Indoor plants tend to grow toward the light, so why do trees outdoors grow straight instead of leaning toward the equator? 
—W. Anderson, Sacramento, Calif.

Edgar Spalding, a botany professor at the University of Wisconsin–Madison, sprouts off:

A plant on a windowsill experiences a stronger light gradient than does a tree outdoors, where gravitational cues can overpower more subtle light-direction cues. Indoor plants get a lot more light on one side than on the other, which activates photoreceptor molecules to a much greater extent on the lit side. This difference is biochemically translated as a growth response, known as phototropism, which makes the plant bend toward the light.

Trees growing at a latitude of, say, 60 degrees are also asymmetrically illuminated because of the slant of the noon sun—approximately 55 degrees at the beginning of the summer growth season—but the difference in light intensity there is smaller and more variable. The modest light gradient experienced by the tree is counteracted by a continuous gravitational influence, known as gravitropism, which guides plant growth upward. The strength of gravitropism trumps phototropism in the tree scenario but not on the windowsill.

The edge of forest gaps provides a good place to observe light-guided tree growth at any latitude. There the effect of a strong light gradient can be seen in the reaching of trees into the gap.