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BI
102 Spring 2003. Tuesday, May
06, 2003 Davison Lecture Notes (last day of class) Chapter 39 Reproduction in Plants
Plants reproduce both sexually and asexually as do
many animals. In sexual
reproduction, plants have for us the technically challenging life cycle
involving an alternation of generations.
An alternation of generations always results from a reproductive
strategy in which spores, not gametes, are the direct products of meiosis.
Spores, by definition, germinate and grow into a new
individual. In land plants,
i.e. embryophytes, spores are produce via meiosis within a sporangium
(spore making/containing sac). Within
a sporangium each spore mother cell (a.k.a. sporocyte) undergoes
meiosis producing four haploid spores.
Each sporangium may contain from one to hundreds of spore mother
cells. Some plants (all
bryophytes, many ferns) have only one type of sporangium and thus only one
type of spore. Flowering
plants, like all seed plants and some of the ferns, have two types of
sporangia and thus two types of spores. The spores of plants germinate and grow into a haploid
generation called gametophytes. Gametophytes
eventually produce gametes (egg and sperm) and following fertilization a
diploid zygote is formed inside the maternal gametophyte.
The resulting embryo is nutritionally dependent on the gametophyte.
In seed plants and ferns, the embryo eventually matures into a free
living, independent sporophyte generation (e.g. an oak tree or a radish
plant). The flower is unique to angiosperms.
The completion of sexual reproduction involving both meiosis
and fertilization occurs in the flower. Flower parts: sepals, petals, stamens,
anthers, filaments, carpels (pistils), stigma, style, & ovary. Floral variation: monoecious, dioecious,
perfect (bisexual flower). Sexual Cycle of Angiosperms (see figs. 39.1, 39.3,
39.5, & 39.6): Typically, four microsporangia
(=pollen sacs) are found in an anther.
Each haploid spore produced within a microsporangium develops into
a two-celled, haploid microgametophyte or pollen grain.
One cell of the pollen grain is called the generative cell
for it will produce (generate) two sperm nuclei, the other cell is the
tube cell. The tube cell
will elongate forming the pollen tube after the pollen grain has
reached a receptive stigma. The ovule is a structure common to all seed
plants. The ovule is many
things depending on its stage of development.
Protective integuments form the outer tissues of the ovule.
When mature the ovule becomes a seed, the seed coat forms from the
integuments. When very young
the ovule contains a megasporagium.
Within the megasporangium a single megasporocyte (megaspore
mother cell) undergoes meiosis. Only one of the four haploid spores produced is a viable megaspore.
The one viable megaspore “germinates,” that is it divides via
mitosis. The megaspore
divides three times producing a gametophyte (megagametophyte) of
eight nuclei but only of seven cells.
This Eight nucleate, 7-celled megagametophyte is better known as
the embryo sac. Following pollination (transfer of pollen to the
stigma) and pollen tube formation, the stage is set for Double
fertilization. The pollen tube delivers the two sperm nuclei to the embryo
sac by penetrating a gap left open by the integuments. One sperm fertilizes the egg cell and the zygote results.
The other sperm fuses with the two polar nuclei in a second
fertilization event. The
triploid nucleus resulting from the second fertilization event represents
endosperm. Ovary becomes the fruit. Ovary is part of the carpel and the carpel is derived from a folded leaf, a leaf that in ancestral gymnosperms supported exposed, naked ovules (as occur in cycads and other gymnosperms today). The carpel and the flower in general are thought to be largely responsible for the success angiosperms enjoy today (250,000 extant species compared to a mere 1,000 extant gymnosperms). The flower (especially the carpel) provides for:
Asexual Reproduction
Natural Vegetative ReproductionPlants reproduce asexually by producing specialized
fragments that can grow into new individuals that are genetic clones of
the parent. This type of
asexual reproduction via fragmentation is nicely illustrated by the
mother-of-thousands or maternity plant (plant shown in class). Man-induced Vegetative PropagationHumans also create plant fragments in propagating
ornamentals and garden plants such as tomatoes. A shoot cutting can be rooted and in some plants even
fragments from leaves can be rooted to produce genetic clones of the
original plant. Other
variations on the theme of fragmentation include natural shoot sprouts
from underground roots (see black locust and sassafras trees) and the
biotechnological practice of plant tissue culture (see fig. 39.11). Plant Tissue CultureIn Plant Tissue Culture (Test-Tube Cloning) a one-to few-celled fragment of tissue is excised from the plant to be cloned. Placed in the appropriate sterile media the cells will grow a new plant, literally in a test-tube (see fig. 39.11). The benefits of Plant Tissue Culture include:
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