Friday, 31 July 2020

Ballonts VI; back from the brink, or still lead balloons?

Over the years, many readers expressed how sorry they were to learn that ballonts, i.e. lighter-than air lifeforms, were almost impossible to achieve on an Earth-like planet. The only way you can have a ballont on such a world is to shape it similarly to actual balloons on Earth: they have to be very, very large. What I had badly wanted was ballonts as aerial plankton floating through forests, or small ballont 'seedlings' released by their parent organisms, or something tiptoeing over the plains, as in the previous post. Such ballonts were an integral part of the Furahan menagerie, until I sat down to do the math. The sobering results are to be found here, here and here.




With that knowledge in mind, I was quite surprised to see a video showing a very small balloon on YouTube. I copied two stills from it to show you, but advise you to see it on youTube.

The video shows a very small balloon, of a size that would lend itself very well to a ballont! Was I wrong to think that small ballonts, or small balloons for that matter, are impossible? The video's narrator says that the balloon weighs 0.3 grams and needs only "naught point five eight litres of helium to float". The video shows a hand releasing a string, or so we assume, as I do not see the string in question, and the image of the balloon then rises. That sounds like a strong assertion that the balloon actually goes up into the air when released. But the video was made for advertising purposes, and advertising and truth are not the best of friends. Beware: the part where the balloon rises after being released looks like an animation rather than live action, and in the rest of the clip there is no actual video of the balloon floating. 

Luckily, they showed numbers: the video specifies a volume of helium, of 0.058 L. Volumes do not depend on weight or mass, so I suppose they meant the volume of the inside of the inflated balloon, as that seems the only thing that would make sense. We can easily find out how large a sphere should be to take up 0.058 L. Turn that into cubic meters, apply the equation for the volume of a sphere (4/3 x pi x R^3 with R as the radius of the balloon), and you get a radius of 0.024 m, or a diameter of 4.8 cm for the balloon. That looks like the size they showed in the clip. Very well, but how much mass can that volume of helium actually lift?

Well, under 'standard' circumstances, meaning 20 degrees centigrade and one atmosphere, the density of helium is 0.179 kg per cubic meter, and of air 1.2019. From that it is easy to calculate the mass of 0.058 L of helium and of 0.058 L of air. Subtract the two, and that is the mass you can lift. You get 0.059 gram. That is almost nothing! Mind you, for a balloon to work it must first lift the mass of its own envelope. In this case, the envelope has to have a mass LESS than 0.059 gram, or else it cannot float. Good luck with that. I am beginning to think that the balloons in the clip were not kept aloft by a bit of string, but that they were held up by a length of stiff wire.     

Still, I wondered if there was anything to be done about the physics. In my previous models, I had used the characteristics of PET for the membrane of the balloon, because that is a strong material that will not let even gases escape. Unfortunately, PET has a density of some 900 kg per cubic meter, so it is only a bit less dense than water.    
Click to enlarge; copyright Gert van Dijk

Here is my old model again. The x-axis shows the radius of spherical balloon in cm and the y-axis shows mass in grams. A balloons works if there is a mass difference between the displaced air and the gas inside the balloon. If that difference is larger than the mass of the membrane, you get lift. In the graph, the red line shows lift: if the values are negative, the balloon sinks, and if they are positive, it floats. This PET-balloon will not float if the radius is smaller than 25 cm; that is a big balloon. Actually, that is MUCH bigger than the balloons you can buy for parties. The membrane of latex balloons must weigh a lot less than the one in my model.

Click to enlarge; copyright Gert van Dijk


Here is the same model, but now the membrane is magically completely massless. That helps a lot! Mind you, that third power is still being difficult: a balloon with a radius of 10 cm can still only lift 5 g. To lift just one gram, you need a radius of 6 cm. Even without such a weightless membrane you cannot have truly small balloons. This raises the question what the mass of a typical children's balloon is, and how small manufacturers can make them?

I asked balloon manufacturers, and they were friendly enough to reply. It turns out that helium balloons of 9 inches can float, and that 5 inch balloons are the smallest ones to float. But they do so weakly and only for a short while, because the helium leaks out though the latex. Now, I refuse to use illogical mediaeval units of measurement, so I will substitute 12.5 cm for the width of five working men's thumbs held next to another. That is a radius of 6.25 cm.  The manufacturer told me that the weight of such a balloon is roughly 0.7 g, if made out of white latex. The colour affects the weight.         

