Primary Growth

People who wish to understand plants should stop worrying about tree rings; they really don’t get you anywhere. Think with me for a few minutes and take a lead from British poet Philip Larkin who turns our attention to leaves, which “begin afresh, afresh.” Set aside rings, at least for the moment.

The Trees

The trees are coming into leaf
Like something almost being said;
the recent buds relax and spread,
Their greenness is a kind of grief.
Is it that they are born again
And we grow old? No, they die too,
Their yearly trick of looking new
Is written down in rings of grain.
Yet still the unresisting castles thresh
In fun  grown thickness every May
Last year is dead, they seem to say,
Begin afresh, afresh, afresh.
Philip Larkin, High Windows, 1967 (Huntington Rare Books 446597)
New stems and leaves on Quercus robur, less than one month’s growth

Yes, Larkin mentions the rings, but only secondarily. Trees are, really, all about new stems and leaves, which we call primary growth. Oaks are masters of this, getting their primary growth done in a single spurt, something like humans. Examining English Oak (Quercus robur), buds formed the previous growing season sat in gestational dormancy for nine months or so. Then, with the earliest promise of spring, more rapidly than you can imagine, each bud breaks out in growth, the tip pushing forth with abandon, creating new stem, and fresh leaf after leaf, until it has reached some nearly pre-destined extent. The frenzy is done in no more than 30 days, and consolidation begins. Each leaf expands to its capacity, toughens up, and begins its life’s work of photosynthesis. The stem itself, tender and even perhaps edible in early growth, goes from green and soft to brown and woody. This has to happen quickly; oaks support thousands of other organisms, many that would dine happily on succulent leaves and stems if there were a chance.

The growing tip, though moving rapidly to lay down the season’s new stem and leaf, got down to a lot of other business. At every leaf, buds of potential were made, defining branching points, nodes – places where things happen. And the new stem’s first few nodes are often full of excitement, because the oak has more to accomplish than feeding itself through the photosynthetic crop of leaves. There is the business of reproduction, which for oaks is a purely sexual encounter.

Singular, or even clustering tassels emerge from side buds at the first few nodes, tassels that hang like sparse knobby pompoms, with each pendent stem barnacled by flower buds, male flower buds. These buds burst open, palming out tiny anthers that fill the air with pollen, the pollen oaks require for pollination and humans despise as seasonal asthmatic irritants.

There is no agent to carry the pollen other than wind and gravity. It takes wide dispersal of pollen for this kind of sex, which means the grains have to sally forth in abundance, reminding me of animated lines from Sebastian Vaillant’s famous 1717 Discours* on the roles of flowers in plants: “the tension or swelling of the male organs occurs so rapidly that the lips of the bud, giving way to such impetuous energy, open with astonishing speed. In that moment, these enthusiastic organs, which seem to think only about satisfying their violent desires, abruptly discharge in all directions, creating a tornado of dust which expands, carrying fecundity everywhere; and by a strange catastrophe they now find themselves so exhausted that at the very moment of giving life they bring upon themselves a sudden death.

A tiny, emerging female oak flower – ready for pollination

Absolutely opposite circumstances hold for female flowers. They are not so common, nor prominent, nor are they so elaborate as the male flowers. Yes, like the tassels they are borne on small side branches arising from nodes, but the female (pistillate) flowers are so demure as to be (in many oaks) difficult to locate when receptive. Vaillant, who was convinced of the importance of males, gave at the least some attention to progress in the female flowers, with his greatest focus on the style and stigma: “These styles, as I was saying, which I compare to the tubes described by Fallope in that they transmit to the small eggs, not the dust particles [pollen grains] ejaculated on them or on their stigmas by the testicles or heads …, but rather the vapor or volatile spirit emitted by these dust particles [pollen grains], which goes on to fertilize the eggs.” He wasn’t aware of pollen tubes; that understanding would await Robert Brown’s work a century later.

Back to Larkin. Our poet was, of course, uninterested in the botany of things, rather he moves darkly in coloring the annual production of new foliage as something of a trick, a quasi-revival, that is futile because even trees, which seem immortal, are bound to perish.

Veering away from the botany allows Larkin to take his own turns, but it is this annual growth, this primary growth, that is how trees are truly made. To a botanist, it’s the greatest excitement. Each year our English Oak sends forth stems from tens of thousands of buds to conquer new light and air. It is this primary growth that creates branching, elongates stems and generates leaves, flowers, and fruit. Primary growth is responsible for the entire structure (we say “architecture”) of a tree. And it only happens at the tips.

When summer wanes and fall approaches, the year’s new twigs have grown their full measure and have hardened off. Next year they join their forebears and begin to make rings. In preparation, with days growing shorter the twigs begin to seal off the connection to the attached leaves, which shut down their sugar-making factories, dismantling the photosythetic equipment and exporting anything of value to the main trunk before the passages close. Once shed of leaves, the youthful twig has transitioned; it will never make its own leaves except through next year’s explosion of activity from the buds in ranks along its length. The twig joins the woody support system on which next year’s crown of leaves will develop. Its growth is now secondary.

That secondary kind of growth, those rings, are about girth and strength and storage and liquid transport and permanence, accumulating mass equal to the task of supporting all of the scaffolding that primary growth creates.

Mature leaves on a mature Quercus robur stem.

Once a stem begins putting on this kind of weight, its youthful days of elongation are spent. Like us, it just gets thicker. Tap a nail into a tree trunk as a child and return in your senior years. The nail will be right there, at the same height above ground. It may be imbedded in annual rings, but it will not have magically stretched higher, further from earth. Once ring-building starts, the stem is fixed in length. It’s like this with humans, once you reach your adult height, your navel will never be any further from the ground.

Leaves Compared With Flowers

A tree's leaves may be ever so good,
So may its bar, so may its wood;
But unless you put the right thing to its root
It never will show much flower or fruit.

But I may be one who does not care
Ever to have tree bloom or bear.
Leaves for smooth and bark for rough,
Leaves and bark may be tree enough.

Some giant trees have bloom so small
They might as well have none at all.
Late in life I have come on fern.
Now lichens are due to have their turn.

I bade men tell me which in brief,
Which is fairer, flower or leaf.
They did not have the wit to say,
Leaves by night and flowers by day.

Leaves and bar, leaves and bark,
To lean against and hear in the dark.
Petals I may have once pursued.
Leaves are all my darker mood
                            Robert Frost

*Search on line for Paul Bernasconi and Lincoln Taiz, transl. 2002. Sebastian Vaillant’s 1717 lecture on the structure and function of flowers Huntia 11(2) ( Original French publication in Huntington Rare Books 477551)

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