'Ulla sanguisuga' at work on a plant louse (a work in progress)

At some 120 pages, The Book is steadily progressing, slow as ever, but getting there. I thought I would let you have a peek at a recent painting. The Book should show at least some of the myriad small animals that make up the bulk of Furahan animal biomass, and so I painted a few. But after that, the final series of paintings will probably show big hexapods only! 

Click to enlarge; copyright Gert van Dijk

What you see here is just a fragment of a painting, and it is even more limited in that it only shows the head and neck of the most important animal on the painting. I turned the layers containing the other parts of that animal to invisible, so there will be something left for The Book. That head belongs to a 'lice eater', an insect-sized animal of the type usually labelled as 'wadudu'. Wadudu are reminiscent of arthropods, but they have a mesoskeleton, not an exoskeleton. Admittedly, the difference in skeleton type is only obvious for the largest wadudu, the size of mice and sparrows. 

Anyway, this 'lice eater' carries the scientific name of Ulla sanguisuga. It should not be difficult to work out where the inspiration of that name came from. 

Its prey consists of 'plant lice'. That is not a clade, but a simple group designation for all the little animals that make a living by sucking plant sap. The plant is question belongs to a major plant clade called the 'poliochromes'; they have a photosynthetic pigment that absorbs light across most of the visual spectrum, explaining why these plants are generally dark grey. 

To mimic the effect of macro photography, I first painted the entire scene as I would normally do, and then rearranged everything in layers that represent distance to the imaginary camera. I left only one layer in focus, and blurred to varying degrees. We now have depth of vision on an imaginary planet; you would think that photography a few centuries into the future would have done away with such blurring, but no, here we are...

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. 

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?

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. 
             

Layers of leaves (Alien plants V)

The last post on alien plants went into some fairly technical details about photosynthesis, and took a look at where the process could be adapted to make it more alien. Today's post has a closer look at just one aspect: leaves.

Photosynthesis obviously depends on catching light and is therefore a process that takes place on the surface of a plant. How much of a surface is needed will depend on many things, such as how much energy is needed. As related before, C3 photosynthesis can only make use of up to 25% of the light falling on them (well, at noon in the tropics, that is). Photosynthesis becomes saturated, doing nothing with that extra light.

For now, let's assume the presence of leaves on a planet of choice as very similar to Earth's broad leaves; flat structures of, say, 10 cm across. They need light, and so face the sun. But not every leaf of every plant will receive full sunlight: the sun moves across the sky (well, as far as the plant is concerned it does), other plants may be in the way, and even its own leaves, placed higher up, will take light away from lower leaves. A typical Earth leaf transmits only 5% of the light striking it. The next leaf down in turn absorbs 95% of the -little- light striking it, leaving only 0.05 times 0.05 of the sun light, or 0.25% of the light striking it.* On Earth the critical level for photosynthesis to be of any use is at about 1% of full sunlight.

It is therefore reasonable to assume that plants would have only one or perhaps two layers of leaves, right? Additional leaves would not contribute anything, and yet trees typically have many more layers of trees. The answer to this riddle is found in the efficacy of photosynthesis and a fact that you might not have considered interesting in this respect: the size of the sun.

Click to enlarge; source: http://en.wikipedia.org/wiki/Umbra

The image above shows the umbra and penumbra as commonly illustrated in astronomy books. The sun is not a point source of light but a sphere much larger than the Earth. Rays of light depart in all directions from any point on its surface, to the effect that there is a conical volume of space behind the Earth where the rays cannot reach, or, in other words, from where an observer can see no part of the apparent disk of the sun. That conical volume of space is the 'umbra' , simply meaning shadow in Latin. Around it there is an area from which an observer can see part of the sun's disk, so that area receives some direct sunlight, but not full sunlight: the 'penumbra' (nearly shadow). Everywhere else receives full sunlight. The length of the umbra cone depends on the diameters of the Sun and the Earth and the distance between them, as a few minutes experimenting with some sketches will show you.

The same applies for objects closer by. All you have to do is to look at the shadow of your hand as you raise it from the ground on a sunny day. Leaves also cast an umbra, an area without direct sunlight, where it would be best not to place another leaf. The length of the umbra can be calculated as explained above, and for Earth the calculations that the umbra is about 108 times the width of a leaf. For a 10 cm leaf that would boil down to 1080 cm, or 10 meters. As we will see the distance is shorter in practice. Leaves may receive enough light in the penumbra to work well. Remember that on Earth photosynthesis is already saturated at 25% of full sunlight, so photosynthesis can work at full capacity even with a fair amount of shade.