Well, that makes sense if you compare it to the magical massless membrane: the magical one could lift one gram with a 6 cm radius, and now we find that the actual weight of the membrane of a balloon of that size is 0.7 g. That leaves 0.3 g for lift. That is just enough.

Today's lesson is that physics still conspires against ballonts. A secondary lesson may be that advertisers can waste your time. I knew that already. Sigh.


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PS this is an additional figure I had prepared but not posted. However, it ties in so well with Abby's comment below that I decided to add it.

Click to enlarge; copyright Gert van Dijk
 

Saturday, 18 July 2020

Tabulae Mortuae III (Archives XIII)



Clich to increase; copyright Gert van Dijk
 It is with some trepidation that I show you this particular dead painting. (If you wish to see more, just use the search function to find 'Archives'.) It is one of the oldest in the series of paintings that later became the Furaha universe. The subject is neither thought out well nor painted well. The sky used to be ochre, but I secondarily, after a year or so, decided to paint it over with bluish tints to improve the demarcation of foreground and background. That helped a bit, but not enough. I admit that I have now decreased the contrast of the background digitally to improve the sense of depth in the painting.

At the time of painting I was still enamoured of ballonts (there are many posts on this blog about 'ballonts'). I was exploring what you could do with them. So here is a ballont species with the ability to hover a bit above the ground, tiptoeing from stem to stem if it chose to do so. If endangered, it was supposed to be able to release its hold and shoot upwards like a cork in water. I did not stop to think how such animals would be able to regulate the upwards force of their balloon. Perhaps it could contract its bladder forcefully, decreasing its volume and compressing the gas inside.

Anyway, I remember that the tree (?) in the background started out as a gigantic slug-like creature moving across the landscape with interesting mushroom-shaped organisms on top. When the painting was redone, I transformed it into a sessile lifeform, but the mushrooms stayed. These can obviously tilt or bend in such a way that they deflect the wind. Although I have no idea what these mushroom organs are actually for, I still like the idea of them forming a buffer against the wind.

You will not be surprised to learn that the tomatl did not make it into the modern, more sensible, Furaha universe. Now they do, and the quality of the paintings have improved too, or so I like to think. It was fun to simply paint whichever shapes suggested themselves, without wondering whether they made any sense or not. Progress...         


Saturday, 13 June 2020

'Bitrophy' II (that's animals with photosynthesis)

In the previous post on this subject, I discussed some basic aspects of the feasibility of an animal using photosynthesis as an auxiliary energy source. Mind you, I never said that it would work! It is too early to reach a conclusion yet, and if you expect this post to finally answer that, sorry, no, not yet...

This post will have a look at some other factors that would have influence the efficacy of ‘bitrophy’, and I plan to try to pull it all together in a third post on this subject. So how do you judge whether or not bitrophy is of value for an animal? I thought that the ‘leaf area’ needed to catch light in relation body size might be a suitable measure: a very large leaf would be expensive in terms of material and metabolic costs as well as mechanical unwieldiness.   

So here we go; let’s walk through some factors.

1. Photosynthesis is inefficient
The inefficiency of photosynthesis, as we know it in Earth plants, is well known and was discussed previously in this blog. To summarise: many wavelengths present in visible light are not used in photosynthesis at all and are thus wasted. Photosynthesis has an inbuilt maximum, meaning that above a certain point increasing the level of light will not lead to more carbohydrate production. Finally, the chemical reaction runs almost as easily backwards as it does forwards, and given the fact that there is much less CO2 (0.04%) in the air than O2 (20%), taking out CO2 and adding O2 was an uphill struggle to start with.        

2. Let there be light

How much light falls on a plant on a planet's surface? Well, obvious influences are how bright the star is, and how far away the planet is from that star. If the orbit is circular, the amount of light reaching the planet as a whole will constant during a year, but it will vary considerably if the orbit is highly elliptical. Then there is axial tilt: if the planet's axis is not perpendicular to the plane of that orbit, the planet will have seasons, and the amount of light will fluctuate during a year. The planet will rotate around its axis, and with day and night comes a halving of the amount of light on any point of the surface (except if there is 'tidal locking’: then one half will be perpetually lit and the other dark).
 
