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If you spend a little time gardening, you soon become aware that plants store energy in their roots, which they collect from the Sun through their leaves. By the end of Autumn, perennials usually have thick roots—the best time for making them into tinctures. In the Spring, many perennials produce new stalks, flowers, and leaves in a burst of growth which depletes their roots, leaving them shriveled up and needing a summer to replenish. Biennials produce no flower or seed their first year. Instead they produce only leaves to gather sunlight to store energy in their roots, which they expend in their second year to flower and reproduce. Some annuals, like potatoes, store up energy in tubers underground, which then power the growth of new stalks and leaves—or supply energy for us when we eat them. There are yet more variations, but you get the idea: plant life is a yearly rhythm of collecting energy, storing it, and using that energy to fuel the start of the next cycle.

I figure the ancients and medievals spent a lot of time gardening or doing its larger-scale variant, agriculture, so all of the above must have been even more obvious to them. How, then, did they describe the yearly flow of energy from Sun to leaves to roots to new plants? What did they call the thing that the plants accumulate and store in their roots? Or did they not think of it this way at all?

diagram showing light energy entering through leaves and being stored in fruits or tubers

Ben Kovitz
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    "Or did they not think of it this way at all?" I think this, but if there is anything in it, it might be found in Vergil's Georgics or Columella. I don't remember reading anything similar at all in Catos' De Agri Cultura. – cmw Mar 31 '21 at 04:21
  • @C.M.Weimer: Or perhaps (natural) philosophers like Epicurus, Lucretius, Aristotle? I could imagine terms like vita, vitalitas, medulla, dunamis or similar being used by some philosopher. P.S. Lovely question. P.P.S. Or perhaps simply sucus "juice", which was also used metaphorically for "strength". – Cerberus Mar 31 '21 at 05:01
  • @Ben Kovitz: This Q. could have been: "Did the Ancients have a Word for Photosynthesis?"; more prosaically, "....Sugar?". OLD gives "saccharon" = "sugar". I thought that the only sweetening agent available to the Romans was honey; perhaps not. – tony Mar 31 '21 at 08:36
  • @cmw If it's anywhere, it must be in those works, or maybe Lucretius or Aristotle, or even Aquinas. (I've only read short excerpts from each.) I got to thinking about this because seeing everything in terms of flow of (conserved) energy sounds like a distinctively modern scientific way of looking at things (e.g. leaves are "solar collectors"), but how do you miss this in the garden? Without some notion of energy, how do you understand the function of leaves? Or why people and animals get tired after physical exertion? Or why eating food helps with that? – Ben Kovitz Mar 31 '21 at 11:01
  • Maybe a Latin word for "fuel" was put to use for whatever plants get from the Sun and store in themselves? Indeed when you burn wood, you're releasing it. – Ben Kovitz Mar 31 '21 at 11:13
  • @tony Maybe your comment (just deleted) provides an important clue: it's not at all obvious that heat, fire, motion, growth, the charge in a battery, the compression of a spring, sunlight, and the state of petrol before it's burnt are all interconvertible. This is the modern conception of energy, enabling us to reason that petrol holds the energy received by plants from sunlight long ago. The ancients surely had no conception of interconvertibility so broad. But didn't they think that plants accumulate something when they sit in the Sun, which they expend when they bloom or bolt? – Ben Kovitz Mar 31 '21 at 12:23
  • Perhaps some philosophers might have thought of it as fire atoms being collected and stored. – Cerberus Mar 31 '21 at 14:55
  • @BenKovitz That's a very mechanical perspective, that is not historically shared. I can't speak for the ancients with any authority, but historically a framework of conflict and balance has been more prevalent; here a life force against the grip of winter, or fertility against sterility. A more common belief seems to have been that nature just is and life just does unless there is something to stop it. –  Mar 31 '21 at 19:43
  • @Servaes That's the kind of answer I'm hoping to see, especially if supported with some ancient or medieval writing that addresses something like what biennials are doing in their first year, or what is the function of leaves. Would you care to post an answer? – Ben Kovitz Mar 31 '21 at 23:02

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Perhaps the word sucus "juice", which was in any case also used metaphorically used for "vigour, essence":

amisimus, mi Pomponi, omnem non modo sucum ac sanguinem sed etiam colorem et speciem pristinae civitatis. — Cicero, Epistola XVIII ad Atticum, 2.

The Loeb edition translates sucum et sanguinem as "vital essence". I don't have a source yet for this from agricultural or pastoral texts.

Cerberus
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    Interesting: I often say "juice" metaphorically for energy, or whatever is the relevant limiting factor that gets consumed by use (e.g. oxygen), when describing attention. – Ben Kovitz Mar 31 '21 at 11:11
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    @Cerberus: Winstedt (Letters to Atticus Vol. 2) gives "sap & blood" = "sucum ac sanguinem". In the phloem tubes sugar, manufactured in the leaves (photosynthesis), is transported downwards as a sweet, watery syrup (sap). This, beloved of insects, who gnaw at the tree-bark. – tony Mar 31 '21 at 12:48
  • @BenKovitz: Exactly! Often used forbattery charge. – Cerberus Mar 31 '21 at 14:54
  • @tony: Voilà! It makes sense. – Cerberus Mar 31 '21 at 14:55