>Object mass wasn’t the only factor the slime mold considered. When the three disks were stacked on top of each other instead of next to each other, the Physaru

>Object mass wasn’t the only factor the slime mold considered. When the three disks were stacked on top of each other instead of next to each other, the Physarum no longer preferred that side. It grew toward the three-disk and one-disk sides at about the same rate, despite their clear differences in mass. This, Novak says, “was a huge hint, or more like a slap in the face, that it must be basing its decisions on something else, too.” It's not clear to me if they are using 'mass' as we understand it from physics, or just the general notion of size of an object. To me the stacking experiment seems to invalidate that its the former. It would probably be useful to do some analysis on airflow inside the petri dish, there could be a very subtle convection and/or radiant heating from the white disks. There's also potential phototropic effects like we would see with plants bending towards light. Both of these inputs would actually be affected by the stacking, where mass is clearly not. (Yes they 'turn off the lights', but you can't take a time lapse movie in absolute darkness) Regardless of the cause, the effects are going to be extremely subtle, and the fact that the slime mold seems to select a direction based on this incredibly subtle effect is still extremely cool.