generation is the gametophyte generation. It is haploid.
The trend in plant evolution is a transition from a
dominant gametophyte, as in the liverworts and mosses,
toward a dminant sporophyte (diploid).
Below is a cross-section of the thallus (a word meaning simple plant
body)
of the liverwort Marchantia. Note the epidermal layer
at the top, and the
photosynthetic layer of cells with visible chloroplasts just beneath
the epidermis.
There is an air pore visible in the epidermis toward the left.
In the middle of
the thallus is a mass of storage tissue comprised of parenchyma cells.
At the bottom, the multi-cellular extensions are scales; rhizoids are
not visible
in this section.
Remember that in liverworts, the thallus or dominant
generation is the gametophyte generation. It is haploid.
The trend in plant evolution is a transition from a
dominant gametophyte, as in the liverworts and mosses,
toward a dminant sporophyte (diploid).
Below is the antheridial head or male sexual part of Marchantia.
This grows out of the top of the thallus, or leafy portion.
The dark ovals are spermatogenous tissues, which will develop
into sperm and be released. Sperm are flagellated, and must
be transferred via water.
Below is the archegonial head or female portion of
the liverwort Marchantia, which also grows up
out of the tahllus. On the left side of the stalk,
hanging down are several archegonia. An egg is visible
in each. Sperm must swim up the narrow neck canal
within the neck of the archegonium in order to fertilize the
egg.
A fertilized egg is called a zygote. The zygote develops into
a mature sporophyte, shown below. Recall that the sporophyte
is diploid, since it develops from the zygote. Note that it is nutritionally
dependent on the female gametophyte, from which it is still
suspended. The darkish cap of cells embedded in the female
gametophyte is called the foot. The dark red oval below the
sporophyte body is the sporogenous tissue. This tissue undergoes
meiosis to yield haploid spores. These are shed when the sporophyte
matures
Mosses are also primitve plants. Their dominant form is
also the haploid gametophyte, and the diploid sporophyte is
dependent on and gros out od the female gametophyte.
Below is the female or archegonial head of the moss Mnium.
Several archegonia are visible standing up in the middle of the head.
They have hollow cavities in which eggs are visible. They are
borne
on the female gametophyte. Once fertilized, each zygote will
grow into
a sporophyte, with a characteristic stalk and capsule. The sporogenous
tissue will undergo meiosis, which will then develop into spores.
These will be shed into the environment.
Below is the antheridial head or male sex organ of the moss
Mnium. The red ovals are spermatogenous tissues, whcih will form
sperm via mitosis. The antheridial head grows on the male gametophyte.
