Measured Against a Tree

Take a tree as the definition of a plant, and write down every way a moss falls short of it. The list comes out fast, and every line on it is true.

What a moss has instead of roots are rhizoids, which anchor it and do essentially none of the absorbing. It takes water across its whole surface, directly, from rain and fog and the air. It has no xylem to lift that water and no phloem to distribute it, which is why it cannot be tall: without plumbing there is no way to supply a top.

And it cannot keep the water it gets. Its tissue simply equilibrates with the air around it. When the air is dry, the moss is dry, and when the moss is dry it stops.

Six faculties, six failures. This is a complete deficit assessment, and there is nothing factually wrong with a single line of it.

Hold on to that list. Every item survives to the end of this zine intact. What changes is what the list turns out to be a description of.

Botany has words for these two ways of dealing with water, and the useful thing about them is that neither one is the name of a defect.

Poikilohydric — from the Greek for varied — means the organism's water content tracks the water content of its surroundings. It has no interior to speak of. Most bryophytes live this way, and the review literature calls it the poikilohydric habit: tissue hydration closely following environmental humidity.

Homoiohydric — from the Greek for same — means the organism holds an internal water status roughly constant against whatever the outside is doing. Vascular plants live this way. So, incidentally, do you.

One matches the world. One argues with it. They are two answers to the same problem, and the words for them are descriptive rather than graded.

Notice what has already happened. Line five of the deficit list — water regulation: none — has a proper name, and under that name it stops being an absence and becomes a strategy. The same fact, relabelled, changes category.

That is not a trick of language. It is what happens when you stop measuring one thing by the standard of another and go looking for what it is actually doing.

Homoiohydry is not free, and it is not simple. It is an infrastructure project that a plant pays for continuously, for its whole life.

To hold an interior against a dry outside, a vascular plant needs roots to reach water that is not at the surface; xylem to lift it, sometimes tens of metres, against gravity; a waxy cuticle to stop it leaving; and stomata — thousands of controllable pores — because a sealed plant cannot take in carbon dioxide, so the seal has to be openable, and every opening is a leak it has chosen.

That is four organ systems in the service of one requirement. And the requirement is unforgiving in a specific way: if a homoiohydric plant loses the argument, it does not pause. It dies. Drying is not a state it can occupy. It is the end of the organism.

The tree's whole apparatus is the price of not being allowed to dry out. Height is what it bought with the plumbing; vulnerability is what it accepted along with it.

So the honest version of the comparison is not tree: capable, moss: incapable. It is two organisms with different fixed costs, different failure modes, and different places they can live.

The moss pays almost nothing to run and can be stopped without harm. The tree pays constantly and cannot be stopped at all. Neither of those is a better deal in the abstract. They are only better or worse somewhere in particular.

Everything so far has assumed the moss lacks something the tree has. The literature on desiccation tolerance says the opposite, and says it plainly.

In their 2007 review in The Bryologist, Proctor, Oliver, Wood, Alpert, Stark, Cleavitt and Mishler set out the evolutionary position: desiccation tolerance is a primitive character of land plants — ancestral, there from the beginning — which was lost in the course of the evolution of the homoiohydric vascular-plant shoot.

The moss did not fail to acquire the tree's solution. The tree discarded the moss's.

Two details make that more than a reversal of emphasis.

First: the vascular plants did not fully let go. The same review records that desiccation tolerance was retained in spores, pollen and seeds. Every seed in a packet is doing the thing the moss does — sitting dry, metabolically stopped, waiting for water. The tree kept the old trick for the one job where nothing else would work, and then spent its adult life unable to perform it.

Second: some went back for it. Desiccation tolerance re-evolved in the vegetative tissue of the vascular plants we call resurrection plants — which is to say some lineages, having given it up, evolved their way back to it, because where they ended up living it was the better answer after all.

Now read the deficit list again. Not one line of it has changed. But the column heading was wrong. It was never faculties absent. It was the older arrangement, still intact.

