Why Are Potato Lateral Roots Mostly At Surface?

This is the same root you saw last week, rising and falling beneath the soil surface. More interesting developments this week:

Secondary (lateral) roots have suddenly developed, but only where the original root is nearest to the surface. It's hard to get a photo because of the shape of the pot, but the root is almost devoid of laterals either side where it is lower in the soil, so farther away from the surface. Why?

• Perhaps laterals responding belatedly (it takes lateral roots some time to appear, because they have to grow from the 'pericycle', deep inside the root) to the water applied to the surface some time back.
• or Perhaps, being principally feeder roots, they just grow when roots come up near the surface (feeder roots do this to gather water from rain showers, among other things)

Just out of interest, I inserted some water around the lower root (where it is furthest from the surface) with a syringe, so that the soil is just moist. Will this water induce lateral roots to form?