arnoldia.arboretum.harvard.edu/pdf/art...cies.pdf
Abstract,
Plant roots can grow anywhere-in the soil, on the surface of the soil, in the water, and even in the air. Except for the first formed roots that respond positively to gravity, most roots do not grow toward anything or in any particular direction. Root growth is essentially opportunistic in its timing and its orientation. It takes place whenever and wherever the environment provides the water, oxygen, minerals, support, and warmth necessary for growth.
Human activities, such as construction, excavation, and gardening, often result in serious damage to trees. In some cases, trees can be inadvertently injured by people who are trying to protect them. Indeed, people can kill trees in hundreds of ways, usually because of misconceptions about root-soil relationships, or because of a disregard of the basic functions that roots perform.
In order to maintain the health of cultivated trees and shrubs, it is necessary to understand the morphology and physiology of tree roots in relation to the aerial portions of the plant. For those who are responsible for maintaining the health of woody plants, this article examines some widely held misconceptions about roots. It describes the typical patterns of root growth as well as their locations and dimensions underground. It also describes the relationship of healthy roots to typical forest soils as well as the behavior of roots adapted to atypical circumstances-growing through deep sands, under pavements, down crevices, inside shopping malls, and in sewer lines.