It would be easy, and wrong, to describe a dried moss as resting. Resting is something a running system does. This is not that.

As a desiccation-tolerant plant dries, its cytoplasm enters what the literature calls a glassy state — an amorphous solid, not a liquid, in which molecular mobility is severely reduced. Work on the xanthophyll-cycle pigments of Syntrichia ruralis was published under the title “Evidence for the absence of enzymatic reactions in the glassy state”, and that title is the finding: in the dry moss, the chemistry does not merely slow down. It does not run.

Nothing is being maintained. There is no idling, no low-power mode, no reduced service. The organism is not doing anything at all, and the reason it survives is precisely that it is not doing anything — a stopped chemistry cannot go wrong.

It is not conserving itself. It has stopped, and stopping is what conserves it.

Say plainly what the cost is, because there is one and it is real. A dry moss is not photosynthesising, not growing, not competing for anything. Line six of the deficit list stands: photosynthesis when dry, stops entirely. Time spent dry is time not spent living, and a moss in a dry climate may be stopped far more of its life than it is running.

This is the trade, stated honestly. Survival bought with the whole of the time.

Here is the part that is hard to believe until you see the gas-exchange traces. The moss does not convalesce. It resumes.

Infrared gas analysis of rehydrating Syntrichia ruralis shows respiration reactivating almost instantly, photosynthesis beginning within a few minutes, and net photosynthesis returning to substantially normal rates within thirty to sixty minutes. Within the first two hours the protein profile has already changed over: the drying proteins are switched off and a set called rehydrins comes up.

There is no ramp, no rehabilitation, no gradual return to duty. Water arrives and the machinery is running again inside an hour.

Nothing had to be rebuilt, because nothing had been dismantled. It was intact the entire time it was stopped.

How long can it stay stopped? Syntrichia can remain quiescent for decades and recover. And there is a stranger record: Roads, Longton and Convey, in Current Biology in 2014, grew new shoots from a core of Chorisodontium aciphyllum that had been frozen in Antarctic permafrost for at least 1,530 years. New growth appeared after three to four weeks. It was the first demonstration of survival on that timescale in any plant; comparable spans had previously been seen only in bacteria.

Keep the two cases apart. The Antarctic moss was frozen, not dried, and cryptobiosis in permafrost is not the same mechanism as desiccation tolerance. The 1,530 years belongs to the freezing case; the decades belong to the drying one. They rhyme, and conflating them would be an overclaim.

None of the previous spread works in open air. Drying and rewetting on that schedule needs a place where water lingers — and such a place exists, a few millimetres thick, over every surface in the world.

Air is viscous and sticks to things. Away from obstacles it moves in smooth sheets; nearer a surface, friction tugs at it and the stream separates into layers of different speed — fast laminar flow aloft, a turbulent zone of eddies beneath it, and then air that gets slower and slower until, immediately against the surface, it is perfectly still, captured by the friction with the surface itself. Robin Wall Kimmerer calls this that small space where earth and atmosphere first make contact. It is the boundary layer.

Because nothing moves there, the layer keeps what it holds: heat radiating back off sun-warmed ground, water vapour evaporating from a damp log, and carbon dioxide. Over a rotting log, where fungi and bacteria are working, the concentration can reach ten times that of the open air.

One number, dated on purpose. Kimmerer gives ambient as about 380 ppm, which was roughly right when Gathering Moss was published in 2003 and is not right now. The ratio is the claim; the baseline has moved since, and this zine quotes the multiple rather than the reading.

So a moss is a few centimetres tall at most, and that is not a shortfall against the boundary layer; it is the entry requirement. Kimmerer puts the height limit exactly where spread four put it: a plant more than a few centimeters high can't keep itself hydrated.

The moss is not a plant that failed to get tall. It is a plant the right size for somewhere a tree can never go.

