Why do plants have tendrils?

Why did evolution "choose" for plants to be green?

• I asked my teacher in Biology and she said I should find out. I know why plants are green (chloroplasts in the cells contain chlorophyll a, b and several other pigments that absorb mostly red and purple light, while reflecting green. Hence leaves appear green). I also know not all plants are green, some contain other pigments and are red etc... but MOST plants are green. So why green? It's in the middle of the light spectrum, it's not even the lowest energy colour. Plants have so many adaptions for photosynthesis, so surely reflecting a colour costs the plant valuable light? What evolutionary value is it to the plant to reflect green light? In fact, why reflect any light at all? Why not just absorb all light?

• Answer:

  Because evolution is based on *inheritance*, sometimes the answer is just an arbitrary "choice" in a very early organism, that is widely inherited simply because it works "well enough" to be useful to a wide variety of organisms. That is enough of a reason to be widespread. The thing that gives chlorophyll its green color, is the magnesium ion at the center of its nitrogen ring. It is a kind of organic molecule called a chelate, which consists of a metal ion in a ring of nitrogen, carbon, and oxygen atoms, and bound to a larger organic molecule. (Another example of a chelate is heme, which has an iron ion at its core, instead of magnesium, and when bound to a protein molecule produces hemoglobin ... and the iron ion gives hemoglobin its distinctive red color, instead of green.) [1] There are a number of variations of chlorophyll, but plants contain almost entirely two types, chlorophyll a and chlorophyll b. They vary slightly in their absorption spectrum, but they all have the same basic shape ... they absorb light in the long-wavelength end of the spectrum (the reds), and the short-wavelength end of the spectrum (the violets), but reflect light in the center of the spectrum (the greens). [1] [2] So the question you are asking is ... why plants are not black (absorbing all wavelengths of light, instead of just the long- and short-wavelength light)? There are some advantages to reflecting light at the the short-wavelength (violet) end of the spectrum. Short-wavelength photons have a higher energy level, and are generally harmful to living things. These high-energy photons have just the right wavelength to break chemical bonds in organic molecules ... like RNA and DNA ... which is why unchecked ultraviolet light causes harmful mutations (like cancers). Our atmosphere shields a lot of the most harmful UV light, but pigments that naturally reflect purple, would also act as a natural shield from the more harmful parts of the remaining spectrum. In fact, there is some evidence that the early earth may have been *purple*, rather than green, as micro-organisms would have used retinal, rather than chlorophyll as the basis for converting light energy. [3] Retinal is a form of Vitamin A that forms the basis for the photopigments in the retinas of our eyes. It is a simpler molecule than chlorophyll, but reflects light only at the violet (purple) end of the spectrum. [4] So the chlorophyll-based photosynthesizers may have been latecomers relative to the retinal-based microorganisms ... but survived because they used wavelengths not used by retinal. But because chlorophyll is a more efficient energy converter, the green photosynthesizers just out-competed their purple rivals, and ended up becoming the dominant form of light-converting life. So there may no further advantage than that. Evolution is often about some arbitrary "choice" that spreads simply because it works. The variations, like chlorophyll a and b, can arise by a mutation in the genes that code for chlorophyll, and can both be co-existing without either becoming dominant. But unless there is some significant newcomer with a distinct advantage, life just may never have discovered anything better (even though we can engineer them in the lab). Life does not "engineer" things ('design' them with a particular goal in mind) ... it "discovers" them, and if they work, a LOT of organisms will inherit it. [5] I highly recommend source [3]. The idea of a retinal-based biome is really interesting. ----