Finally, part of the surface will receive rays of light at a glancing angle, while others receive sunlight perpendicular to the ground, delivering much more energy. You can estimate that the average amount of light is only about one quarter of the maximum (I can expand with some fingures later). And that is at the top of the atmosphere. Below that, you have atmospheric scatter, clouds, and the shade of mountains, other plants, of being under water, etc., etc.

In the previous blog I used the local maximum amount of light to calculate how large a 'leaf' an animal would need to power its 'minimal metabolic rate' (MMR). Well, if you wish to account for the average amount of light, you should make that area four times as large! That means doubling the length of its side, if the leaf is square, or its radius, if it is circular. Of course, you can decide that such large leaves are unworkable, and you can limit the animal to the tropics. Or you can have it shut down and 'hibernate' through the night. 

3. Energy for an active lifestyle
The MMR only powers the energy needs of an animal ding nothing except being alive. More activity requires more energy, and Alexander's book mentions that the average energy need is about three times the MMR. So, if you wish to cater for that, you should increase the leaf area three times; that means increasing the radius by a factor 1.7.

4. More active animals

In the previous post, we learned that the MMR depends on body mass through an exponential function. The exponent was close to 0.75 for all animals, so that doesn’t matter. However, a multiplication factor differed greatly between animal types: mammals (and birds) need much more energy than some other animals.

Click to enlarge; copyright Gert van Dijk
The image above shows three schematic animals: a mammal, a -warm- lizard and a crustacean, all with a mass of 1 kg. I did not bother to refine the shapes, but they should be recognisable.

Click to enlarge; copyright Gert van Dijk

Here they are again, but now with leaves of the right size to cater for an MMR under maximum light. Remember that if you wish to take astronomical and activity problems into consideration, the radius of the leaf should be made 3.4 times as large (1.7 times 2). However, even with the not-adapted leaf, it is obvious that the mammal needs a ridiculously large impractical leaf. The crustacean's leaf looks more acceptable.  

5. Body size
MMR depends on the mass of an animal, and we have set leaf area to follow MMR. But mass is itself a function of size: increasing the size of an animal by a factor x will increase its mass by a factor of x to the third power. For instance, doubling the size will increase the mass eightfold. In reality things are more complicated, as you cannot simply increase all dimensions of an animal by the same amount and expect it to work. For instance, legs need to become thicker. This was explained in earlier posts, here, here and here. If you want more, here and here are posts on the same them devoted to the giants of  Game of Thrones.

Is anyone still there? If you are, we can work out how MMR responds to size as opposed to mass. First, mass is length to the power of three, and second, MMR depends on mass to the power of 0.75, so MMR increases with length to the power of 0.75 x 3 = 2.25.

Does this matter? Yes: say we double all aspects of an animal’s size. The radius of its leaf is doubled, and the area of the leaf becomes four times as large. Its mass increases by a factor 8, but its MMR only by a factor 4.76 (that’s two to the power of 2.25). That particular MMR would require an increase of the radius of the leaf of a factor 2.18 (that’s the square root of 4.75). But doubling the size yielded an increase of the radius of a factor 2.0, slightly too little. So, larger animals need extra large leaves, but the additional increase is not dramatically large. The influence of size on relative leaf area is not all that strong, but still, if you want your bitroph to have a relatively small leaf, the animal itself should be small.

Click to enlarge; copyright Gert van Dijk
The image above shows three sizes of mammal, warm lizard and crustacean, if 0.1 kg, 1 kg and 10 kg. You can see that the leaves are relatively larger for the big animals than their smaller cousins, but the differences between the three sizes are not impressive.            

Well, that concludes this post. It seems that the required 'leaf area' needs to be very large, and I doubt that having such a large structure would be worth it under most circumstances. But what about other circumstances? I will think it over and try to find solutions. No doubt, readers will have suggestions too.

Wednesday, 3 June 2020

Tabulae Mortuae II (Archives XII)

While I am preparing figures for the next instalment of the ‘bitroph’ series of posts, I thought I would post another ‘dead painting’, again an example of a plant with leaves as large as the sails on a sailing boat.

