Alongside the horizontal green roof, in recent years the vertical axis, aka green walls have been becoming more popular across the developed world. They are as described, walls covered by vegetation with an earth or soil component for plants to grow in, generally, though not always, with a water system integrated into the wall. Examples from around the world show how green walls can be found at all scales, from the small fence wall to the very large, growing on the side of corporate buildings.   Green walls are a popular alternative in cities where parks, and other green spaces are not available. They serve a variety of functions, including reducing heat in cities and the urban fabric; reducing rain water or grey water run-off, since plants help to purify water; urban gardening, and also, of course, as literal gardens, tended for the same gardenly reasons as their horizontal cousins.