I wrote a Matlab program to have a look at how the umbra and penumbra could look for some artificial leaves. The distance between the Earth and the sun is 149,597,870,700 meters and the diameter of the sun is 1,392,684,000 m., both according to Wikipedia. A leaf takes up half the area of a 10 by 1-0 cm square area. In the program, this meant that I could paint half the pixels in a square area black denoting the leaf. All the program does is to cast ray from a raster of points on the sun's disk to all points on the leaf area, and to see which rays are intercepted and which are not. I did that for three distances behind the leaf: 0.5, 1 and 5 meters.

Click to enlarge; copyright Gert van Dijk

And here is the result of a simple roughly circular leaf. Half a meter away our leaf casts a recognizable shadow. I calculated how large the area is that receives less than 25% of full sunlight. That value of 25% is randomly chosen but helps to indicate deep shadow. Depending on the efficacy of photosynthesis, the value could indicate the lower limit of light for photosynthesis to work if it is particularly inefficient, or perhaps the point at which its efficacy becomes impaired. At half a meter an area of 85% of the original leaf receives less than 25% of full light, while at one meter the area decreases to 69%; at 5 meters it is 0%.

Click to enlarge; copyright Gert van Dijk

Let's try with a differently shaped leaf. After all, the umbra depends on the width of the leaf, so a leaf with a thinner shape should do better. This cross-shaped leaf was somewhat disappointing, as its values for 25% full light were only slightly better than for the circular leaf. For 0.5 meter the value was 81% of leaf area, for 1 meter it was 65% and at 5 meters it is 0%. Clearly, some more shape experimentation is needed.

Click to enlarge; copyright Gert van Dijk

This 'clover' has more space between its petals. Does it work better? Yes it does: 0.5 m results in 62%, 1 m in 31%, and 5 meters as usual results in 0%.

Click to enlarge; copyright Gert van Dijk

Finally, here is the ultimate feathery leaf, designed to have thin strands, while its area is still the same as that of the others. Here are the values: for 0,5 meter, only 11% of the leaf area receives less than 25% of full light, and at 1 and 5 meters the value is 0%.

Click to enlarge; source here

So, what does all this mean for the design of trees on other worlds? Firstly, like on Earth, you can have multiple layers of leaves and still have enough light trickling down for lower leaves to be useful. The shape of leaves is also important. Apparently some Earth trees use this effect: the outer or upper leaves of olive trees are thinner than the leaves lower down, which makes sense in view of the experiments above.

An interesting consequence is that the distance between leaf layers would depend on the apparent diameter of the sun's diameter as seen from a planetary surface. Doubling the diameter would half the length of the umbra, so leaves could be closer together and still receive an adequate amount of light. 

Should your alien trees have a few layers of leaves or multiple ones? Theoretical considerations on Earth suggest that fewer layers work better when the amount of light is low to start with: the absolute level decays very quickly with the number of layers. For alien worlds, 'low light' can probably be rephrased as a low capacity to make use of available light. That could be low light with good photosynthesis or good light with poor photosynthesis. The effect of changing the saturation point is more difficult to predict. On Earth, where photosynthesis saturates at only 20-25% of full light, shadows may still leave enough light. But if photosynthesis could use up to 75% of all light, the top layers might generate all the energy needed, so more layers would be superfluous. Then again, the plant might well have evolved to use all that energy, so perhaps lower layers would still be useful.

No doubt, additional demands, such as transport of metabolites and structural stiffness will complicate the picture. Nevertheless, on Furaha various plants carry their leaves in umbrella-like shapes, in which two or three layers of leaves form a nearly completely closed canopy, closing off the sky to potential competitors that might grow up beneath them.                               

* This example is taken from the excellent book 'The life of a leaf' by Steven Vogel

Fiddling with photosynthesis (Alien Plants IV)

'Alien Plants IV'? Where are the other 'alien plant' posts? Well, 'Alien plants I' and 'II' were published a long time ago, and 'Alien Plants III' was not labelled as such: that would be the post 'The black, black grass of home...' posted one year ago. That one was more serious than the first two, and  dealt with the colour of plants on Earth. To be succinct: green does not equal photosynthesis.