Click to enlarge; copyright Gert van Dijk

The painting is basically static: there is not a single animal about to enliven the scene, the horizon is completely flat, and the landscape is not exactly spectacular. There are some puddles on the ground, suggesting recent rain in an otherwise dry environment. Only the somewhat unnatural looking clouds add a bit of drama. All this may sound as if I am reviewing someone else’s work, not my own. That is because the painting is old enough to mean that I no longer have a strong image in mind of what I was aiming for. For painters such an ‘intended image’ can obscure judgement of the actual painting. I used to hold paintings up to a mirror to get a fresh look (I now do that digitally, without an actual mirror).

So it is up to the plants together with the clouds to provide any visual drama. The plants grow from underground roots, forming a regular succession of stems resembling telephone poles. In the scene, two roots met and formed special variants of their normal stem. The two entwine one another, and now form a botanic union.

I guess that we are looking at sexual reproduction. I have no idea what happens next; seeds drifting on the wind? Nuts borne by animals? I do remember that new plants form numerous roots that grow out in all directions.

I still like the idea of these underground roots traversing the landscape with maniacal precision, although such linearity looks unnatural. The scientific name of this species, by the way, was ‘Mania predictabilis’. Perhaps I should have envisaged a landscape where these plants are more numerous, and they all criss-cross the otherwise empty landscape.

Unfortunately, the large ‘unileaves’ won’t work for reasons outlined previously. If I do use the idea again, it will be easier to start a whole new painting than to alter this one. That’s why this is a tabula mortua: this painting is dead; it’s no more; it’s expired; it’s an ex-painting. 

Friday, 8 May 2020

It's a plant! It's an animal! It's a bitroph!



Click to enlarge; Source: wikipedia

Several years ago, a species of sea slug had its day of fame on internet sites specialising in scientific news. Those sites all showed a bright green flattened blob. like the image above. This sea slug was green because it performed photosynthesis, which animals are generally not supposed to do.

I guess everyone interested in speculative biology sat up straight, because a lifeform that is part animal and part plant exudes ‘alienness’ through every pore. But was the flow of alienness coming out of those pores accompanied by oxygen, as in plants, or by carbon dioxide, something more befitting an animal? 

The slugs of the genus Elysia get their photosynthetic ability by feeding on algae. Algae, as the well-informed readers of this blog will know, perform photosynthesis in intracellular organelles called chloroplasts. The slugs eat the algae, but rather than simply digesting the chloroplasts too, they envelop then through phagocytosis, and keep them alive, in their own bodies. From then on the chloroplasts are called ‘kleptoplasts’, or ‘stolen plasts’.

It turns out that the photosynthetic slugs can live quite well in the dark, so they do not critically rely on photosynthesis. They do use photosynthesis as an auxiliary power source, mostly when they are starved anyway. When the slugs are kept in the dark AND starved, the number of kleptoplasts decreases, so the slugs then apparently disassemble the then useless chloroplasts and get a final energy boost from the hapless organelles (Cartaxana et al  2017).

Plant-animal combinations are not novel in speculative biology. Actually, there is a group of creatures  on Furaha called, for the time being, ‘mixomorphs’. They probably share characteristics with plants as well as with animals. The ‘probably’ is in there because I always had the uneasy feeling that a plant-animal combination might not work. After all, Earth is not filled with such creatures, doing whatever it is ‘plantanimals’ do when they are not just sitting in the sun. Does their absence mean that they do not make sense?

The concept of animals performing their own photosynthesis certainly sounds like a good idea. Earth plants take in carbon dioxide (CO2), water (H2O) and sunlight and turn them into carbohydrates. Because they turn nonbiological material into carbohydrates, they are called ‘autotroph’. Animals cannot do that and require some ready-made carbohydrates as a source of carbon, making them ‘heterotroph’. By breaking up those carbohydrates animals get materials for their own bodies, producing H2O, CO2 and energy. An animal is a plant in metabolic reverse, in a way.

Why not do what the slug does and cut out the middle man? This plant-animal chimaera could use photosynthesis as an auxiliary and cheap way to store free energy in carbohydrates, giving it an edge over animals that have to hunt, chew and digest to get any carbohydrates. They would even have an edge over plants in that a major problem with photosynthesis for plants is that there is so little CO2 in the air. The animal part of a chimaera would produce more then enough CO2 to boost photosynthesis of the plant part.

