The roots follow the nutrient

One of the things that surprised me a lot when I first started doing hydroponics was the extent that the roots grew. The tomato plant's roots would follow the flow of the nutrient and each plants roots would become entangled with the plant down stream of it. The tomato plants that were nearest the nutrient tank would grow roots that followed the nutrient right down into the tank.
It was common to have root masses that towards the end of the growing season in September would totally fill the 4" PVC pipe so that the nutrient would get close to running over the side right out of the holes in which the plants were set. I have a picture of myself pulling out a combined root mass something like six feet long.
One thing I discovered is that if the nutrient flowed more rapidly it seems to stimulate more root growth than if it flowed more slowly. So after the first couple of seasons I cut down the flow rate of the nutrient appreciably by limiting the flow into the tubes to a 1/4 inch feed line. This slowed down the growth of the roots so that I didn't get channels filling up and threatening to overflow.