil and traditional seeds that are more robust and suitable
for local food. The marketing of seed industry is one of the causes of
genetic erosion.
BIBLIOGRAPHY
PRISCILA ANDRESSA CORTEZ1 and SANDRA MARIA
CARMELLO-GUERREIRO 1,2(received: November 10, 2006;
accepted: January 17, 2008) Revista Brasil. Bot., V.31, n.1, p.71-79, jan.-
mar. 2008 Ontogeny and structure of the pericarp and the seed coat of
Miconia albicans (Sw.) Triana (Melastomataceae) from “cerrado”, Brazil
http://www.scielo.br/pdf/rbb/v31n1/a08v31n1.pdf
Website Gerhard Leubner Lab Royal Holloway, University of
London 2005. The Seed Biology Place
http://www.seedbiology.de/structure.asp
The World of the Plants
http://www.botanical-online.com/llavorangles.htm
Catherine Steinbauer, eHow Contributor What Are Three Seed
Adaptations?
http://www.ehow.com/info_8494069_three-seed-adaptations.html
Amsel, Sheri. “Adaptations.” Pollination and Seed Dispersal
Adaptations. Exploring Nature Educational Resource. © 2005 - 2012.
July 22, 2012.
http://exploringnature.org/db/detail.php?dbID=45&detID=2792