Click to enlarge; source: wikipedia

Autotroph + heterotroph = bitroph
There is a nice scheme on Wikipedia explaining the full nomenclature of how lifeforms get energy and carbohydrates. There are three big two-by-two divisions, shown above. These result in six fragments of phrases: hetero- vs. auto-, chemo- vs. photo-, and organo- vs. litho-. There are eight possible combinations. Our garden-variety plants (sorry for that pun...) are ‘photo-litho-auto-troph’, while ordinary animals are ‘chemo-organo-hetero-troph’.

This nice scheme seems to cover all the possibilities, creating a challenge for speculative biology lovers: where should we classify animals that can photosynthesise? Note that there already are lifeforms that cannot build their own carbohydrates and yet use photosynthesis: photo-litho- and photo-organo-heterotrophs. However, they are all bacteria, and to increase the ‘alienness’ level we want creatures we can see without a microscope, and that we can stroke, or supply with compost. Or both. Also, as these creatures would run both energy pathways, they do not fit in the scheme. They might be labelled ‘autoheterotroph’; I can't say I much like the term ‘plantanimal’. Let’s introduce ‘bitroph’ to emphasize the dual energy principle (without also adding 'photo-organo-litho-chemo-').            

Bitrophy in practice

'Bitrophism' needs consideration of energy requirements. The first question is how much energy you get from a leaf, or a standardised area performing photosynthesis.  Luckily, that information was already available on my bookshelf, in ‘Energy for animal life’ by the late R. McNeill Alexander (if you want to give your speculative biology a scientific edge, get his books). 
   
In bright sunlight the flux of light on the surface of the Earths is about 1000 Watt per square meter, and with that light intensity the rate of photosynthesis reaches a maximum of 21 Watt per square meter. This ratio of 21 to 1000 shows, again, how inefficient photosynthesis is. Mind you, this light flux is the maximum value in Alexander's biome, which was England. Just outside the atmosphere you get 1370 Watt per square meter. Obviously, seasons, clouds, latitude, and the time of day all influence the amount of sunlight the surface actually gets. For now, let’s go with that value of 21 Watt per square meter.

The next question is how much energy an animal actually needs. That also depends on many things, such as its activity, but it's minimum level is largely fixed: the ‘minimal metabolic rate’ describes the energy requirement of an animal doing nothing, except being alive. This rate depends on two factors.

The first is the type of animal: warm-blooded animals such as birds and mammals burn energy at much higher rates than other groups, such as lizards, fishes, etc. For two animals that have the same mass, a mammal uses almost 5 times the energy of a lizard (even one warmed up to 37 °C), and 12 times the energy of a crustacean at 20 °C.

The second factor is mass: a 100 kg animal will use more energy than a 10 kg one. However, it needs less than 10 times as much. As Alexander remarked: ”Weight for weight, it is a great deal cheaper to feed elephants than mice.”  The relationship between minimal metabolic rate (MMR) is an exponential one, and has the form

MMR = a (body mass) ^ b

(formatting is difficult here; the '^b' part means 'to the power of b'

The exponent ‘b’ differs somewhat between animal groups, but lies close to 0.75. The fact that it is less than 1 explains why large animals have a lower metabolic rate per kg than small ones. The factor ‘a’ is the one that differs between animal groups (it is 3.3. for mammals, 0.68 for warm lizards, and 027 for crustaceans.

Click to enlarge; copyright Gert van Dijk
          
The image above provides the Minimal Metabolic Rate the rate for mammals, (warm) lizards and crustaceans, all ranging from 0.1 to 1 kg. The crustaceans burn the least energy, and bigger animals need more energy than small ones.

But we wanted to get to photosynthesis; remember that one square meter of photosynthetic area provides 21 Watts, so I provided an additional y-axis on the right, which is simply the left y-axis divided by 21. The right one tells you how many square meters of photosynthetic area we need for each point on the graph. A 1 kg mammal will need about 0.16 square meters of ‘leaf’. That corresponds to a square with sides of 40 cm. Examples of 1 kg mammals are seven-banded armadillos, muskrat, pine martens, platypuses, meerkats and European hedgehogs. Just picture one of those them with a 40 cm by 40 cm parasol to catch sunlight. A large fruit-eating bat may also have a mass of 1 kg; it needs a large wing area anyway; hmmm...