And now the part that is better than any of that. The layer is not simply found and moved into. Friction makes it — so anything that increases friction deepens it. Moss leaves are long, narrow and held upright, which slows the air moving past them; species in dry places carry dense hairs, long reflective leaf tips, minuscule spines. Every one of those textures thickens the still layer overhead. Kimmerer states the rule twice, and the second half is the astonishing one: the greater the resistance, the deeper the boundary layer — and the larger the boundary layer, the larger the moss can be.

Read that as a loop, because it is one. The moss builds the habitat that then permits it to be bigger. The ceiling on its size was never a property of the moss. It was a property of the layer — and the layer is something an organism can thicken by being the shape it is.

Which is why mosses are not uniformly small. They run from crusts a millimetre high to wefts ten centimetres tall, a disparity Kimmerer likens to the one between a blueberry bush and a redwood — and the difference tracks the depth of the boundary layer in that habitat, not the quality of the moss.

One last thing, and it is generous. Boundary layers nest. A rock in a moist forest has its own, sitting under the umbrella of the forest's; the trees slow the wind and their shade holds back the drying, and the mosses on that rock grow lush and tall. So the tree that wins every contest for light is also, intending nothing by it, the reason there is a layer deep enough for a moss to be large in.

The ruler was never the tree. But the shelter, quite often, is.

There is one more inversion in the same shape as the last, and it is hiding in plain sight: the green thing you call “the moss” is not the same kind of body as the green thing you call “the tree.”

Land plants alternate between two generations. The gametophyte carries one set of chromosomes and makes the sex cells; the sporophyte carries two and makes spores. Both exist in every land plant. What differs is which one gets to be the plant.

In a moss, the green cushion is the gametophyte. The sporophyte is the little stalk with a capsule on the end that grows up out of the cushion — attached to it, nutritionally dependent on it, and short-lived. In a tree, it is the other way round and not by a small margin: the tree is the sporophyte, and its gametophyte generation has been reduced to a handful of cells inside a pollen grain and an ovule.

The moss is not a small version of the tree. It is the other half of the life cycle, living in the open, at full size.

That little stalk has one job, and the last spread explains it. Spores cannot disperse in still air, and they cannot germinate in their own parents’ carpet — so the seta is a periscope, lifting the capsule up out of the boundary layer and into the moving air above, where eddies can pull the spores out and carry them off. Kimmerer records that its length is strongly correlated with the depth of that layer: tall setae in forests, short ones in exposed places. The dependent generation is the one built to leave.

So the two organisms are not even the same part of the same story. One kept both generations visible and let the older one stay in charge. The other put nearly everything into one generation and reduced the other almost to nothing.

Twice now the comparison has broken down in the same direction. Not because the moss turns out to be secretly grand, but because tree was never the general case. It was one specialisation, being used as a ruler.

Mosses in the genus Sphagnum build bogs. They acidify the water they sit in, hold many times their own dry weight in it, and decay so slowly in the resulting cold acid anoxia that their own dead bodies accumulate underneath them for thousands of years. That accumulation is peat.

Peatlands cover roughly 3% of the world's land surface and hold on the order of 500–550 gigatonnes of carbon — approximately twice the carbon held in the biomass of all the world's forests.

The wording matters and is doing real work. The comparison is against forest biomass — the wood and leaves and roots — not against forests including their soils. The looser claim, “peatlands store twice as much carbon as all the world's forests,” travels much further than the careful one and is not the same statement. We are using the careful one.

Still, take the careful one seriously. The organism with no roots, no vessels, no height and no control over its own water is holding twice as much carbon in its slow wet archive as every tree on Earth is holding in its wood.

Not because it is stronger. Because it does not decay, and because it has not stopped, and because there has been a great deal of time.

Which is the last thing the deficit list got wrong. It was scored on capacities — what can this thing do, right now, compared to that thing. It had no column at all for persistence, and persistence is where the moss keeps its entire answer.

A piece about an organism that survives by shutting down is about four sentences away from becoming something horrible. So, plainly: