STUDY GUIDE
Plant Life Cycles
In lab, we tried to copy down and make sense of the life cycles of mosses,
ferns, gymnosperms, and angiosperms. This study guide will hopefully help you
understand plant life cycles and their evolution.
Understanding Haploidy, Diploidy, and Alternation of Generations
One way to better understand the life cycles of plants is to compare them
to the life cycles of humans. Remember from prior lectures and labs that a diploid
cell is one that has two sets of chromosomes (2n chromosomes), one set inherited
from each parent. A haploid cell is one that has only one set of chromosomes (n
chromosomes.) In humans, sperm and eggs are haploid.
Study Question 1:
Are most of our cells haploid or diploid? What is the type of cell division
that can turn a diploid cell into a haploid cell?
Study Question 2:
Do our sperm and eggs divide into multicellular organisms before
fertilization? Does the egg divide after it has been fertilized? What does
fertilization do to the chromosome number in an egg? Do humans normally have
any multicellular haploid phase in their life cycle?
Alternation of Generations:
Instead of producing sperm or egg directly, meiosis in plants in the diploid
sporophyte stage produces spores. These are single cells which can be male or
female and can divide. When these spores divide by mitosis, they make haploid
gametophytes. It is the gametophytes that produce sperm or eggs. Sperm can
fertilize eggs, and the resulting zygote can develop into a new sporophyte.
In other words, while humans directly create gametes by meiosis, and
these gametes can be fertilized without dividing further, plants reproduce
differently. In plants, there's a delay and a great deal of mitosis before gametes
are produced to be fertilized. This delay is the gametophyte generation. Plants
alternate between sporophyte and gametophyte generations.
STUDY GUIDE
Plant Life Cycles
Study Question 3:
In humans, we can get, at most, four sperm from a single meiosis event. Is
this true in plants, which use meiosis to produce male and female gametophytes,
which then divide more to produce sperm and eggs? Why or why not?
Alternation of Generations in Bryophytes:
In bryophyte mosses, the gametophyte is the majority of the moss's
structure, near the ground. It produces eggs and sperm in special chambers, and
sperm can be dispersed from one gametophyte to another by rain or other water
to fertilize an egg. Once an egg is fertilized, it grows into a diploid sporophyte that
is still attached to the gametophyte. That sporophyte makes haploid spores that
become new gametophytes.
Alternation of Generations in Ferns:
In ferns, unlike bryophytes, the gametophyte stage is reduced to a small
prothallus. Fertilization occurs under or near the ground, and the resulting
sporophyte is the majority of the plant. A big, leafy fern we see in the woods is a
sporophyte, and its spores can be found under its leaves.
Study Question 4:
On land, there's a higher chance of sunlight and other environmental
dangers causing mutations. Suppose a mutation "breaks" one copy of a gene at
some random point in a plant's life. Which kind of plant, a bryophyte or a fern,
would seem to be at more risk of suffering severe consequences from losing one
working copy of a gene? Why?
Alternation of Generations in Gymnosperms and Flowering Plants:
Matters become a little more complicated when we reach the first seed
plants. In gymnosperms, as in ferns, the sporophyte is the main phase of the life
cycle. However, the haploid gametophyte is even less prominent. We'll use pines,
STUDY GUIDE
Plant Life Cycles
a conifer, as our example. A pine tree is mostly diploid sporophyte, and makes
male and female cones
In the male cones, meiosis occurs to give rise to spores, and these spores
give rise to pollen. This pollen is the male gametophyte, making sperm cells. In
the female cones, meiosis also gives rises to spores, but the gametophyte those
spores make is contained entirely inside the ovule (future seed). This female
gametophyte produces an egg that is fertilized by the sperm cells, giving rise to a
new sporophyte embryo. In other words, the gametophyte stage of a gymnosperm
is reduced to a tiny portion of the life cycle - pollen and part of the ovule.
Flowering plants are similar to gymnosperms in this way; they, too, reduce
the role of the gametophyte to pollen and the egg-producing cells of the future
seed. One difference is that in flowering plants, the seed is protected in an ovary.
Study Question 5:
Is the outside of a pine cone diploid or haploid? What about the inside of a
mature pine seed? The inside of a pollen grain? The petals of a flower? The seed
of an apple?
Summary Table:
Group of Plants
Bryophyte mosses
Ferns
Gymnosperms
Angiosperms
Mostly
gametophyte or
mostly
sporophyte?
Where is/are the
gametophyte(s)?
Where is the
sporophyte?