Plant systematics, or the scientific classification of plants, is a science that I’m fascinated with. I think you’ll find this article cool if you like plant science even a little bit. I began studying this subject by simply thinking about trees, and I’m hoping to spark your curiosity for this subject with them too.
I think it’s fascinating that plants become trees as their ultimate form. This article briefly explores and talks about different plant lineages evolving into trees, and the different ways they achieve that, with awesome pictures.
I don’t need to get too deep into the weeds in order to explain what that means. OK, maybe a tiny bit into the weeds. First, we need to establish a baseline and define some terms.
The study of classifying organisms is commonly called taxonomy (or plant systematics, doesn’t roll off the tongue though). In the past, organisms used to be classified solely by their physical traits (or morphology). Think Darwin’s classifying finches in the Galapagos back in the early 1800s by their beaks. While it’s a great foundation and lens through which to classify organisms, nowadays, genetic analysis is used to more specifically classify organisms, though their morphology still plays a role, it isn’t the primary characteristic. This study is more specifically called ‘phylogeny’ (pronounced phy-lah-gen-ee).
A sequence of evolution is based on phylogenetic studies of large groups of organisms over time, which is called a lineage. You can also think of a lineage as a family tree.
Phylogeny has found interesting and surprising relationships between plants that we would have otherwise never known if we were to just look at their morphology. Using information from fossilized plants and phylogenetic analyses of whole lineages, the origins of certain traits (or combinations of traits) can be approximated along their timeline.
Phylogeny has led to the answers to very compelling questions. When did plants first begin evolving tree-like forms? And what are the different ways they can achieve it?
Convergent Evolution
And this is where we need to define convergent evolution. The term is sort of a weird one, so I’ll try to describe it in a few ways:
When analyzing lineages, scientists can identify traits that appear on multiple separate lineages. When that trait or characteristic functions similarly to another trait found on a separate lineage, we say both lineages converged on the same solution.
A quick example of convergent evolution is the development of fins in both fish and mammals. Two very different groups of organisms reaching the same evolutionary solution.
Both lineages took different evolutionary paths to achieve the same thing. Different lineages can take longer to achieve the same thing, and at different times throughout natural history.
We are using the notion of convergent evolution to guide the explanation towards how trees achieve their final form.
Tree-like Form
There are many definitions of what a tree is. I think it's appropriate to think of a “tree” as a classification of form. A plant achieves tree-like form when it assumes a growth pattern. You know the pattern. Plants that grow with tree-like form are similar in many ways; they have a “trunk” in order to support a larger size that sometimes branches. All in order to optimize for photosynthesis. But they fundamentally differ in the ways they achieve tree-form.
Why they Differ (Lineages)
Since there are many lineages at different times that have developed tree-like form, it makes sense that they differ. You would call both a palm and an oak a tree, right? Both lineages of these two example plants, the oak (eudicot lineage) and the palm (monocot lineage) have both converged onto growing tree-like form.
Image source (modified by Tree First)
The above chart is a very zoomed-out and simplified representation of the lineage of the entire plant kingdom. Don’t worry about studying it too closely. Just know, when we zoom in, each of those lineages becomes very complex, almost fractally. But from this far away, I’ve labeled where tree-like form has evolved several different times.
Tree-form has been converged upon in many different lineages. Even within closely related lineages, sometimes totally different growth structures are used to achieve tree-form. The common denominator across all plants that achieve tree-like form is the growth of a trunk-like structure, which we’ll call the trunk.
How they Differ (Trunk Formation)
Common Forms
The way trees build their trunk is a great example of different lineages converging on achieving tree-like form. These are two of the most common ways trees achieve their tree-like form:
From L. H. Bailey, “Palms and Their Characteristics,”. A woody-monocot stem cross section (left) next to a woody-dicot stem cross section (right)
Most of us are roughly familiar with the cellular structure of wood. You know, wood-wood. The regular kind with rings. In a phylogenetic sense, you could call it the most “modern” wood, having evolved most recently. It is capable of achieving the largest tree-like forms to date.
Compare that to the cellular structure of wood-wood to what I call palm-wood. It doesn’t form rings, it doesn’t form true heartwood, nor does it form bark. The entire cross-section of palm-wood is actively conducting, unlike wood-wood, where only the sapwood is. Palm-wood is also a wet, almost spongy texture.
Left: Palm cross section highlighting spongy fibers (Image source) | Right: wood-wood
Less Common Forms
Most of the big trees we interact with have one of those two types of wood. But there are even more ways tree trunks can form! Check out some other cool trunk cross sections.
Tree-ferns generally don’t grow beyond 16 feet tall, and don’t have nearly the geographic range as other trees. Tree-ferns evolved the earliest of any other currently alive trees, and you could consider this a primitive tree-like form, having evolved millions of years before any other (see lineage chart above for “lycophytes and ferns”).
Left: tree-ferns (image source) | Center: tree-fern cross section (image source) | Right: tree-fern cross section (image source)
Cycads are a group of ancient plants within the gymnosperm lineage. Interestingly, when currently alive species of cycads are compared with fossilized cycads from the past, there isn’t a significant difference between them. Some call them “living fossils”. While they can resemble palms, they’re very distantly related. They begin their own lineage all the way back when gymnosperms were first appearing (check the lineage chart above). That’s around 300 million years before palms.
Most currently alive species of cycads don’t grow to have tree-like form, but a small handful of them certainly do.
Left: Encephalartos woodii cycad with tree-like form (image source) | Center: Cross section of Encephalartos woodii (image source) | Right: Diagram of generic cycad trunk anatomy (image source)
Bananas use an interesting trunk growth pattern to achieve tree-like form. Bananas are considered monocots just like palms, although they are classified in different orders. Despite being closely related, the cross sections of these two groups of plants don’t resemble one another at all. Banana trunks (pictured above) are formed by fleshy leaf tissue growing around a central core.
Left: Musa ingens or giant banana (image source) | Center: Cross section of a Musa species (image source) | Right: Cross section diagram of a Musa species (image source)
The cross sections really make this convergence on tree-like form obvious. The plants use whatever means necessary, by shortest evolutionary path possible, to evolve to withstand environmental pressures. Sometimes brand new tissues are evolved, like the cork cambium of wood-wood. Other times old tissues are adapted into new roles, such as the modified leaves which form the banana’s trunk.
Cats and Dogs
It’s easy to think all trees are pretty similar. And sure, broadly speaking yeah they are. They share a lot of plant traits. Tree-like form has been achieved in many different ways throughout natural history.
After pointing out how distantly related trees have convergently evolved common traits, it sort of begs the question; are there other cool examples of convergent evolution in other family trees? Absolutely. Tons of them.
There are other instances of animals achieving ultimate-forms too. Check out “carcinization”, a phenomenon where multiple family trees evolve crab-like form. Pretty insane. Maybe then, I should coin the term “arborization” for what I’ve explained in this article. You heard it here first.
Would you say that a cat and a dog are pretty similar? They both have four legs, a mouth, and other common mammal traits. But just how close are they actually? What if I told you that dolphins are more closely related to hippos than cats are to dogs?
This is why I think taxonomy is so compelling and full of mindblowing connections. We simply could never make these connections without the use of hardcore genetic science. It allows us to come to the conclusion that currently, the ultimate form of land plants are trees. But we sort of already knew that intuitively, right?