I'm trying to design a humanoid or anthropomorphic organism that would be classified as a plant. The main argument people mention is that it wouldn't be able to produce enough energy to survive. I've done some basic calculations but I want to check it with someone else.

The average amount of sunlight in a square meter is equal to $120W$ or $120$ x $24$ x $60$ x $60 = 10368000 Jm^{-2}$ in 24h.

To increase photosynthesis, they need more surface area. These creatures would be slightly larger than a person. Average area of an adult human male is $1.9m^2$. Slightly larger with tail and features, lets say it's area is $2.5m^2$. Since only about part of them, lets say 35%, can be in the sun at any given time, photosynthetic area is only $0.81^2$. This wouldn't be enough. Instead, if they had grass-like fur the area would increase exponentially. Average $30mm$ length, $2mm$ thick base, $200mm^2$ area per blade, $250,000 blades/m^{2}$, photosynthetic area x $50 = 40.5m^2$ x $120W = 4860W$. Photosynthetic efficiency is at least 2% so they would produce $97.2W$, the same as what the average person uses. But ps efficiency can get higher, if it was 5% they would produce $243W$.

Did I make a mistake in my calculations?

Note that if this is not a sufficient energy source, they are mobile and are capable of digestion, meaning that they could get energy from external starch and glucose.

(Edit: I recalculated some things, using the sunlight duration by WMO and lowering the amount of sun exposure to 35% not 50%.)

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    $\begingroup$ The surface area of the organism is irrelevant. What matters is the area of the cross section illuminated by the sun. You cannot magically multiply the available energy by extracting it multiple times. This is why very few plants grow on the forest floor. In other words, to extract the energy of the sunlight falling on 60 m² of land you actually need to cover 60 m² of land. A human covers about 0.5 m² of land... $\endgroup$
    – AlexP
    Commented Jul 30, 2018 at 16:32
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    $\begingroup$ Just like only about half of the person can be exposed to the sun, only half of each grass blade will be exposed. A good a portion more will be shaded by other blades and probably wouldn't be much better than a single large 'leaf'. Also, how does each grass blade have 400 square mm of area if they are 30mm by 2mm? They're basically rectangles, so wouldn't that be 60 square mm per side? $\endgroup$
    – Giter
    Commented Jul 30, 2018 at 16:48
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    $\begingroup$ @Giter Do note I've converted the units in edit based on what I thought the OP was saying so they could be off. The original was an area of 2cm which isn't an area at all. $\endgroup$
    – Ash
    Commented Jul 30, 2018 at 16:56
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    $\begingroup$ I can't help repeatedly thinking that this question, as written, is in point of fact one for either a Mathematics one since you ask for a check of your calculations or the Biology for a check of your basic assumptions concerning photosynthesis. $\endgroup$
    – Ash
    Commented Jul 30, 2018 at 17:17
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    $\begingroup$ Relevant: what-if.xkcd.com/17 $\endgroup$ Commented Jul 30, 2018 at 21:14

1 Answer 1


Not sure if this belongs in discussion... I'm attempting to answer the titular question about whether or not a animal-plant is viable at all, and I'm going to do it using napkin math. I'm not a biologist.

Some plants eat bugs or other kinds of meat; they just digest it very slowly, and they move very slowly. Venus flytraps, for example, spend a couple of weeks generating the necessary potential to snap shut. So if your plant creature is to move around quickly enough to be mechanically competitive with animals, it's going to need to digest its environment much more quickly.

So, I'm going to defer to XKCD for the ammount of energy a green cow would get in a day from sunlight (less than 2 million joules), and assume that the plant man is taking in about as much sunlight as a cow. This website: http://sustainability.blogs.brynmawr.edu/2012/07/31/understanding-energy-part-1/ says that a person might take in about 2000 calories per day (8.4 million joules). So since the plant-man is pretty big, we might say that it needs triple what a person needs. So, 25 million - 2 million = 23 million joules, or about 5500 calories.

The plant man will have to digest quite a bit of food per day, so he's going to need a strong solvent in his gut. Also, as I understand it, plant cells are adapted to store energy for long-term slow-release, rather than the way animals do it using sugars for quick release. However, if we can accept some limitations, a plant can nonetheless achieve locomotion as long as it is constantly digesting food at a steady rate; it would just have to pull simultaneously from a large quantity of slow-release energy stores to make its motions, and the motions would have to be done quite differently. So I'll defer again to my understanding of venus flytraps. Rather than a complex network of sinews which contract to produce motion, such as what we see in animals, the plant would have an array of lobes which can be snapped to either concave or convex. So the plant man can move slowly by snapping his lobes in small quantities, or quickly by snapping them all at once in a location. So, rather than having muscles which pull at a joint internally, it is the surface of the creature which deforms to produce motion, and so it would look quite a bit different externally (the joints would be arranged around these changes, so when it bends its arm, the arm would become a rounded concavity around the joint rather than display a bulging bicep).

So, the big limitation is that the motion is done by storing energy prior to snapping lobes, which means that there is a fixed limitation on how frequently a lobe can be snapped, directly dependent on the rate of energy release from the cells, and the number of cells available to draw from. If you pluck one of the leaves from your plant person, he won't die, but he'll move slower.

Well, that was fun speculation for me, but I hope it leans towards what you were looking for!

  • $\begingroup$ This was cool to read but the question wasn't whether they are viable, it was if I correctly calculated the rate of energy production through photosynthesis. Also you massively overestimated the size. I said slightly larger, not triple the size. I will keep this all in mind when designing the creatures in depth. Idealy they would store glucose and the like in pockets in their body and move it around through xylem and phloem like blood. TL;DR: Really good, not what I was asking. Moderators, don't delete this. $\endgroup$
    – ZoneWolf
    Commented Jul 31, 2018 at 6:16

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