Anyway, as I found it difficult to imagine how large that actually is, I assembled a mock animal with a mass of 1 kg (the volume can be calculated because the animal consists of spheres and cylinders; its density is 1.05). I used mammal characteristics to calculate the disc it needs to provide the energy for its MMR.

Click to enlarge; copyright Gert van Dijk

The image above shows such a 'Disneius solamor'. The small squares on the ground are 1x1 cm, and the larger ones 5x5 cm. The animal is 21 cm long, and the radius of its dark green 'sun disc' ('antenna'? 'leaf'?) is 22 cm. It needs that to power its MMR. A general human provides additional scale. Hm; the animal does not look very elegant, and that large 'leaf' looks rather vulnerable.

But we are not done yet. The calculations so far used maximum light settings, which is not realistic. And how about the effect of mass? How about animals that are thriftier with energy than mammals? How about more efficient photosynthesis? I suspect that this post may already have passed the 'maximum allowed complexity per unit of enjoyment ratio' (MACPUOER), so I will stop here. But I will very likely return to this theme.
       

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PS. Although I welcome the large number of questions the blog has recently received, many had nothing to do with the post under which they were asked, and many could easily have been answered by using the blog's search options. So from now on I will be less likely to answer such questions.  Surely you would prefer me to spend my time working on The Book or on writing posts?

Friday, 10 April 2020

'Tabulae mortuae' (Archives XI)

Or, in English, 'dead paintings'.

The Furaha project started with oil paintings without much forethought. The reason to decide to paint something was that I thought it would look nice. Well, that obviously resulted in some designs that with hindsight simply did not make sense. As I explained in the previous post, one design involved plants with enormous leaves. That idea is gone, and so the paintings that show them are no longer useful. Let's say they lived out their lives. I will show a few in this blog. Note that they are NOT typical of current paintings; they are just stuff found in the archives.


Click to enlarge; copyright Gert van Dijk
Here is one. It really needs a better separation of foreground and background, but never mind about that. The animal in question was called a 'Mesencephalon meditans'. That name tells you it was inspired by the human brain stem (as seen from the back). Those into neuroanatomy might recognise several brainstem details, such as the 'pons'. The text regarding this animal mentioned that it might look as if it was lost in thought, but the animal would be more likely to be lost in a more general sense. That's what you get if you leave off the cortex.        

I still like the overall shape and lines of the tree. But how would it respond to wind? Would it turn around so the stem could face the wind, and the sails would flap and flutter? 

Mind you, this painting was done in oils, and for The Book it would need a digital makeover. In some cases, I used the basic idea of an old painting but changed almost everything to produce a new one. This particular dead painting was in fact resurrected. Parts of the landscape survived, and so did a much modified 'Mesencephalon'. The tree, however, did not...

Monday, 6 April 2020

Finally, Furahan plants! ('Plants VII', also 'Post #250', and 'Twelve years on')


Click to enlarge; copyright Gert van Dijk

Experience taught me that posts about plants do not attract many readers and do not generate many comments. If I wanted to maximise interest, I would probably do better to keep plants in the background and focus instead on big fierce animals with lots of teeth, or spikes, or thagomisers. But I write these posts because I like to learn (and teach, I guess).

So, this post will be about Furahan plants. For those diehards who wish to read up on the subject, see the list of posts at the end of this one. The reason to write it now is that I am working on a chapter on plants for The Book. Doing so forces me to think about the specifics of the object I am working on and to make some decisions. For instance, the wish to paint early explorers, who look at the planet Furaha from their spaceship, forced decisions about how artificial gravity and the aesthetics of interior spaceship design. Likewise, having to paint trees forced me to collapse the uncertainties about Furahan plant life into ‘facts’, although it is more like pruning fantasies: only one remains.

The very first sketches involving Farahan plants showed shapes something like the one above. (This is a quick inelegant sketch made for this post; I will show paintings of such tree designs that are now wholly defunct in a later post.) They usually had very thick trunks and had a few gigantic leaves. They were obviously alien and, I thought, visually quite appealing. But the decision to set the threshold for biomechanical aspects of Furahan life at its minimum level of ‘feasible’ dealt as much a death blow to these large leaves, as when it killed ballonts.

So Furahan plants have Earth-sized leaves, making them rather mundane. Why? Plants, as all organisms, have to find compromise between conflicting demands. If the only requirement would be to provide a place for photosynthesis, then a large thin surface would do, resulting in something resembling a bed sheet held up at a right angle to the rays of the sun. Well, that’s not what plants look like, and there must be a reason for that...

