A container of water that is treated with herbicide is placed just underneath the lower roots of a plant. Now this poisonous mixture isn`t enough to kill the plant outright but will kill it over an extended period. Will the plant continue to try and grow more roots into the poisoned water or will it "write off" the roots already in the poison and continue to grow roots elsewhere?

Good question. Are you asking knowing the answer, or are you looking for one? I love plants. They're so cool. They grow towards or away from the things they need, but I don't know what the would roots do, only the above-ground plants. (Science project involving a home-built rotating cage filled with dirt with seeds in the middle... cage rotates, plants grow in spirals, always trying to reach the "sun"... won first place in the science fair. )

I don`t know the answer Loth.

I might have to develop a little experiment, then, Roos. However, my hypothesis would be that it would depend on whether the roots were also able to reach "right" water, and exactly how fast the plant was being killed. Oh, and also, whether the herbicide was synthetic. That's based on a (probably fanciful) assumption that the roots wouldn't just grow OUT to whatever was there. Roots are more likely to search out (even minute) pockets of "air" in soil and rocks (since that's where water will go), taking advantage of loose soil and cracks in rock. If the root system were bare except for the possibility of the water below, it may just dip in and kill itself off for lack of anywhere else to grow (and anything to support it). If the root system were in soil, it would probably be more likely to grow further into the soil, since "drowning" in the water isn't ideal anyway, and might then also kill itself off if the poisoned water were the only water made available. It might also depend on the mechanism of the herbicide.

The purpose of the root system is to take water and minerals to the plant generally, so the roots in the area containing the herbicide would carry the herbicide into the plant and weaken or kill it, depending on the size of plant, strength of herbicide, amount available, etc. Many garden weedkillers work on this principle, and it may take several applications to finally kill the plant, but if as stated 'there is enough to kill the plant over an extended period' then the plant will be killed!

Thanks for the answers people. Aramis so what you are saying is that the plant will keep taking in more of the poison? There won`t be any discarding of the poisoned root? I would love to know the results of your experiment Lothruin.

Well, I killed a couple spider plant shoots, Roos. Gravity won. I don't think the plants can keep the roots out of the water if the water is where gravity will put them. The bits of plant growing UP will move toward things like sunlight, but the roots, at least in an air and water situation, just hang into whatever you put directly below them.

Loth, those spider plants certainly appear to be ideal for this particular question, the speed at which they grow in particular.
I`m thinking myself of experimenting. Maybe using an ant farm type set-up so I can view the results, that might give the plants half a chance to grow away from the poison and grow into the good water. Perhaps diluting the poison in varying degrees and placing the poisoned water in small tubs placed 10 mm beneath some flyscreen,fine enough to hold soil but not so as to stop roots, at the bottom of the ant farm. Using mature stronger plants might also help, as might using different types of poison, perhaps petrol,herbicide etc.
I hope the plants being used as guinea pigs appreciate that this is all being done to them for scientific purposes!