Plant Cuttings

Newton thwarted by the ascent of sap

Is botany nothing more than a matter of physics?
Image: Laurel Jules/Wikimedia Commons.
Image: Laurel Jules/Wikimedia Commons.

As a botanist, I was understandably excited by the notion that the BBC (British Broadcasting Corporation) was broadcasting a radio programme that purported to debate (and hopefully answer…?) the question, ‘what is the point of plants?’.

However, as episode 5 in series 11 of ‘The Infinite Monkey Cage’, a show that’s advertised as a ‘comedy and popular science series’, my hopes weren’t too high of a serious answer, even if it did feature the University of Oxford (UK)’s Professor Jane Langdale as the representative plant biologist. But the real shock came from University of Surrey (UK)’s Jim Al-Khalili, Professor of Physics and Professor of Public Engagement in Science, whose main contribution to the debate was to advise that botany is all a matter of physics nowadays since photosynthesis can be reduced to quantum phenomena. Having never really gotten on with physics, I was almost forced to conclude that I’d now have to give up botany henceforth. A doom-and-gloom scenario if ever there was.

But! I soon perked up again when University of Sheffield (UK)’s Professor David Beerling revealed that world-famous physicist Sir Isaac Newton had investigated the plant biological phenomenon of transpiration. Now transpiration – ‘the process of water movement through a plant and its evaporation from aerial parts’ – is an important natural process, not only to the biology of plants but also to global water fluxes in the hydrologic cycle as it is estimated to constitute 80–90%* of terrestrial evapotranspiration.

Fortunately(!), Newton didn’t establish the mechanism of transpiration (that matter was seemingly not settled until 1895 with Dixon and Joly’s Cohesion–Tension Theory of the ascent of sap)**. But the fact that Newton didn’t solve it convinces me that physicists don’t have all the answers to botanical phenomena; there is still a role in plant science for the non-physicists amongst us.

Evidently, when it came to botanics, natural philosopher Newton was much better with things that go down rather than those that go up. Phew! And there’s more post-Newtonian good news. Newton was also ‘famous’ – post-humously! – for his clandestine investigations into the realms of alchemy. Alchemy – ‘the medieval forerunner of chemistry, concerned with the transmutation of matter, in particular with attempts to convert base metals into gold or find a universal elixir’ – is, arguably, the opposite of science, and more akin to magic. Which seems entirely fitting somehow because, to me, transpiration and the ability of 100-metre tall trees to move water to their topmost branches seems like a kind of magic. And, if having wonder and mystery – ‘magic’ if you will – alongside serious, sober science was good enough for Newton, then perhaps it should be good enough for us too.

* For balance, we should mention that the magnitude of the contribution of transpiration to global evapotranspiration in one paper has been questioned, as a large overestimate.

** This is not the only view of the mechanism of sap ascent. There are other suggestions out there, as considered by Melvin Tyree and Bob Johnson.

[For a review of the history of the Cohesion–Tension Theory, Harvey Brown’s ‘The theory of the rise of sap in trees: some historical and conceptual remarks’, which attempts to provide the fuller picture of this biological phenomenon for physicists (‘in non-technical terms’…), is suggested – Ed.]


  1. Thank you for this comment.
    Another nice example of the need for botanists to study botanical phenomena….



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