Click to enlarge. Copyright 2007 University of Chicago. From: Nature's palette by David Lee

As can be seen from the absorption spectrum of chlorophyll above, photosynthesis does not use the green portion of the spectrum, so that portion gets reflected for us to see. In doing so plants ignore much energy potentially available to them, as green is right in the part of the spectrum where the sun emits a lot of light. You might think that photosynthesis would evolve to make the most of the light falling on it, and, if so, you would predict that Earth plants should be purple (see the 'black grass'  post for speculations why some bacteria are purple but plants are not).

Some people wonder whether we can predict the colour of plants on a planet by looking at the spectrum of its sun. Earth's example definitely suggests that we cannot, so I personally see no problems with filling hypothetical planets with plants of just about any colour; well, as long as the absorbed colour is present in that sun's spectrum, of course. A perfect photosynthesis process would be able to use light of every frequency equally well, with the effect that such plants would be grey or black.

After writing the 'black grass' post I returned to the question why it is difficult to come up with alien-looking plants. Intuition suggested that there would be only so much you could do with plant shapes: flat leaves fixed to the ends of a branching structure seem so sensible that they are probably universal, so plants everywhere would look similar. Perhaps so, but intuition is not a reliable predictor in science, so some old-fashioned studying was called for. I recommend 'The Life of a Leaf' by Steven Vogel, who also wrote a fine book on biomechanics.

The fun part will be designing new plant shapes, if possible, but before we get to that there is some work to do, I am afraid. This post starts with photosynthesis on Earth, to find out if it can be tweaked to produce plants with a high degree of 'alienosity'.

Click to enlarge; Based on Long SP et al. Can improvement in photosynthesis increase crop yields? Plant, cell , and environment 2006; 29: 315-330

1. Efficiency of photosynthesis
The job of photosynthesis is to take water, CO2 and light, and turn out carbohydrates to use as energy sources and building materials, with O2 as a leftover waste product. Although the total energy capture by photosynthesis outranks human power consumption by far, photosynthesis is less efficient than the photovoltaic process used in solar panels. Photosynthesis is surprisingly inefficient.  The image above is based on analyses done by scientists looking for ways to improve crop yield. The 'black grass' post explained that only a portion of sunlight is used for photosynthesis, and the papers show that portion to be about half of the available energy. The graph above states the efficiency of each step, which is which fraction of energy gets passed on to the next step. The efficiency of the first step is 0.5: of 100% light to start with, 50% is left. That's a big loss. 
   The efficiency of the next step is 0.9. In terms of the original amount of light 45% goes on to the next step. And so it goes on, multiplying all the efficiency factors in turn, step by step, until only about 5% of the original energy is left at the end. As I said, not impressive at all. I should add that this holds for the so-called C3 photosynthesis type. The C4 type does better, managing to end up at 6 to 6.5%.  That does not seem like a big improvement, but it is still up to 30% better than C3 photosynthesis.      

One biochemical step deserves additional mention: 'photorespiration'. The reactions that take in H2O, CO2 and light to turn them into sugars and O2 are not exactly simple; an important enzyme capturing CO2 is ribulose-1,5-bisphosphate carboxylase oxygenase (no wonder that it is called 'Rubisco'). Rubisco deserves to be known, if only because it is probably the most common protein on Earth. Its job is to speed up the reaction binding CO2 that ultimately ends in O2. Oddly, Rubisco binds quite readily with O2, driving a process in the wrong direction! This backwards process is called 'photorespiration' and has puzzled biologists a lot. Its presence suggested that it might have some use, but apparently plants do quite well in artificial atmospheres without any O2 at all, so photorespiration seems to be a gigantic and puzzling waste.

2. Bright light: photosynthesis saturation
As if the above series of limitations is not enough, there is another one: photosynthesis saturates. Photosynthesis normally increases with the level of light but only up to a point. If light intensity increases beyond that point, photosynthesis cannot increase with it (it may apparently even decrease to protect the plant). Whether this is an important limitation depends on where you are: to catching the maximum amount of light to reach the Earth's surface, you will have to stand at the equator, at noon, on a clear day. The C3 type of photosynthesis can only use about a quarter of the light there! If you were to add that step to the image above, the scheme would start with a giant loss of 75% right at the start. Seen in that light (pun intended) the overall efficiency of 5% becomes an even less impressive 1.25%.

Then again, it is a bit unfair to set light at noon in the tropics on a cloudless day as the standard. Living at higher latitudes, clouds and shadows from mountains or leaves will limit the amount of light that reaches a plant, so in many cases the saturation point will never be reached. That is fine for those plants, but the tropics are still there, and photosynthesis could do a lot more for tropical plants  if their saturation point would lie at a higher intensity.

