AP Biology
Mr. McDowell
1)
Several spore cells are shown in the diagram and in the photograph above. These specialized
haploid cells (1N) are the product of meiosis and each cell nucleus (2) contains only a single set of
chromosomes. Spores are the first cells of the gametophyte generation. They are released into the
air from the sporophyte capsule. Dehydration of these cells, by the air, is prevented by a thick wall (1)
that covers their outer surface. Spores are produced in large numbers as only a few will be lucky
enough to reach environments suitable for germination and growth. Go to number 2 to continue.
2)
Wind blown spores (1) that land in favorable environments, germinate and grow into thread-like
filaments.These filaments, called protonema, resemble filamentous green algae. Cells of the
protonema stage of the gametophyte generation contain chlorophyll and are capable of
photosynthesis. As the protonema grows, via cell division, bud-like growths appear along the filament.
These buds eventually differentiate into small leaves and the bud takes on the appearance of a very
small moss plant. Go to number 3 to continue.
3)
Moss gametophytes (figure 3 ) lack vascular tissue. This causes them to be small in stature because
they lack supporting and transport tissues.. Most reach a height, when mature, of only 1 to 2 inches.
They thrive in moist shady environments and depend on water to carry sperm cells from the male
plant to the female plant. The male plant has a cup shaped head where several sperm producing
organs are located (see male head above). The leaves of the female head point upward and surround
the female egg producing organs. This shape is ideal for catching water droplets (see female head
above). When it rains water lands on the male head picking up sperm. When some of this water
splashes over onto the female head fertilization becomes possible. Go to number 4 to continue.
4)
The sperm producing organs of the male plant are called antheridia (see figure 4 and label 1 above).
In the photograph several antheridia are scattered over the surface of the male head. Hairs between
and around the antheridia reduce evaporation and help to keep the delicate sperm, contained within
the antheridia, from drying out. The antheridia are fragile structures and rupture when struck by falling
drops of rain. This event mixes water and sperm together forming splash droplets. These droplets
often land on the head of a neighboring female gametophyte. Go to number 5 to continue.
5)
The egg producing organs of the female plant are called archegonia (see figure 5 and label 1
above). In the photograph several archegonia are scattered over the surface of the female head. Note
how leaves and hairs bend up and over the archegonia. This arrangement helps to trap splash
droplets and to prevent dehydration between rain storms.The archegonium is bottle shaped and
produces a single egg (see figure 6 and label 2 above) near the base of the structure. The neck of
the archegonium remains closed until the egg is ready for fertilization. Go to number 6 to continue.
6)
An egg cell is identified by label 6. in the diagram and by number 1 in the photograph. It is produced
within the base of the archegonium. At maturity the archegonium's neck opens forming a pathway for
the sperm cell to reach the egg. The egg cell is haploid (1N) like all of the cells of the gametophyte
generation. Go to label 7 to continue.
7)
A sperm cell is identified by label 7. Flagella, shown in the diagram, make fertilization possible by
propelling the sperm cell through splash droplets to the open archegonium and egg within. The
direction the sperm moves is thought to be the result of chemical attraction. The sperm cell like the
egg cell is haploid (1N). Go to label 8 to continue.
8)
The zygote is a single diploid (2N) cell formed by the fertilization of the egg by the sperm (see
number 8 in the diagram). Remember, fertilization brings together two haploid (1N) cells giving rise to
one diploid cell (2N), the zygote. This cell is the first cell of a new sporophyte generation plant.
Immediately after fertilization the zygote begins to divide forming an embryo. Go to number 9 to
continue.
9)
The embryo is formed as the original zygote cell divides. This process forms a cluster of (2N) cells
that are dependent on the gametophyte for their nutrition. As growth continues the embryo extends
upward and eventually emerges from the archegonium at which time it is exposed to the surrounding
air. As can be seen in diagram (10), the sporophyte stage is recognizable as a capsule perched at
the top of a long supporting stalk. Go to number 10 to continue.
10)
In the diagram (10) and in the photograph (1) the sporophyte is shown attached to and growing out
of the top of the gametophyte (2). The sporophyte is composed of two main parts, the seta (4) and
the capsule (3). These are labeled red in the photograph. The seta, a stalk-like structure, elevates
the capsule away from the gametophyte plant. The capsule is the spore producing organ of the moss
sporophyte. Go to number 11 to continue.
11)
Label 11 identifies the sporophyte capsule. A longitudinal section of the capsule is shown in the
photograph above. Line (1) in the photograph points to a spore chamber. Here haploid spores (2) are
formed as diploid spore mother cells divide by meiosis. Line 3 above, points to the operculum. This
structure covers the outer end of the capsule. When the capsule is ripe the operculum falls away
allowing the spores to be released. With luck some of these spores will reach favorable environments
and start new gametophyte generations.