1. 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:
Eggs are produced in female reproductive structures called archegonia
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
In mosses (phylum Bryophyta), the gametophyte is a small green plant with leaflike structures that
grows in moist or boggy places
A slender sporophyte grows out of the parent gametophyte
Each sporophyte bears a sporangium, within which haploid spores are produced by meiosis
Ripe spores are released and fall to the soil, where they germinate and grow into gametophytes
Female gametophytes produce a small, flask-shaped archegonium at the top of the plant, while
male gametophytes produce antheridia at their tops
Sperm are released following a rain, and they swim into the neck of an archegonium, one sperm
fertilizing one egg
After fertilization, the diploid zygote develops into a young sporophyte within the archegonium
The cycle begins anew when the embryo grows out of the archegonium and differentiates into a
sporophyte
Liverworts and Hornworts
Liverworts (phylum Hepatophyta) resemble the human liver, and their life cycle is very similar to
that of mosses
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
A mature fern plant produces spores by meiosis in clusters of brownish, dot-like sporangia (called
sori) under the leaves

2. 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)
Antheridia and archegonia develop on the underside of the gametophyte, and eggs and sperm are
produced
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
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
The most familiar representative of this group of plants, called gymnosperms, is the pine tree
The pine tree, the diploid sporophyte generation, bears specialized sporangia called cones
Male cones produce microsporocytes, which undergo meiosis to form haploid pollen grains, each
of which is a male gametophyte
Female cones produce megasporocytes, which undergo meiosis to form the haploid female
gametophytes, each of which contains an egg
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
Sperm produced within the pollen tube fertilize the egg, and the zygote grows into an embryo
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
The seed germinates, and the embryo continues to grow and eventually becomes a new pine tree
Angiosperms form protected seeds
Angiosperms are flowering plants
The conspicuous flowering plant is the diploid sporophyte generation
The flower contains male stamens and one or more female pistils
Meiosis produces haploid microspores within the anther of each stamen
Microspores develop into haploid pollen grains, which are the male gametophyte
At the base of the pistil is the ovary, which holds the ovule
Each ovule contains a single megaspore which produces the female gametophyte, which, when
mature, is called the embryo sac
Eight cells develop within the embryo sac, one of which is the egg
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
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
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
The embryo is surrounded by nutritive tissue and the rest of the ovule, which constitutes a seed
The ovary ripens into a fruit, which attracts animals, which play a role in dispersal of the seeds
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. 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

3. While feeding on the nectar of one flower, pollen adheres to the insect’s body, and is transported to
the next flower visited
III. 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)
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
IV. 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