
One of my psychology professors recently returned from Moscow (Russia). He is working with NASA and the Russian space program to find out WHY they should grow plants in space. At this point they CAN but aren't sure WHY. There are the obvious reasons like if the astronauts want a nice fresh salad, but they are looking at the non-nutritive value. For example, people like to go outside. When feeling stressed out a nice walk through the trees is calming.
A community garden is a popular thing in some communities. Why? It is easier to buy a tomato at the store.
A community

garden in a prison has been shown to reduce violence. Why? Is it because they can work together towards something instead of... well, whatever they do in prison.
Gardens in war zones have been grown. During WWI soldiers grew plants in the trenches. One girl in my class said that while she was in Iraq a soldier had had someone back home send him some grass seed and he planted along the top of the dirt barrier that protects from shrapnel. Why?
The point is that during high stress situations, humans have been shown to have a tendency to turn to plants. SO, why have plants in space? Wouldn't it be nice to have a nice ficus tree aboard the space craft?