>>11391646i refrained from replying with my thoughts but i'll humour you
we can assume there is sufficient diffusion in the plant's fluids, so there is probably a uniform concentration of electrolytes etc. that's why it can grow upwards as well as down.
the grout is lower than the bricks, so water will pool there first, and the plant will grow there. Also, the grout is more permeable than the brick, so diffusion of nutrients from the soil below will occur there faster, so roots will grow there faster. you can see that it doesn't grow exclusively in the grout, just faster. there are smaller roots growing on the brick.
i'm certain plant growth follows the most favourable thermodynamic path, i.e. where the chemical potential gradient is largest. it follows that the areas with the highest concentration of nutrients (the grout) would have the most growth.
i can't prove it but i'd like to see a better explanation
