With the plants detailed here (I’ve decided to call them ‘plant-etoids’
in what I hope will induce groans) I worked backwards. Such an exercise,
while not speculative evolution in the purest sense, is nonetheless a
useful tool, for it allows us to consider how things we would normally
consider impossible seem to be, at the very least, possible. Even if I
am correct in all my speculation, plant-etoids are still very, very
unlikely to exist.The first problem space plants would face is adapting to conditions. As [REDACTED] pointed out, such a plant would quite possibly go into stasis, at which point while it would, indeed, be a PLANT! IN! SPACE!, it would hardly be anything particularly significant. No, to have space plants—and, therefore, plant-etoids—the plants would have to be able to cope with near vacuum, cold, and radiation. [REDACTED] suggested the use of a planet slowly losing its atmosphere.
We have a planet like that in our solar system—Mars. According to some scientists, Mars may at one point have had a magnetic field. When it lost its magnetic field, the solar wind from the Sun stripped away the Martian atmosphere. Such a process would be slow, allowing gradual evolution, and would also result in an increasing amount of radiation reaching the surface. Mars, of course, did not go this route. If life ever existed on the Red Planet, it is either underground, in the ice, or simply extinct.
Escaping from the planet, I feel, would be best provided by volcanic activity or a meteor strike. While volcanic activity might seem like something that would result in huge amounts of heat, given the lack of air there would be no real opportunity for convection, and a reduced opportunity for condution via the air. Instead, the soil and rock on top of the volcano would be conducting the heat.
Water collection initially appeared to be an insurmountable problem. However, I believe I developed a plausible solution. The bladderwort has orb-like structures that it uses to catch prey. I propose that a plant on a planet with a gradually thinning atmosphere would be forced to develop something along the lines of an analogous structure. Ice would be surrounded by plant matter, after which the plant would develop an air-tight coating and generate metabolic heat to melt the ice. The limited space available would force the water to remain in a liquid state. Roots could then be extended to absorb the water. Dessication could, of course, be solved by a waxy covering, but plants without them are apparently already capable of surviving vacuum for a day or so.
Nutrition could also be assured in such a way. If the plant could survive in very poor soil (on a Mars-like planet, I would say this is quite possibly a given), then by capturing the ‘dirty snowballs’ they could also absorb amino acids and various other elements necessary for survival. These comets could be caught by having the plant-etoid extend structures made of spongy plant matter. When a small comet impacted it, it would then surround it in air-tight plant matter and absorb water and nutrients. In otherwords, it would use a similar method as aerogel. This spongy plant matter could evolve by natural selection; if the plant-etoid’s ancestors grew around bits of ice to absorb them, then a comet embedded itself in the plant would probably be absorbed, too. Sunlight, in turn, could be absorbed by the spongy plant matter.
Reproduction would be asexual and happen via budding, I would expect, though I suppose that if the plant-etoid’s existed in large concentrations sexual reproduction would also be a possibility—as they would be if the sperm and eggs were suitably protected from the elements. Certainly the plant-etoid’s would have a slow metabolism, so I would expect zygotes to last a rather long time.
The plant-etoid would not need inhale or exhale carbon dioxide or oxygen. Instead, it would operate by consuming all the oxygen produced for cellular respiration, and consuming all the carbon dioxide it produced for photosynthesis. Since the plant-etoid would have to have very, very slow metabolism, gradually it could accumulate more and more carbon dioxide (or oxygen), allowing gradual growth and reproduction.
The ideal shape, I think, for the plant-etoid would be a sphere. Such a shape would allow large amounts of area to be exposed to sunlight without risking decreased exposure if the plant-etoid turns sideways after getting hit hard enough. The aforementiond spongy tissue would cover the outside of the plant and be photosynthetic. It would also be only the outer layer that was alive; as the plant-etoid grew larger and larger, it would accumulate heartwood, or something similar, in the center, like a tree. Even after a plant-etoid died, it would still float on, potentially food for other organisms. Over a long period of time, the plant-etoid’s could act like clearers, as small micrometeorites would get stuck in their spongy tissue, but not digested.