my experience says a plant's roots' priority is to grow down first in search of water, followed by potentially intense branching anywhere from 1-6 inches behind the root tip. the way i've seen the roots grow (hydro setup), it seems their first priority is finding water, growing with almost no side branching until they hit the reservoir. once they hit the reservoir, the roots tend to continue to grow rather rapidly in length (i'm saying length is comparable to downward growth since they hit the bottom of my res pretty quickly), with more and more branching as you get closer to where the roots meet the plant, however right at the base of the plant, the roots tend to get THICK with less branching. the "downward" growth slows after a time, and more delicate side roots continue to grow as the plant enters flowering as the side branches (of the roots, not the above ground plant) develop their own side branches. before and during the few weeks as the plant enters flowering, the addition of mychorrizal fungi causes EXPLOSIVE root growth in the form of extensive side branching.
this is all from personal observations of the strain MI5 in a hydro setup, so it's possible other mj strains have different root growth "priorities" but i'd be willing to bet this sort of growth is seen across the species.
one other thing to consider is root trimming. roots will terminate themselves if they end up drying out. the tip (where all new length comes from) will shrivel up and essentially kill itself so other roots can use the plant's energy to search out more nutrition. in my hydro setup, similarly to a plastic pot, roots almost never get the chance to self prune because they aren't exposed to the air long enough. think of it like this - you've lifted up a pot and found roots growing out the bottom, coiled around in that dark, humid area. however, if you lifted that pot off the ground and allowed good airflow to the drainage holes, roots might start to poke out, but they'd quickly stop growing and self prune. this is MUCH more likely to happen in an airpot, and a good thing. those coiled roots at the bottom of your pot aren't absorbing nutrients as efficiently as roots buried in media, since they're competing against each other to absorb nutes. anyway, now i'm rambling and lost my focus on what i was trying to say, so i'm going
