LECTURE 13
Patterns of Reproduction in Plants
Lecture Outline
I.
What are the general patterns of reproduction in plants?
A.
Plants have two multicellular generations that alternate to create their life cycles.
 In animals, the diploid phase of the life cycle is multicellular, and the haploid
phase of the life cycle is represented only by sperm and eggs
 In plants, there is a cyclical alternation of generations: a diploid, multicellular
sporophyte generation utilizes meiosis to produce haploid spores, which
grow into a multicellular gametophyte generation that produces gametes,
which unite to form a diploid zygote, which matures into a sporophyte
 Gametophytes are haploid and use mitosis to produce gametes in specialized
reproductive structures:
o Eggs are produced in female reproductive structures called archegonia
o Sperms are produced in male reproductive structures called antheridia
 In nonvascular plants, and several phyla of vascular plants, spores are
produced in spore cases called sporangia
 The haploid generation dominates in nonvascular plants, such as mosses,
whereas in vascular plants, such as ferns, conifers, and flowering plants, the
sporophyte generation dominates
B.
In nonvascular plants, the gametophyte generation is dominant.
 The nonvascular plants comprise three phyla: mosses, liverworts, and
hornworts
 Mosses
o In mosses (phylum Bryophyta), the gametophyte is a small green plant
with leaflike structures that grows in moist or boggy places
o A slender sporophyte grows out of the parent gametophyte
o Each sporophyte bears a sporangium, within which haploid spores are
produced by meiosis
o Ripe spores are released and fall to the soil, where they germinate and
grow into gametophytes
o Female gametophytes produce a small, flask-shaped archegonium at the
top of the plant, while male gametophytes produce antheridia at their tops
o Sperm are released following a rain, and they swim into the neck of an
archegonium, one sperm fertilizing one egg
o After fertilization, the diploid zygote develops into a young sporophyte
within the archegonium
o The cycle begins anew when the embryo grows out of the archegonium
and differentiates into a sporophyte
 Liverworts and Hornworts
o Liverworts (phylum Hepatophyta) resemble the human liver, and their life
cycle is very similar to that of mosses
o Hornworts (phylum Anthocerophyta) has elongated sporophytes that
protrude like horns from the creeping gametophyte
C.
In vascular plants, the sporophyte generation is dominant.
 Whisk ferns, club mosses, horsetails, and ferns do not form seeds
o A mature fern plant produces spores by meiosis in clusters of brownish,
dot-like sporangia (called sori) under the leaves
o Ripe spores disperse when the sporangia burst and those that settle on
moist soil will germinate, or sprout, into haploid heart-shaped
gametophytes (each one called a prothallus)
o Antheridia and archegonia develop on the underside of the gametophyte,
and eggs and sperm are produced
o Because eggs and sperm ripen at different times on the same gametophyte,
sperm from one plant usually swim to and fertilize eggs on an adjacent
gametophyte, producing a diploid zygote
o The zygote develops into an embryo, and then matures into the large fern
plant with which you are familiar
 Conifers, cycads, ginkgoes, and gnetophytes form naked seeds
o The most familiar representative of this group of plants, called
gymnosperms, is the pine tree
o The pine tree, the diploid sporophyte generation, bears specialized
sporangia called cones
o Male cones produce microsporocytes, which undergo meiosis to form
haploid pollen grains, each of which is a male gametophyte
o Female cones produce megasporocytes, which undergo meiosis to form the
haploid female gametophytes, each of which contains an egg
o Air-borne pollen grains get stuck in the sticky sap produced by the female
cones, and when the pollen grain comes in contact with the female
gametophyte (called an ovule), it produces a pollen tube that slowly grows
into the ovule – to the egg
o Sperm produced within the pollen tube fertilize the egg, and the zygote
grows into an embryo
o During this time, the female ovule enlarges, and grows into the familiar
pine seed, which falls from the cone when it dries up and the scales open
o The seed germinates, and the embryo continues to grow and eventually
becomes a new pine tree
 Angiosperms form protected seeds
o Angiosperms are flowering plants
