Chapter 34, pages 733-738
Basic Botany-5 20:45-25:51
I. Seed Development
A. Recall the mature ovule
1. Structure of the embryo sac at fertilization
a. Egg Cell
b. Polar nuclei
c. Synergids
d. Antipodals
2. Embryo sac is surrounded by the nucellus
3. The nucellus is surrounded by the inner and outer integuments
B. Double fertilization of the embryo sac
1. Fusion of egg and sperm to form the diploid zygote
2. Fusion of the two
polar
nuclei with the
sperm
nucleus to form the
triploid
primary
endosperm
nucleus
C. Embryonic Development
1. Primary endosperm
nucleus
(3N)
divides rapidly by
mitosis,
forming a
multinucleated
mass of
endosperm
a. No further divisions of the primary endosperm nucleus occurs
b. Zygote nucleus remains quiescent until the primary endosperm has fully developed
2. Zygote grows into a full embryo
a.
First few
mitotic
divisions produces a
filament
4-8 cells long (filament called a
suspensor)
b.
Cell at the
micropylar
end elongates, pushing the rest of the filament further up
into the
endosperm
c.
Cell at
chalazal
end begins
mitotic
divisions at right angles to the filament (this
cell called a
proembryo)
d.
Further divisions of the proembryo produce a
globular
structure at the chalazal
end (called
globular
embryo)
e.
Globular structure grows into a
heart
shaped structure (this is the beginning of
cotyledon
formation)
f. Each branch of the heart grows into a cotyledon
II. Three Types of Seeds
1. Characteristics
a. Two cotyledons - the food source for the seed
b. No endosperm present
2. Structure
a. Hilum - oval scar where funiculus attached to the seed
b. Funiculus - stalk or "umbilical" cord of the seed
c. Micropyle - small opening in the seed coat below the hilum
d.
Raphe
- ridge above the hilum. The main conducting tissues of the
developing seed go through this area.
e.
Chalaza
- area above the raphe where the conducting tissue of the developing
seed
fans out over the seed
f. Integument - seed coat
g. Embryo
(1) Shoot - the part of the plant that will grow up and above ground
(a)
Cotyledons
- seed leaves, two of them, containing all the nourishment
for the seed.
(b)
Hypocotyl
- portion of shoot below the
cotyledons
that attaches to the
embryonic root of
the
radicle
(c)
Epicotyl
- portion of shoot above the cotyledons that contains the
apical
meristem
(2) Radicle - embryonic root that will turn down and grow into the ground
1. Exactly like the common bean seed EXCEPT:
a. Cotyledons are thin and leaf-like
b. Cotyledons become the first foliage leaves after the seed germinates
c.
Cotyledons are embedded in the
endosperm
and the cotyledons do not contain
any food material
d. The endosperm is the food source of the seed
C. Wheat Seed
1. Characteristics
a. One cotyledon
b. Endosperm present - the source of nourishment for the seed
2. Structure
a. Endosperm
(1) Two layers
(a) Aleurone layer
i) Single layer of cells
ii) Contains proteins and fats but little or no starch
i) Nutrient region of the seed
ii) Filled with starch grains
b. Embryo
(1) Shoot apex
(a) Contains rudimentary (early, non-functional leaves)
(b) Coleoptile - sheath covering the shoot and leaves
(2) Root apex
(a) Contains the root tip and the root cap
(b) Coleorhiza - sheath covering the root apex
(3) Scutellum
(a) Shield-shaped structure that
occurs between the
endosperm
and
shoot and
root
apicies
(b) This is the single cotyledon of the seed
(c) Function
i) Outer cells secrete enzymes that digests the food in the endosperm
ii) Digested food moves through the scutellum to the growing embryo
iii) Remains
inside the seed during germination and does not
emerge
and become a green leaf as the
castor
bean seed does
3. Terminology for manufactured wheat products
a.
Polished
white rice removes the
caryopsis
coat,
aleurone
layer and some
endosperm
b. White Flour - the ground endosperm of the wheat seed
c. Wheat Germ - the scutellum, shoot and root apicies
d. Bran - the caryopsis coat and aleurone layer that has been polished off the wheat seed
e. Whole Wheat Flour - the entire wheat kernel ground up into flour
f. Wheat Flour - flour that comes from wheat, generally unbleached white flour
g.
Bleached
Flour - white flour that has been bleached white with a chemical
agent
III. Seed Dissemination
A. Adaptations for dispersal
1.
Wind
dispersal - various types of
wings
or
plumes
on the fruit are used to catch the
wind and carry the fruit to new locations
a.
Memberanous
envelope within the fruit contains air so that the fruit will float -
sedges,
an example
b. Coarse, loose fibrous outer coat - coconuts
c. Most seeds will float until they become hydrated
a. Spines, hooks, barbs, etc. adhere to the animals coat
b.
The seeds of
fleshy
fruits pass through the
digestive
tracts of animals and birds
and are deposited with the
feces
of the animal
(1) Weed seeds through the digestive tracts of horses and cattle
(2) Tomato seeds through the digestive tracts of humans
(3) Toyon and Madrone seeds through the digestive tracts of Cedar Waxwings
c. Seeds are buried by animals - squirrels
IV. Seed Dormancy and Germination - Beal Experiment
A. Experiment
1. Started in 1878
2. Buried jars containing seeds of several different species of plants
3. Opened jars at 5 to 10 year intervals
B. Results of experiment
1. Most seeds remained viable for at least 10 years
2. Moth mullen germinated after 90 years
C. Archeological Deposits
1. Oriental lotus from archeological diggings it appears to be viable after 1000 years
2. Other species from cold
and
anaerobic
archeological deposits, it appears, are
viable
after long
periods
V. Factors for breaking seed dormancy
A. Moisture - seeds must be hydrated
B. Oxygen - seeds need oxygen to burn food for energy
C. Light - some seeds need light after they are hydrated to cause germination to occur
1.
Seed
coats that are resistant to water and oxygen
permeation
must be removed so
that
water and
oxygen
can
penetrate.
2. Scarification is the process of removing the impermeable seed coat
3. It can happen naturally
as in the intestine of an animal or artificially by
etching
the
seed with
acid
or mechanically rubbing off the seed coat
E. Cold Period
1. After being
hydrated,
the seed needs freezing or near freezing temperatures for a
certain minimum
time
2. This certain minimum
amount of time at this cold temperature signals the seed that the
winter is
past and it can
germinate
without being frozen by frost or cold weather
3. This happens naturally
with most wild seeds that are in the ground over the winter.
They become
hydrated in the fall and go through the winter and germinate in the
spring as
a result of experiencing the cold temperature.
4.
Stratification
is an artificial method of giving seeds a cold period. The seeds
are
spread
out on wet cheese cloth. The sheets of cheese cloth are stacked on
top
of one
another and placed in a dark cold refrigerator for approximately two
months
depending upon the species. The wet cheese cloth hydrates the seed
and the
cold period makes it ready to germinate.
F.
Chemical
Inhibitors -
abscisic
acid (plant hormone) as well as other chemical inhibitors
exist within the
seed
coat or fruit that prevents the seed from
germinating.
These must be
leached
out of the seed before it will
germinate.
G. Heat or Fire -
Cones
of certain of the
pines
remain closed after the seeds become
mature. When
a fire comes through the area, it heats up the cone. After
a few days, the cone
begins to open, releasing its seeds. These seeds
begin to germinate
as soon as the first heavy rains begin to come after
the fire. The
knobcone
pine is a good example of this.
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