One reason to grow upward is also just the need for oxygen. Mushrooms do this too - they tend to colonise and begin to fruit in high CO2 concentrations, but eve

One reason to grow upward is also just the need for oxygen. Mushrooms do this too - they tend to colonise and begin to fruit in high CO2 concentrations, but eventually want to escape it. And you can see the negative side effects of not being able to do this in mushrooms too (if the CO2 is too intense they grow too fast relative to their thickness [trying to reach air] and become nasty and stringy looking)