|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|
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.
Earlier posts on alien plants
Alien Plants I
Alien Plants II
Alien Plants III
Alien Plants IV
Alien Plants V
Alien Plants VI
"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?
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.