Video Transcript
The ornamental fern
Polypodium, shown in the figure, reproduces by alternation of
generations. Which structure produces female
gametes?
This question asks us about a
multicellular organism that reproduces through alternation of generations. Alternation of generations is a
very interesting phenomenon whereby an individual organism alternates between two
distinct forms, usually sexual and asexual, or haploid and diploid, within its own
reproductive life cycle. This pattern of reproduction is
often observed in plants, such as the ornamental fern Polypodium, which is
shown in the image. Letβs review the life cycle of
these plants to try and answer the question regarding which structure produces
female gametes.
A mature fern is also called a
sporophyte and is composed entirely of diploid cells, which is often represented as
2n. Remember, βdiploidβ means that the
cells contain two complete sets of chromosomes, one set from each biological
parent. If you have ever turned over the
leaf of a fern, you might be surprised to find little spotlike structures, which are
called sori. Each of the sori contain diploid
cells called spore mother cells. Spore mother cells undergo meiosis
to produce haploid spores, which are stored in a structure called a sporangium.
You may recall that a haploid cell,
which is often represented as n, is one that contains only one set of chromosomes,
half the number of most other body cells. The haploid spores are eventually
released from the sporangium, and those that land on a suitable surface can begin to
grow. The spores grow into small plants
called gametophytes, which are full of haploid cells. You might recognize the word
βgameteβ in gametophytes, which refers to the fact that gametophytes are responsible
for the production of sex cells, both sperm cells and egg cells.
The gametophyte contains specific
structures that produce each of these types of gametes. The female gamete, which is
otherwise known as an egg cell, is produced in a structure called the
archegonium. The male gametes, the sperm cells,
are produced in a structure called the antheridium and can travel to the archegonium
of the same or adjacent plants in order to fertilize an egg cell. Once fertilization has occurred, a
diploid zygote is formed, which can grow and develop into a mature sporophyte able
to initiate the life cycle all over again.
Now we know the answer to the
question as to which structure in a fern will produce the female gametes, or egg
cells. The female gametes are produced in
the archegonium.
