Overview of Plant Classification: Vascular and Nonvascular Plants

 Introduction

So far throughout this series, we’ve  been using the word “plant” to refer  

to essentially any living organism that carries  out photosynthesis. 

we all sure 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  

Algae

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 seaweed groups:  

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. 

Vascular Nonvascular Plants

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.