Video Transcript
The figure shows the life cycle of
a fern. Identify the stage that corresponds
to reproduction by spores. (A), (B), (C), (D), or (E).
The question asks us about a group
of multicellular organisms, ferns, that reproduce through alternation of
generations. Alternation of generations is a
pattern of reproduction whereby an organism alternates between two distinct forms:
usually sexual and asexual or haploid and diploid within its own reproductive
lifecycle.
Let’s review the life cycle of
ferns, which is summarized in the diagram provided by the question, so that we can
work out the stage that corresponds to reproduction by spores.
A mature fern is called a
sporophyte and is composed entirely of diploid cells. Remember, a diploid cell contains
two complete sets of chromosomes, one set from each biological parent, which is
often represented as two 𝑛. The underside of fern leaves often
contain spotlike structures called sori. Each of the sori contain diploid
cells called spore mother cells. Spore mother cells undergo a type
of cell division called meiosis to produce haploid spores, which are stored in a
structure called a sporangium. We can see meiosis occurring in
stage A of the diagram provided by the question.
Remember, a haploid cell, which is
often represented as 𝑛, contains only one set of chromosomes. This is half the number of most
other cells, including the diploid spore mother cells, that halved their genetic
material through meiosis to produce these haploid spores. The 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. This process of spore reproduction
is represented in the diagram in stage (B).
Gametophytes are responsible for
producing sex cells, which are otherwise known as gametes: the sperm cells and the
egg cells. This is shown in stage (C) of the
diagram. A sperm cell produced by one
gametophyte can either fertilize an egg cell of the same plant or an adjacent plant
to produce a diploid zygote, as represented by stage (D). The zygote eventually develops into
a mature sporophyte, full of diploid cells, initiating the life cycle once more, as
shown in stage (E) of the diagram.
Having reviewed the life cycle of a
typical fern plant, we can answer this question correctly. The stage in the diagram that
corresponds to reproduction by spores is (B).