o The conspicuous flowering plant is the diploid sporophyte generation
o The flower contains male stamens and one or more female pistils
o Meiosis produces haploid microspores within the anther of each stamen
o Microspores develop into haploid pollen grains, which are the male
gametophyte
o At the base of the pistil is the ovary, which holds the ovule
o Each ovule contains a single megaspore which produces the female
gametophyte, which, when mature, is called the embryo sac
o Eight cells develop within the embryo sac, one of which is the egg
o After a pollen grain lands on the pistil (either wind-borne or carried there
by an insect or bird), the pollen grain produces a long tube that grows
down the style of the pistil
o The pollen tube penetrates the ovule, and one sperm produced within the
tube fertilizes the egg nucleus, producing a diploid zygote, which
undergoes mitosis and develops into an embryo plant
o Another sperm nucleus from the pollen tube combines with two nuclei in
the embryo sac to produce a triploid endosperm nucleus, which develops
into nutritive tissue to feed the embryo
o The embryo is surrounded by nutritive tissue and the rest of the ovule,
which constitutes a seed
o The ovary ripens into a fruit, which attracts animals, which play a role in
dispersal of the seeds
o When the seed reaches favorable conditions, it germinates, and the
enclosed embryo will grow into a new plant
II.
How does pollination take place?
A.
III.
Animals and wind help pollinate plants.
 The two primary means for accomplishing pollination involves the wind and
insects, but birds, such as hummingbirds, and mammals in the form of tropical
bats are also effective pollinators
 Flowers have evolved showy colors and fragrant nectars that attract
pollinators in search of food
 While feeding on the nectar of one flower, pollen adheres to the insect’s body,
and is transported to the next flower visited
How do seeds develop, become dispersed and germinate?
A.
Seeds develop from ovules, and fruits develop from ovary walls.
 The seed consists of the embryo plant, the nutritive tissue surrounding the
embryo, and the outer covering of the ovule, which hardens into the seed coat
 While in the seed, the embryo develops embryonic leaves, called cotyledons
 Monocots, such as corn and wheat, develop a single cotyledon, while dicots,
such as beans and peanuts, develop two cotyledons
 A fruit is a mature ovary of an angiosperm, consisting of the seeds, the tissues
connected with them, and their coverings
B.
Animals, wind and water help in seed dispersal.
 Many fruits are edible, and attract animals that consume them and scatter the
seeds in their fecal droppings
 Some fruits have sticky surfaces, and stick to an animals body, to be deposited
somewhere later
 Some fruits, such as those of dandelions and maple trees, are blown away by
the wind
 Some fruits, such as coconuts, are carried away by the ocean
C.
Germination begins when the seed takes up water and begins to sprout.
 The first step in seed germination occurs when it absorbs water, which helps to
break open the seed coat and also stimulates metabolism in the embryo
 Usually the first portion of the embryo to emerge from the germinating seed is
the radicle (young root)
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IV.
Next, the epicotyl, the portion of the shoot above the cotyledons, undergoes
rapid elongation
In grasses, the first leaves are protected by a sheath called a coleoptile
The hypocotyl, the shoot below the cotyledons, next begins elongation
Multiplication of the cells (by mitosis) in the tips of the stem and roots
continues growth of the young seedling
What are types of vegetative propagation in plants?
A.
Underground runners and stems are two types of vegetative propagation.
 Vegetative propagation is an asexual reproductive process in which a new
plant develops from a portion of a parent plant
 Some plants, such as irises and grasses, use underground stems, called
rhizomes
 Other plants, such as strawberries, have horizontal stems that grow above the
ground
 Underground tubers, such as potatoes, can grow a new plant from each of it
“eyes,” which are lateral buds
 In some plants, such as African violets, new plants may arise from portions of
a leaf
B.
Using cell culture techniques, scientists can grow plants from individual cells.
 Scientists are able to take individual cells or groups of cells from a plant and
use them to grow a new plant on a petri dish