No one really knows what a tree is - Max G. Levy

Consider these six plants.

It might not seem like going out on a limb to call them all trees.

But botanically speaking, only three make the cut.

Take a guess which.

Generally, true trees are tall, woody plants

that have leaves and one load-bearing trunk.

They begin their lives with primary growth,

where a soft stem develops upwards until secondary growth kicks in

and the stem bulks out into a woody trunk.

From there, trees keep growing taller from their crown,

and wider, so new rings expand their trunks annually.

Working off that rubric, identifying a tree might seem simple.

But not so fast.

What makes or breaks a tree can come down to some pretty specific characteristics,

based on how the plant develops as a result of how it evolved.

We only see trees among seed plants, which consist of two groups:

gymnosperms and angiosperms.

All gymnosperms are woody plants, and some of those are trees.

All angiosperms are flowering plants—

some of which are woody, and some of those are trees.

Plants assume a variety of forms.

Tree is one of them.

There are also shrubs, which grow as clusters of larger, woody stems—

like lavender, an angiosperm, and ephedra, a gymnosperm.

And then some angiosperms, like mint, are herbs,

because they grow from herbaceous, non-woody stems.

But mint and lavender are actually part of the same family—

one that also includes hardwood teak trees.

That’s because these planty forms are all evolutionarily tangled up,

and all trees don’t comprise one closely related group,

like insects or mammals.

Many plants actually charted entirely different evolutionary paths

to reach tree status.

Apple trees, for example,

are closer kin to rose shrubs and herbaceous strawberry plants,

than to avocados and guava trees, which are also pretty distantly related.

Over millions of years, some plant lineages became less tree-like,

while others became increasingly arborescent.

But not every lineage has what it takes to become a tree.

All plants exhibit primary growth.

And all gymnosperms also display secondary growth,

making them woody.

However, angiosperms tend to fall into two groups,

and only one yields woody plants.

All angiosperms sprout early leaves distinct from those that grow later;

some— dicots— sprout two seed leaves, while monocots sprout just one.

Monocots and dicots also grow differently from there.

Monocots don’t undergo woody secondary growth,

so none of them are trees.

Some dicots, however, do, and some woody dicots are trees,

though plenty aren't.

But why can dicots grow wood and be trees— and monocots can’t?

Well, the earliest angiosperms were probably woody,

much like their non-flowering relatives, the gymnosperms,

which likely evolved first.

Dicots seem to have kept the propensity for woodiness embedded in their DNA,

even as the characteristic has switched off at different points,

while monocots seem to have lost it completely.

So, okay, back to these six.

Each has its quirks, but these are the true trees of the bunch.

The Brazilian grape tree just looks like that

because its fruit grows directly from its trunk and branches—

a trait called cauliflory.

Giant baobabs’ bulbous trunks store water.

And bristlecone pines, some of the oldest trees,

grow slow and sturdy in their cold habitats,

twisted by millennia of high winds.

The remaining three plants grow in tree-ish shapes

but lack critical qualities.

All three are angiosperms, and more specifically, monocots.

Their stems are tall and thick, but made of herbaceous primary growth,

not wood from secondary growth.

Bananas have pseudostems with soft centers,

surrounded by hardened, overlapping leaves,

and are related to bird-of-paradise and ginger plants.

Joshua trees are succulents, like agaves.

And palms are pretty closely related to grasses.

However, among angiosperm dicots that do produce wood,

the line between shrub and tree can be blurry.

Characteristics like height and trunk diameter

might be used to reach a verdict.

But because those metrics change over time,

a plant like juniper may switch from shrub to tree within its own lifetime.

Plant evolution is twisty and complex, and tree-ness is no exception.

And even the arborescent plants that don't technically make it

into the official tree club

are certainly tree enough by other measures.

So, no shade to them.