Why are plants attracted to radium?

A 1909 issue of Popular Science Monthly contains a piece about how plants react to radium. Since radium emits gamma radiation, which kills cells and mutates DNA, they don't generally respond well. So why are these plant roots curving towards it?

We've already established that radium's overall effect on the world is mixed at best, but one thing it did leave behind is cool science experiments. After its discovery in 1898, scientists all over the world got samples and started experimenting. This particular experiment exposed plants to radiation in many different ways. Most of the plants were blighted, but there was one unusual result.

Why are plants attracted to radium?

A group of seeds were grown in tap water. A sealed tube of radium bromide was placed next to them. As they grew, the roots curved toward the radium. If they were leaves and branches, it might be tempting to say they grew towards a nourishing energy source. Actually, it's the opposite. Researchers had already noted that, when placed between electrodes, roots would curve toward the positive electrode. This was not because the positive electrode was more nourishing; actually, it inhibited growth while the negative electrode spurred growth. When one side of the root grows faster than the other, the overall root curves toward the slower-growing side. The behavior was called galvanotropism.

Here we are seeing, according to the author, the same thing. The plant isn't curving due to radiation, instead the radiation from the "rays of radium" is causing a charge gradient in the water, with electrons on the far side of the container and positively charged ions gathering on the side closest to the radium. Root growth is stimulated on the negatively-charged side, and inhibited on the positively-charged side, and the roots curve towards the radium.

Top Image: Nicolas Perrault III

[Via Effects of the Rays of Radium on Plants]