3. Shadows: the photosynthesis compensation point      
Plant cells burn molecules with the help of oxygen to free stored energy and use that for their metabolic needs, exactly like animal cells. This process is called cellular respiration and does the opposite of photosynthesis. As the amount of light decreases, photosynthesis will be less effective and produce less oxygen, while cellular respiration keeps using it a stable rate. At some shadowy light intensity the two processes are matched: the compensation point. When light levels drop beyond that point, plants become net users of oxygen and energy instead of producers. Plants can survive that state and in fact do so every night, but over time there must be a net profit. There are many places, such as the floor of dense forests, where it permanently too dark for photosynthesis to work.

"It's photosynthesis, Jim, but not photosynthesis as we know it". 
With all this in mind there seems to be ample opportunity to tinker with the process and design an alien photosynthesis. Mind you, photosynthesis could well be even less efficient on an alien planet than on Earth, and that possibility should not be dismissed out of hand. World builders have a strong tendency to design super-organisms, better than what Earth has to offer, but that is not very realistic. For once I will follow the flow and aim to improve on Earth's state of affairs. The following list concerns my suggestions how to improve on off-the-shelf photosynthesis:

Alien photosynthetic to-do list
- Have your photosynthesis process use a larger portion of the light falling on it
- Increase its affinity for CO2 (abolish photorespiration!) and improve reaction speed 
- Increase its saturation point so it can use intense light
- Lower the compensation point so it can work with less light.    

This 'to-do list' assumes that there are numerous biochemical pathways that can take in CO2, H2O and light and produce carbohydrates. Such processes may be centred on completely different pigments, sensitive to other wavelengths.

Click to enlarge; copyright University of Chicago. From: Nature's palette by David Lee

The illustration above has nothing to do with photosynthesis itself, but illustrates that there are many pigments in vision that are sensitive to varying wavelengths and to varying ranges of wavelengths. The pigment of the nectar-varying bat is interesting in that it is sensitive to a very broad range of light with a broad peak in the green area. A pigment like that, used for photosynthesis, would result in plants using light best where there is most of it, without throwing the rest away. Such plants would probably be a boring dark purplish grey.   

You may well ask whether all this biochemical tinkering will make plants look different. If they still look like Earth plants but grow faster the exercise loses much of its appeal, doesn't it? I think they would look different: if leaves can use all light falling on them, that will have consequences for any leaves underneath; simple blobs or needles might replace complex leaves; the ability to have fewer leaves might induce trees to grow higher; plants might continue to grow through winter, etc., etc.

Click to enlarge; by Ghedoghedo

Of course, apart from biochemistry different biomechanical design principles will also result in differently looking plants. To see whether that approach yields interesting choices, we may need to travel back to the Silurian and Devonian and have a look at designs principles that came into being when land plants first struggled against gravity. Changing designs and changing plant biochemistry ought to result in enough 'alienosity' to please anyone. We'll see...

The black, black grass of home...

Black? I hoped that by substituting 'green' with 'black' in the title of this evergreen ('everblack'?), your mind would create an image in which plants are suddenly no longer green but black. Plants hardly ever feature as more than background material in science fiction. SF artists may try to come up with odd plant shapes, but the general colour of your generic SF plant is green. Personally, I also often used green plants in my Furahan paintings without conscious thought.

Click to enlarge; copyright Gert van Dijk

What you see here are 'blackgrasses' on Furaha. In effect, they are largely brownish, but in this case at least I did not fall for the 'plants are green' trap. There are exceptions to ubiquitous greenery though, and the best-known one is probably Well's Martian 'red weed'. Anyway, perhaps it is time to think a bit harder about the colour of plants, first on Earth, then elsewhere. This post will be a bit technical; sorry for that. For more thoughts on this issue, see here and here.

The 'green, green grass of Earth' may trigger associations chlorophyll and photosynthesis ('chloro' means 'green' and 'phyll' is derived from 'leaf' in classical Greek, so it means 'green leaf stuff'). Photosynthesis concerns the trick of capturing the energy in light and transferring it to chemical energy (ATP), and chlorophyll is at the centre of that trick: it captures a photon, setting loose an electron that sets a cascade of motions going. As Chlorophyll is green, you might think that a green colour is good for photosynthesis. It is not: green light is almost useless for photosynthesis using chlorophyll.