Two very important factors determining leaf size turn out to be temperature and humidity. Leaves catch light, and unfortunately that warns them up too. Even though leaves are very good at reflecting infrared light, and do not therefore warm up that easily, excess heat is still a big problem. One reason for that is that (on Earth!) photosynthesis becomes less effective at temperatures above 26 degrees. Leaf size is important for that because the air around a leaf forms a ‘boundary layer’ slowing heat exchange. This layer is bigger for large leaves, so large leaves run the risk of warming up too much. You would expect that plants in hot climes would be small, right? Maybe, but Victorian scientists had already noticed that the biggest leaves are found in the tropics, right where they shouldn’t be.


Click to enlarge; copyright as indicated; source

Leaves have tricks to cope with overheating: as the figure above shows, fake leaves in cooling experiments cooled more when they had lobed, leaflike, edges than when the edges were straight. Apparently, bits of leaf closer to an edge cool down better. Another way to stay cool is to have water evaporate from the leaves. Unfortunately, that requires lots of water, so this trick is best reserved for humid regions where water is readily available. Cooling isn’t always beneficial though: at night or in cold climes low temperatures can damage leaves, so then the ability to keep warm becomes important.

In short, leaves have overheating, freezing and water loss to contend with, all of which are affected by leaf shape and size. So how do you balance all those demands? In 2017 scientists put it all together by studying 7670 species of plants worldwide (Wright et al 2017), and finally managed to understand why big leaves are found in the tropics, right where you think they shouldn’t occur.
 
Click to enlarge; Wright et al 2017; source here

This figure and its legend say it all. Leaves can be big if there is lots of water to cool the leaves during daytime and also if it doesn’t get cold enough at night to harm the leaves. Basically, we are talking about tropical rain forests. That explains the circumstances under which leaves may get large, but not yet which benefit they derive from that. The authors say they think that large leaves need less twig mass, which is good because twigs do not contribute to photosynthesis. They also think that large leaves help when temperatures are marginal.

I expect that wind has an impact too, but I found surprisingly little information of the impact of wind on optimal leaf size. The reason for that lack might be that the really leaves I had in mind, from towel-sized ones, through bedsheet-sized leaves to small-sailboat-class giant leaves, do not occur on Earth. I guess that the typical tree branch anatomy, with each leaf attached by a stem to a twig that is attached to a bigger twig, etc., is a trick to absorb forces. If forces are absorbed at each level, the next level only has to carry part of a bigger load it would otherwise carry in its entirety.

Click to enlarge; Copyright Vogel et al 2009; source
Regardless of that, leaves have nice mechanical tricks to reduce the force of the wind. Some leaves take on conical shapes in a strong wind, and in other species all leaves on a twig together bundle up and reduce wind drag. The shape of the leaf even helps bring such curling about.

So where does that leave (pun intended...) those truly alien plants with giant leaves like sails? Well, nowhere. Their remaining niche would be somewhere where leaves suffer no ill effects from heating up or cooling down, or where the wind cannot harm them. The planet Furaha has wind, and its leaves do not like heating up very much. In that way they are Earth-like. So they have leaves in a form of flat thin sheets of tissue on stalks, connected to twigs, etc. It’s all rather Earth-like and boring.

But do not worry; the fact (’fact’...) that Furahan photosynthesis is more efficient than Earth’s ridiculously inefficient system ensures differences in how they look, and so does the ‘fact’ that Furahan photosynthesis responds to different portions of their star’s spectrum than Earth photosynthesis. Of course, there are the architectural differences in overall shap and trunk design too. But more on that later.

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Earlier posts on alien plants
Alien Plants I
Alien Plants II
Alien Plants III
Alien Plants IV
Alien Plants V
Alien Plants VI

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"Twelve years on"; Yes, I wrote the first post in this blog in 2008. This is also the 250th post I have written on Furahan Biology and Allied Matters. I intend to pick up the pace a bit from the extremely sedate rate of new posts you have enjoyed the last few years, so no big celebrations right now. Perhaps just a small applause for keeping the blog -largely- alive for twelve years?              

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The main site has now moved to a new host, and some things broke while moving. They always do. I will repair them in the coming weeks.