So far throughout this series, we’ve been using the word “plant” to refer
to essentially any living organism that carries out photosynthesis. But I’m sure we can all agree
that there’s a world of difference between various photosynthetic organisms like algae,
mosses, grasses, and trees. So how do we organize and classify plants? If we start at the beginning
of the evolutionary history of plants, we can look at algae. Algae is a term we use to describe
a large group of photosynthetic organisms that are not actually all related to one another.
Many organisms that are sometimes referred to as algae, like cyanobacteria and diatoms,
don’t really resemble the things we normally think of as plants. In fact, cyanobacteria
don’t qualify as plants at all, because they’re unicellular prokaryotes, where plants
are multicellular eukaryotes. Similarly, diatoms, like other protists, although they are eukaryotic,
are also unicellular and therefore lack the structure we normally associate with plants,
so they’re classified totally separately, and referred to as plant-like protists.
Seaweeds and kelps are another story. There are three different groups of seaweed:
red, green, and brown, and they are all capable of photosynthesis. These organisms can also be
referred to as macroalgae, so they’re still not technically plants. But if you look at a seaweed,
it has blades that look like leaves, a stipe that looks like a stem, and holdfasts
that look like roots. So due to these and many other morphological and genetic similarities,
we can see that certain kinds of algae must be the ancestors of the land plants we’re more familiar
with, and it is the case that true plants started to develop on land around 500 million years ago.
There are two different major groups of true plants. These are vascular and nonvascular plants.
Nonvascular plants are usually small and relatively simple in their structure. This is
because they lack the xylem and phloem vascular tissues we learned about earlier in the series.
Without vascular tissues, these simple plants have no way to transport water and nutrients
long distances through their bodies, so they don’t have long stems or complicated structures.
Nonvascular plants also don’t have roots, but instead hold onto their substrate using tiny
hairs called rhizoids. As you might expect, since their structures and reproduction are
much less complicated, nonvascular plants are much older on the evolutionary timeline
than vascular plants. Some examples of nonvascular land plants are mosses, liverworts, and hornworts.
On the other hand, the more complex and more recently-evolved vascular plants
have complex structures. Vascular plants have xylem and phloem, which, as we learned, facilitate
the transport of water and nutrients through a plant’s body, allowing it to grow very large.
These vascular tissues also allow for more complex structures like roots, long stems,
and branches. Examples of vascular plants include ferns, conifers, and flowering plants.
Now that we know the basic differences between vascular and nonvascular land plants,
let’s use the next few tutorials to take a closer look at these groups, and specific types of plants
within them, so that we can understand their structures and their reproductive systems,
as well as how these different capabilities arose from an evolutionary perspective.