Plant Diversity

This week I want to start a series of articles spanning the next few months delving into the world of plants. I've written briefly about different species of plants, their uses, and some of their importance in a healthy ecosystem, but now I want to dive into the different structures, types, and functions found across them. Most people understand that trees, bushes, grasses, ferns, mosses, crops, and flowers are all considered plants. While many of these look vastly different from each other they share many common visible structures and internal mechanisms. This week I’m going to start with an overview of the variety of plant classes in the hopes of giving a basic intro.

Moss (Bryophyte)

Hornwort (Bryophyte)

First, let me provide a definition of a plant. Plants are embryophytes. The name comes from the way they protect and nurture the embryo inside the parent structure. This definition excludes algae and other things that may have been included with a less specific definition. Most plants are terrestrial (living on soil), but some species have evolved back into the water (such as water lilies and duckweed). All are complex, multicellular organisms. All plants are non-motile (they can’t move). Photosynthesis is their primary means of producing energy, but a small number are parasitic. Plants also display an alternation of generations between a haploid gametophyte (a multicellular generation with a single set of chromosomes) and a diploid sporophyte (two sets of chromosomes present).

Quillwort (Lycophyte)

Clubmoss (Lycophyte)

Plants can be further divided into two groups, those reproducing and spreading through spores: bryophytes (liverworts, mosses, and hornworts), lycophytes (clubmosses, spikemosses, and quillworts), and monilophytes (ferns and horsetails); and those reproducing and spreading through seeds: gymnosperms (conifers, cycads, gnetophytes and ginkgoes), and angiosperms (flowering plants). Plants can also be divided into those with vascular tissue (specialized tissue in the plant the transports water and nutrients around): lycophytes, monilophytes, gymnosperms, and angiosperms; and those without specialized vasculature: bryophytes. (Don’t worry about understanding the ins and outs of plant reproduction or tissue yet. I’ll make sure to cover them in detail in later posts.)

Fern Frond (Monilophyte)

Horsetail (Monilophyte)

Bryophytes include mosses, hornworts, and liverworts. They are grouped together based on their lack of vascular tissue, and are therefore referred to as non-vascular plants. However, they do not form a monophyletic group (a group made of an ancestor species and all its descendants). Instead they are more likely a paraphyletic group (a group made of an ancestor species and its descendants minus one or more monophyletic groups). In this case they are made of embryophytes minus the tracheophytes (plants containing vascular tissue). Bryophytes lack true leaves, stems, and roots. They are among the most primitive of land plants. They require almost constant moisture to keep from drying out as they lack many features that more advanced plants use to keep from drying out.

Lycophytes are represented by clubmosses, spikemosses, and quillworts. They still reproduce primarily through spores, but they have vascular tissue, and therefore; leaves, roots, and stems. They are more advanced than the bryophytes.

Monilophytes are made up of ferns and horsetails. They reproduce through spores, have vascular tissue, leaves, roots, and stems. They are the closest relatives to the seed plants.

Welwitschia 

(Gnetophyte, Gymnosperm)

Ginkgo (Gymnosperm)

Gymnosperms are composed of four different groups: conifers, cycads, gnetophytes and ginkgoes. Gymnosperms are more advanced than the previous groups because they produce seeds instead of spores for their primary reproduction. Gymnosperms are known as naked seed plants because their seeds are not enclosed. Even though they’re typically in cones for general protection, the seed itself is not encased. Conifers are made up of many common and important species including pines, firs, cedars, and junipers. They represent many of the largest plants on earth. Cycads generally have stout, woods trunks with a crown of leaves at the top. They can be confused with palm trees, but they are not closely related. Gnetophytes have some characteristics, such as vessel elements, not found in other gymnosperms. Ginkgoes are represented by one species of plant, Ginkgo biloba, and have been present on earth for millions of years virtually unchanged.

Quaking Aspen (Angiosperm)

Rose (Angiosperm)

Angiosperms make up everything else. They are the most common plants, representing some 250,000 species on earth. They have flowers, vasculature, stems, roots, leaves, and enclosed seeds. Orchids, maples, oaks, roses, apples, grasses, and many other common plants are all angiosperms. It’s important to keep in mind that not all flowers are showy and easily noticeable. When’s the last time you looked at a field of grass and pointed out its flowers? I assure you they are present, and when I get to the post on flowers I’ll cover it in much more detail.

Of course this is just the tip of the tree when it comes to all the fascinating aspects about plants. I realize that not many things are explained in detail here, but that’s not really the point for this post. The importance here is to understand the great diversity found in the world of plants. They started as very simple organisms, basically land versions of green algae. From there they evolved up to giant redwoods and flowering roses. There are many common characteristics, of course, but also a great deal of differences in them. Over the next few months I hope to introduce and explain features found in plants, and explain the differences.