Straggly growth and spreading upwards has nothing to do with the light. This is yet another optical illusion that we have been under for years. Plants grown in air have a specific physiology which includes a distribution network of tubing. This tubing network allows for gas exchange and gas transportation throughout the plant. So for example gaseous Oxygen needs to be transported to the roots from the upper region of the plant. CO2 needs to move from the underside of the leaf to the reaction chambers on the upper side. There is another important gas called ethylene (C2H4) that is present inside the tissues in very small concentration. As it turns out, this gas is actually a hormone and it's used to regulate plant growth and also to regulate cell death.
There are lots of environmental conditions under which the plant needs to change it's growth rate. This normally happens under environmental stress. When the plant is under these stress the concentration of ethylene rises and various mechanisms respond to the concentration rise. One of the most extreme environmental stresses a plant can face is that of being flooded. Flooding the plant traps gasses, prevents their movement and causes buildup. In general, gasses are about 10,000X less soluble in water than in air. That's why CO2 uptake is such an acute problem for aquatic plants. When the plant is flooded ethylene is trapped and it's concentration build. The reaction of the plant is to immediately grow upwards to reach the surface where there is air and where access to atmospheric gasses are in greater abundance. In effect, the plant is building a snorkel by having straggly growth.
There are a couple of ways that we fight the straggly growth. One way is to simply cut the growth. The new leaves that grow are submerged leaves and they are better adapted to a flooded environment, so they tend to grow less straggly. The penalty though is that cut leaves do not produce food, so recovery is slow. Another way is to have better flow. More flow across the leaf means better diffusion of gasses so ethylene can be removed more quickly and CO2 and O2 also have better movement.
