Video Transcript
During the life cycle of the
ornamental fern, Polypodium, a zygote is produced. What plant form does it develop
into? (A) A haploid gametophyte, (B) a
haploid sporangium, (C) a haploid sporophyte, (D) a diploid sporophyte, or (E) a
diploid gametophyte.
This question asks us about a
multicellular organism, the ornamental fern, Polypodium, that reproduces
through alternation of generations. Alternation of generations is an
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. Let’s review the life cycle of
ferns so that we can work out the plant form that a zygote produced by these ferns
develops into.
A mature fern is also called a
sporophyte and is composed entirely of diploid cells. Remember, diploid means that the
cells contain two complete sets of chromosomes, one set from each biological parent,
which is often represented as 2n. The underside of fern leaves often
contain spot-like structures called sori. Each of the sori contains diploid
cells called spore mother cells. Spore mother cells undergo meiosis
to produce haploid spores, which are stored in a structure called a sporangium. Remember, a haploid cell, which is
often represented as n, contains only one set of chromosomes, half the number of
most other body cells.
Haploid spores are eventually
released from the sporangium and those that land on a suitable surface begin to
grow. These spores grow into small plants
called gametophytes, which are full of haploid cells. Gametophytes are responsible for
the production of sex cells, or gametes: the sperm cells and the egg cells. The sperm cells produced by one
gametophyte can either fertilize the egg cell of the same plant or an adjacent plant
to produce a diploid zygote. The zygote eventually develops into
a mature sporophyte full of diploid cells, initiating the life cycle once more.
Now, we can answer this question
correctly. The zygote that is produced in the
life cycle of the ornamental fern develops into (D), a diploid sporophyte.
