In the previous tutorial we divided true plants into vascular plants
and nonvascular plants. Let’s now zoom in on the nonvascular plants,
which we can refer to as bryophytes, so that we can better understand them. The bryophyte group
is actually an informal way of talking about three very similar types of nonvascular plants:
mosses, liverworts, and hornworts. While distinct, all three of these types of plants
have enough characteristics in common that we can talk about them together.
Bryophytes are very sensitive to moisture, as they don’t have a vascular system to bring water
up from deep in the soil. You’ll usually find bryophytes living in cool, moist places, like
how moss grows on tree bark in dense forests or on rocks next to streams. However, they don’t need to
be completely submerged in water, giving them an evolutionary advantage over algae and other plant
ancestors, so these were the first types of plants to ever exist outside of some body of water.
Even though bryophytes are considered to be relatively simple plants, their forms
show significant diversity between species. For instance, these are two different types of moss.
And while this hornwort is somewhat similar in structure to moss, we can also see how different
they are. If we also look at a liverwort, we can really begin to understand the variety that
exists within the bryophyte group. For example, the rhizoids in mosses are multicellular, while
only unicellular in liverworts and hornworts. Some liverworts are parasitic, while mosses are not.
And there are a number of other subtle differences in their structure and organization.
Since bryophytes are nonvascular plants, they aren’t differentiated into leaves and stems as
easily as vascular plants are. Instead, we refer to the “body” of a bryophyte plant as the thallus.
Bryophytes, like essentially all plants, go through sexual reproduction. But plant life cycles
and reproduction are a little more complicated than what we’ve discussed for animals in the
biology series. Plants demonstrate heteromorphy, or a quality in which they have two genetically
and morphologically distinct generations that alternate. The two plant generations are a haploid
gametophyte and a diploid sporophyte. As we learned in our study of genetics, haploid refers
to any cell with a single set of chromosomes, like our sperm and egg gametes, and diploid refers to
cells with two full sets of chromosomes, like all of our normal somatic cells. The difference
is that plants utilize organismally distinct haploid gametophytes, which would almost be as
though our sperm and egg cells formed totally separate biological organisms unto themselves.
In fact, with bryophytes, the haploid gametophyte is the dominant generation,
or the generation that we’re most aware of seeing. The entire thallus or “body” of this moss
is the haploid gametophyte. The gametophyte sprouts from a haploid spore,
and it grows into a number of rhizoids, which are the things that look like tiny stems and
roots. But remember, the rhizoids aren’t actually stems or roots because they lack vascular tissue.
Once the gametophyte thallus is mature, it will sprout two kinds of reproductive “heads” called
antheridia and archegonia. Antheridial heads have multiple antheridia, or places where sperm
cells are produced through mitosis, rather than the way they require meiosis in humans,
because the gametophyte is already haploid. Archegonial heads have multiple venters where
egg cells are produced, also through mitosis. When there is sufficient water in the environment,
the sperm cells can swim to the archegonial heads and fertilize the egg cells.
Once an egg is fertilized, the zygote and then embryo develops inside the venter where the
egg was formed. Eventually, the embryo will emerge from the neck of the venter and grow
into a diploid sporophyte. In this particular case, the diploid generation actually never
detaches from the thallus of its haploid parent. These little stalks at the tips of this moss are
the sporophytes. The head of each sporophyte is called a capsule. Meiosis takes place within the
sporophyte capsule, cutting the chromosome number in half, and then haploid spores are released from
the capsule to start the cycle all over again. Now that we’ve examined nonvascular plants,
and received an introduction to plant reproduction, it’s time to shift our focus
to the different types of vascular plants, so let’s move forward and do just that.