BROADLY speaking, there are two ways of ‘growing’ plants useful to man. The first and most obvious is to treat the plant as an agricultural crop, that is, to clear the ground of existing vegetation, prepare it by digging, ploughing, or other treatment, and then to sow or plant the crop, which is kept ‘clean weeded’. The second is employed where large tracts of natural vegetation are useful as a whole, or contain useful Species, as with pasture grasslands or forest. Here the primary attention required is to see that the regime of exploitation, by the grazing of stock or by felling, is not such as to impoverish the vegetation as a whole, or to favour useless at the expense of useful species. Besides regulating the regime, active interference to these ends is sometimes required.