BIO 120
Field Natural History
Spring
LECTURE 14
Plant Reproduction
I. Introduction to haploid and diploid life stages.
1. The life cycle of all plants has two stages – haploid and diploid.
a. The diploid stage means that there are two sets of chromosomes. Abbreviated as 2n.
b. The haploid stage means that there is only one set of chromosomes. Abbreviated as 1n.
c. Eggs (1n) and sperm (1n) are often called gametes. When they join together (fertilization)
the newly formed 2n cell is called a zygote.
2. Alternation of generations – Sequential change in the life cycle of plants between the diploid
and haploid stage.
3. Sporophyte - the diploid (2n) generation.
a. The sporophyte generation eventually produces spores, which are haploid.
4. Gametophytes – haploid individuals produced by germinating spores.
a. The gametophyte produces haploid gametes, e.g. egg and sperm.
b. The gametes fuse to form a diploid zygote – the sporophyte.
5. The major trend in alternation of generations from primitive plants to the more advanced
flowering plants is for plants to spend less time in the gametophyte generation and more time
in the sporophyte generation.
II. Alternation of Generations.
A. Vascular plants with seeds.
1. The sporophyte is the form of plants that you are most familiar with.
a. In plants, the growth of the zygote produces an embryo. Encased in what we call a seed.
2. The seed is a very important structure that permitted the successful colonization of land.
a. The seed structure serves as a protective, drought-resistant enclosure for the embryo,
enabling it to be dispersed to a new locality by wind, water or animals.
3. The pollen grain represents another important adaptation that allowed for the colonization of
land by plants.
a. The pollen encapsulates the male gametes in watertight packages that frees plants from the
need to use water to transport the sperm to the egg for fertilization.
B. Seedless, plants without vascular systems – the mosses.
1. Although mosses live on land, they have never really freed themselves from their dependence
upon an aquatic environment. There are 2 main reasons for this.
a. Most of the mosses lack well-developed vascular tissue (phloem and xylem).
b. They still have flagellated sperm cells which must swim to the eggs.
2. Basic structure of a moss.
a. Typical moss is actually the gametophyte generation.
b. It is haploid and it dominates the life cycle of mosses.
c. The sporophyte is the stalk and capsule.
3. The gametophyte generation is nutritionally independent.
a. The sporophyte is usually dependent upon the gametophyte.
b. Reverse in flowering plants – the sporophyte nutritionally independent.
c. The gametophytes – the pollen and egg , are produce by the sporophyte and dependent
upon it!
4. The sperm of moss have two flagella. When released, they swim to the egg.
a. They swim through a film of moisture provided by rain or a heavy dew.
BIO 120
Field Natural History
Spring
C. Lower (Seedless) Vascular Plants - The Ferns.
1. One major difference between the mosses and the ferns is size.
a. The ferns have a true root system, whereas the mosses do not.
b. The ferns have a well-developed vascular system, can grow taller.
2. The sporophyte generation of ferns is quite obvious – they are the fronds.
a. And they occupy a greater portion of the ferns life cycle.
3. One other distinction is that the sporophyte and gametophyte generations are separate and
nutritionally independent.
4. But, the ferns are still dependent upon water for reproduction.
a. The sperm swim through a film of water on the small (1-2 cm across) gametophyte.
b. Once again, they are dependent upon rain or heavy dew.
5. Moreover, the small gametophyte are not vascularized like the larger sporophyte.
a. Therefore, they cannot live in drier areas, but are limited to moist areas.
D. Vascular Plants with Seeds- The Flowering Plants – (Angiosperms)
1. Three major life cycle modifications that contributed to the success of seed plants on land.
2. The gametophyte generation of seed plants became even more reduced than in ferns or
mosses.
a. Small gametophytes of seed plants are protected from desiccation by being retained within
the moist reproductive tissue of the sporophyte generation.
3. Pollination (by air) replaces swimming as the mechanism for delivering sperm to eggs.
4. The seed evolved.
a. The seed relieves the female half of the life cycle from dependence on water.
b. Protects the embryo against dehydration.
c. Zygote is packaged with a food supply and a protective coat. This protects the embryo
from drought, cold, whatever.
d. The seed replaces the spore (1n) as the stage of the life cycle that disperses the offspring.
e. The seed can travel farther, by wind, water or animal away from the parental environment.
f. The seed has survival value because it has resources to begin life in what may be a less
than optimal environment.
E. Why does alternation of generations still persist?
1. Diploid plants have a back-up copy of chromosomes.
2. Diploidy may have been favored because of the harmful effects of UV radiation.
3. Dominant gametophyte generation of mosses and ferns is haploid.
a. If the only set of alleles is damaged, the gametophyte (which produces the egg and or
sperm) may produce only faulty or mutant gametes.
4. A diploid plant has two sets of alleles for the same gene. This second copy provides a
backup in the event that one set of alleles is damaged.
a. The second copy can allow an individual to have two slightly different versions of a
gene.
b. This either broadens the range of the plant, or allows it to cope with a change in the
environment.
5. Why does the haploid phase still exist?
a. Haploid phase can screen out mutant genes without severe impacts on the life cycle
of the plant.
b. Since the gametophyte is haploid, no alleles can “hide” from the environment. Only
the most competent alleles persist.
c. Only those gametophytes that are genetically competent in the existing environment
can persist and combine to produce a new sporophyte.