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To elaborate on the title, can alien plant life on an Earth-like planet look similar to ours, or would plants on an alien planet be radically different, unrecognizable, or incomparable to Earth plants in the eyes of a human traveler?

I've heard things about different pigmentation based on what wavelength of light the planet's mother star emits, but I'm talking more about the broad strokes. Would leaves still look like leaves? Would trees still look like trees? Or would they be as different to Earth plants as some aliens in harder science fiction are to us?

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  • $\begingroup$ Evolution is largely dependent on external stimulus and since you mentioned plant and Earthlike, chances are there are certain recognizable features but you should never underestimate the word diversity. $\endgroup$ – user6760 Sep 19 '16 at 5:52
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Assuming the same principles of competition and selection are in play, and that plants evolved, there should be various obvious analogues between Earth and Xeno plant life (plant being defined as sessile phototrophic organisms). The exact details – the shape, colour, structure – will depend on local circumstances.

On Earth, leaf morphology correlates with water and light availability, because various shapes when combined in overlapping clusters are optimized for catching light. The shape of the leaf also determines how much light reaches leaves below and at what distance that penetration reaches zero. (You may have heard of Planet Furhaha, a worldbuilding ecology project; here is the study the author did looking at light penetration and leaf shape.)

Colouration of xeno-analogue photosynthesizers will be determined by available wavelengths from the star and absorption by the atmosphere.

Extraterrestrial photosynthetic plant-type life may look quite look different in color because they will have evolved their own pigments based on the colors of light reaching their surfaces. Nancy Kiang of NASA's Goddard Institute for Space Sciences has modelled the light reaching the surfaces of Earth-sized worlds orbiting their host stars at distances hospitable to Earth-type life, where liquid water could exist on a planetary surface, where depending on the star's brightness (and color) and the planet's atmosphere. Kiang found that "plants" on Earth-like planets orbiting stars somewhat brighter and bluer than the Sun might look yellow or orange, and even look bluish by reflecting a dangerous overabundance of more energetic blue light. On the other hand, plants on planets orbiting stars much fainter and redder than the Sun might look black. (Source; referencing this.)

And it's reasonable to expect similar growth strategies would be used: grow up and/or grow out. Plants can avoid being shaded out by competition, to the point of stifling the growth of competitors, by growing taller or wider (or both).

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I think leaves are close to optimal, thus expect leafy plants. Color could be quite different, leaf shapes could be quite different as well. They might not have conventional bodies, they could easily end up being supported by strengthened vines or could be similar to ferns. Thus it might look quite similar.

On the other hand, I could say, there could be photosynthetic mobile life forms. They don't have to be plants like our plants. In this world, it is possible that there are no tree like creatures exists. But being pinned to the ground has quite a lot of advantages but it is not impossible for a world like this to develop. Flying photosynthetic creatures would be quite nice.

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Plants would look very similar to earth plants. The biggest difference would likely be in flowering plants since flowers are based on animals, but I'd suspect very similar adaptations...

But "Plant" isn't really the correct way to look at these things. Plants will only ever be descended from the plants we have here on earth. There will be things on other worlds that share in the role plants play here on Earth, but they won't be the same. They may in fact be wholely different. You might find many of the characteristics of what you would think are plants to be from what we normally associate with animals and vice verca. There is a reason things go the way they do and that generally forms the same type of biology, but just because plant cells are better for plants and animal cells are better for animals doesn't mean you won't find the plant like cells in what you'd consider an animal.

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