A Sort-of Distinction Between Flowering Plants
If you spend any time at all identifying wildflowers, you’re going to run into the terms monocot and dicot. It’s a basic, biological distinction between two groups of flowering plants, sort of. Why “sort of?” Because it does help to identify plants, but it’s not perfect. In this case I mean perfect in the sense that we humans like everything to fall neatly into categories which are easier for us to remember. The problem is that nature doesn’t give a hoot about categories. When organisms in nature and multiplying and diversifying, they’re concerned with the availability of food, camouflage, predators, competitors and Crassulacean acid metabolism; the last thing they’re thinking about is “what neat, concise category are we heading for?”
OK, just a quick digression. In biology, the whole category thing can be traced back to a guy named Linnaeus who, as a good Christian, believed he was putting everything God created (pretty literally everything) into its proper category. Since we love categories, and God created everything, and God was created in our image, it made sense to think that he created stuff to fit into categories.
Over time, though, both the “neat category” and “God created everything” theories fell apart and left us with a real mess. We call it nature. Things weren’t all created all-at-once by a single entity, so they weren’t created to fit into some pre-existing schema. That caused a major shift in taxonomics: instead of figuring out where everything fit in the pre-existing schema, we had to create a new schema and try to force everything into it.
That’s where cotyledons come in. The key, defining difference between monocots and dicots is the number of cotyledons they have. Monocots have one and dicots have two. And that leads to an important question: What’s a cotyledon.
For starters, it’s not the first thing that’s going to jump out at you when you see a flower in a field. In fact, you have to look pretty hard, and at the right time, to see it at all. But it’s still an important distinction because even though the number of cotyledons isn’t obvious, there are secondary distinctions that are.
A cotyledon is kind of like a leaf. Leaves, as you know, supply plants with energy through photosynthesis. It works great, when it works, i.e. when a plant has leaves and sunshine to photosynthesize. But what about when a plant is young, still a seed, without any leaves and buried where the sun don’t shine (Underground! Not the other place, although...).
At this stage, there’s not much photosynthesis going on. Fortunately, plants plan ahead. While they’re producing their seeds, they store up some nutrients and pack them into the seeds’ endosperm. Yeah, the same endosperm we hijack to make bread and tortillas.
So in its early stages, a plant doesn’t need to photosynthesize, it can just draw energy from its own endosperm. It needs a process, though, to extract that energy and that’s where the cotyledon(s) come(s) in. Nature, being efficient, adapted a tool plants already have in order to extract nutrition from the endosperm. It’s called a cotyledon and it’s what biologists call a “leaf-like structure” (you were expecting Latin, weren’t you?).
To get back to the point, flowering plants have either one or two cotyledons. Except water lilies, which have either one or two cotyledons.
That is the genus water lilly itself has either one or two cotyledons, depending on how you look at it. It’s like one of those drawing that can be either a candlestick or a face. Some biologists see two cotyledons fused together, others see one cotyledon with two lobes.
Other than water lilies, each genus of flowering plants (angiosperms, to use the technical term) have either one or two cotyledons. So biologists frequently divide angiosperms into two categories: monocots and dicots.
As I mentioned earlier, the distinction has to do with more than just the number of leaf-like structures sucking nutrients from the endosperm. There are other differences as well. Unfortunately, this being nature, the differences are -- to paraphrase Barbosa -- more like guidelines than laws.
Monocots don’t make wood. Just to keep this from being a clear-cut distinction, though, some do produce a sheath that looks an awful lot like wood. Palm trees, for instance. The trunks sure look like wood, but they’re actually made up of overlapping leaves, like shake shingles. Except shake shingles are wood, but you get the point.
The key differences are:
Monocots
Dicots
1 cotyledon
2 cotyledons
To quote Barbosa again: “Yeah, we knew that one.”
Monocots
Dicots
Flower parts in threes
Flower parts in fours or fives
Leaf veins parallel
Leaf veins reticulate
Stem veins scattered
Stem veins in a ring
Roots adventitious
Roots from a radicle
No secondary growth
Usually some secondary growth
Pollen has one pore
Pollen has three pores
Flowers are divided into parts, like petals, sepals and stamens. In monocots, the number of petals, sepals or stamens is usually three or a multiple of three. In dicots, it’s usually four, five, or a multiple of four or five.
Leaves have veins running through them. In monocots the veins are usually parallel to each other. In dicots, they’re generally reticulated, that is, branching from one to another.
A plants roots can grow from a single point, or branch out from nodes along the stem. In monocots, they tend to do the latter. In dicots, the former.
Some plants grow outward by making wood or bark, others don’t. Monocots are the latter, most dicots are the former.