Click to enlarge; from Wikipedia

Remember that what we call white light is a composite of a range of wavelengths in the electromagnetic spectrum, ranging from deep purple through blue, green, yellow and red to deep red. It is no coincidence that we call that portion 'visible light'. When light falls on an object some wavelengths are absorbed, while others are reflected. If an object looks green to us, that means that green light is reflected, meaning it is NOT used by photosynthesis. Above is an image of the absorption spectrum of two chlorophyll variants. A peak at a specific wavelength means that light at that wavelength is absorbed and used by chlorophyll. There are peaks in the red and blue parts of the spectrum, but not in the green portion of the spectrum. Does that matter? The answer depends on whether there is in fact a lot of light in that part of the spectrum, so let's compare the absorption spectrum of chlorophyll with the light output of the sun.

Click to enlarge; from Wikipedia

The atmosphere selectively absorbs some wavelengths, so what is relevant is how much of each wavelength reaches the Earth's surface. That is shown above. Try to find the visible part of the spectrum , from about 350 to 750 nanometer. You will see that there is a lot of light there.

Click to enlarge; from here

Here is a graph combining all the previous information. Note that the title states that chlorphyl is well-adapted to use solar energy. Well, yes, in the sense that it is roughly sensitive to light in the area where there is most energy. However, there is this big conspicuous gap, meaning that lots of light is unused by plants, mostly of the green variety. In fact, Earth plants would get on quite well if the sun did not emit all that energy at green wavelengths. (We would not like that though, as there would be about 40% less light to see with, and our ability to see details would be harmed as that depends to a large extent on green light; but that is another matter).

All this suggests that chlorophyll is not the best of all possible light absorbers on Earth. To make the most of sunlight, you would want a molecule that is responsive to a much broader part of the spectrum. Such a molecule would reflect very little light, so it would be black or at least very dark. You can also argue that, if chlorophyll can get away with using just part of the spectrum, so could another molecule. In fact, chlorophyll is not the only molecule used in photosynthesis.

Click to enlarge; taken from this site

'Bacteriorhodopsin' is a well-known example, occurring in some bacteria. Above is a graph in which its spectrum is overlaid on that of chlorophyll. This pigment has a rather broad absorption spectrum, but with a peak at precisely the spot where you would want it, meaning where the sun puts out much light: in the green / yellow parts of the spectrum. As a result, it looks reddish. There is a theory that these bacteria formed mats overlying the very first plants. The only light they let through was at wavelengths the bacteria did not use, so chlorophyll evolved to pick up just those wavelengths. This is a fascinating idea, but one thing that has me worried is the following: if chlorophyll could evolve such a detailed sensitivity to particular wavelengths, why did it stop evolving once plants outgrew those bacteria? Wouldn't it have made sense to retune it afterwards? If it would be sensitive to green light as well we would have black plants, and if it would be mostly responsive to green light plants would be purple.

I have no idea how easily evolution can tinker with light-sensitive molecules to shift their absorption spectrum. Still, light-sensitive pigments occur in vision as well as in photosynthesis, and there is an astonishing variety of pigments for colour vision in the animal kingdom. It seems that such pigments can evolve readily. But chlorophyll seems just to be sitting there, blind to all that glorious green light. Adding insult to injury, the chemical steps following chlorophyll are also singularly inefficient. Part of the problem seems to be that there is very little CO2 in the atmosphere, and that the molecule that takes in CO2 to strip the carbon from it leaving O2, quite readily works in the wrong direction, so the work is partly undone. After all this, the calculated efficiency of chlorophyll photosynthesis is on the order of 4-7%. Seeing how life on Earth depends on photosynthesis, that is a bit worrying.

In summary, the blindness of chlorophyll to green light suggests that other molecules with a different absorption spectrum could be just as efficient (or inefficient). It also suggests that you cannot tell the colour of plants well from the spectrum of the sun in their solar system. If you apply that reasoning on Earth, you will proclaim that Earth's plants are purple...

click to enlarge; copyright Chris Webb or Scientific American

In order to show something besides graphs, I will show a nice image used on the cover of Scientific American. I found it on the website of the artist (Chris Webb) right here.

In another post I may go into the consequences of all this for alien plants. For now, just consider some parallel evolution going on, resulting in different groups of 'plants' using different pigments. Forests on such planets need not show shades of green only, but could sport a riot of colours. Ah! The green, blue, yellow, purple grass of home...