I spent the entire month of May sorting through the past and throwing away old fruit. The discarded material filled several dumpsters in total, but I was careful to space it out, so as not to overload the university's rubbish pickup schedule. It was fruit that I had collected more than 20 years ago, as part of my dissertation research, which was to study the mineralisation processes within the hackberry (Celtis occidentalis) seed.

During the mid-1990s, I collected thousands of hackberry fruits from trees all across the Midwest. I chemically analysed each seed in order to formulate an equation relating the hackberry's mineral make-up to the summer temperature under which it grew. My bigger goal was to develop a model that would allow scientists to reconstruct past climate conditions by analysing the chemistry of recovered fossil seeds. The model is still in use, while the laboratory analyses that led to it have faded from my memory.

I somehow scraped funding together to move thousands of spent hackberry fruits from laboratory to laboratory - first from California to Georgia, then on to Maryland, and finally all the way to Hawaii - but I am now running up against what many research museums and libraries are facing in America: the high cost of storage.

My next move is overseas, and I don't have an expense account to ship 20 extra boxes of carefully preserved old samples. And so, into the garbage they go.

While clearing out my laboratory, sorting regret from satisfaction, I made a detailed list of everything that I threw away, including where it originally came from and when it had been collected.

I'm glad I did, because in doing so, I stumbled upon something I'd nearly forgotten: the lone box of hackberry samples that was labelled in my father's handwriting, instead of mine.

In 1993, my father collected hackberry fruits for me. My task that year was to observe the development of the seed over the course of the growing season, and I had earmarked several trees in South Dakota for that purpose. During a rare visit home to neighbouring Minnesota, I saw with new eyes the fine specimen of C. occidentalis that graced the south-western corner of my parents' property.

I asked my father if he wouldn't mind pulling off a few fruits every week throughout the summer, and he obliged. From May to September, he visited our hackberry tree twice each day, carefully recording the weather conditions, and also sampling, first flowers, then green fruits, then ripe, then withered, all placed into small plastic vials. Hundreds and hundreds of fruits - each week's harvest wrapped in a sheet of paper describing its yield.

My father was a scientist, and I grew up in his laboratory. Maybe I am like him, but he is not like me. My father was a physicist, while I am a biogeochemist. I live to study plants, and he has never had more than a generic interest in biology. And yet my father spent the better part of his 70th summer observing a single tree and, in the end, gave me a hundredfold more than what I had asked for.

My father can no longer write. He is 92 now, and he cannot make his hands work. He cannot walk, or even stand, and he can barely see. He is not certain what year it is, but he is sure that I am his daughter, and that my brothers are his sons, and he treats us just as he always has. He still knows me, and for this I am grateful.

When I visit him these days, we sit in the same house that I grew up in, but we don't talk about science anymore. Just one year ago we could, and he explained friction and inertia to my son, letting him push on his wheelchair to demonstrate.

But numbers confuse him lately, and so we talk about poetry instead. My father's schooling during the 1930s was heavy with memorisation; eight decades later, he is reaping the benefits. It is positively amazing to see how much of Longfellow's forest primeval he remembers from "Evangeline", indistinct in the twilight though he may be.

As with many Midwestern families, great distances pervade our relationships - both literally and figuratively. We never really talk to each other; instead we box up our hurts and longings and store them for decades, out of sight but not forgotten.

It is summer in Minnesota as I write this: four blessed weeks of bird song and fireflies made all the more beautiful by the fact that it will not last. This year, my father and I have spent it inside, reading.

I take out our old tattered giant golden book of Aesop's Fables, and I start with the stories about dogs because they were always my favourite. I recite the text with the inflection my father used 40 years ago when I was six and still pretending that I could not read, so that he would not stop reading aloud to me. When I near the end, I pause, and my father recites the moral word for word.

In the fading light, we offer each other words that were carefully written by dead strangers, because we know them by heart. We also know that children eventually leave. Even when they do come home, there is always the end of the day, of the week, of the summer, when they fly away to the other side of the world, off to a place where you cannot follow.

This month I am leaving Minnesota, and the United States, relocating yet again, to build a new lab and start over a fourth time. Compared with my previous moves, I am taking very little with me. The dead fruit of my early career has now been discarded. Instead, I carry in my luggage a delicate pile of paper. It is the small bundle of notes written in my father's handwriting that I recovered from the box of hackberries he collected.

The notes are precious because they constitute proof - proof that my father thought of me every single day and must still do so. Proof that I am his, our shared last name written on every page. Proof that no one in the world knows that tree the way he and I do.

Our hackberry tree still stands, tall and healthy, near the western edge of Mower County. It should outlive both of us, growing stronger and greener even as we inevitably wither and fall. The tree will remain in my parents' yard, and the notes describing what it was like 20 years ago will go with me, although its fruit will not.

I am taking with me only what I can't live without, and the utility of these letters is clear. This collection of papers, filled exclusively with symbols and dates and botanical terms, is all of the things that my father and I have never said.

NEW YORK TIMES

• The writer is the author of the memoir Lab Girl and a professor at the University of Oslo.