|
SEEDS
I. Double fertilization
A. Following pollination the pollen tube grows down through the style. It
enters the ovule through the micropyle and one sperm fuses with the egg to
form the zygote and the other sperm fuses with the two nucleate polar cell to
form a primary endosperm nucleus
B. Presumably this double fertilization ensures
that energy is not wasted forming stored food unless fertilization has taken
place
II. Seed development:
A. Primary endosperm nucleus divides many times to form endosperm
(stored food)
B. Zygote develops into the embryo, the young sporophyte
C. Integument forms the seed coat
D. Ovary wall (pericarp) and sometimes other structures develop into the
fruit
III. The embryo
A. The embryo consists of:
1. 1 or 2 cotyledons (seed leaves)
2. A stem like axis with apical meristems at both ends
B. In some embryos only an apical meristem occurs above the cotyledon(s)
C. Other embryos have an embryonic shoot with one or more leaves above the
cotyledon(s). Such a shoot is called an epicotyl
1. Plumule = the epicotyl plus these young leaves
2. Hypocotyl = stem like axis below the cotyledon(s)
3. Radicle = the embryonic root
D. In most dicots the cotyledons are fleshy and they
absorb all the stored food (endosperm) before the seed is shed. These fleshy
cotyledons occupy most of the volume of the seed, e.g. peanuts are mostly
cotyledons with a tiny hypocotyl visible
E. In dicots with large amounts of endosperm the cotyledons are thin and
membranous, and they absorb stored food from the endosperm
F. In most monocots the single cotyledon, called a scutellum,
may function as a food storage, photosynthetic, and food absorbing structure.
It is embedded in the endosperm, where it absorbs food digested by enzymatic
activity, which it moves to growing regions of the embryo
1. In monocots both the plumule and radicle are enclosed by sheath-like
structures called the coleoptile and coleorhiza,
respectively
IV. Embryo development
A. Following fertilization the zygote divides to form two cells
1. The lower cell at the micropylar end will ultimately give rise to a
stalk like suspensor
a. In addition to pushing the developing embryo into the endosperm, the
suspensor also appears to absorb nutrients from the endosperm. It also
appears that the suspensor manufactures substances that are utilized by
the embryo during its early growth
2. The upper cell will ultimately give rise to the embryo
B. As the embryo grows the three primary meristems begin
to form:
1. Protoderm - differentiates into epidermis
2. Ground meristem - gives rise to ground tissue called
cortex or pith if in the center
3. Procambium - differentiates into the vascular
cylinder which contains xylem and phloem
C. As the embryo matures the axis continues to elongate and it may remain
straight or become curved inside the seed
D. During embryo formation there is a continuous flow of nutrients from the
parent plant, resulting in the formation of stored food reserves in the
endosperm or cotyledons
E. Eventually the funiculus, the stalk connecting the
ovule to the ovary wall, separates
F. The seed loses most of its water and the seed coat hardens
V. Seed germination
A. Seed germination is dependent upon a number of factors, particularly
water, temperature and oxygen. Relatively few seeds require light for
germination
B. Most seeds contain only 5-20% water so they must take up or imbibe water
to resume metabolic activity. Enzymes become activated which then begin
digesting the stored food inside the seed
C. Early metabolic activity is primarily anaerobic but as soon as the seed
coat swells and bursts, aerobic respiration begins
D. The seeds of different species tend to have different optimum
germination temperatures, but most will germinate between 25-30oC
E. Some seeds must undergo a series of biochemical changes called after-ripening,
before they will germinate
1. E.g. some seeds must be cooled for a minimum period of time before
they will germinate. This prevents seeds of colder regions from germinating
in the fall so they won't be killed by harsh winters
2. Some desert plant seeds may require significant amounts of rainfall to
wash off germination inhibitors. This prevents them from germinating when
there is insufficient water for the seedlings to become established
3. Some seeds, such as those found in southern California, require a heat
treatment from fire to stimulate germination
|