The shading of the ground by leafy vegetation will, of course, greatly diminish and sometimes suppress evaporation from the soil-surface; thus very nearly fulfilling the same conditions referred to above (chap. 7, page 111) in discussing the effect of natural vegetation in rendering tillage unnecessary; the beating of rains, and the formation of surface crusts, being alike prevented. This fact is of essential importance in contributing to the welfare of crops sown broadcast, where subsequent cultivation is impracticable. _Weeds Waste Moisture._—The injurious effects of weedy growth among culture plants are in most cases due quite as much to the appropriation of moisture that should have gone to the crop, as to the abstraction of plant-food, to which the injury is generally attributed. This is much more obvious in the arid region, where during the dry summers every pound of moisture counts, than where summer rains obscure this influence. It has led orchardists in California almost to an excess of clean culture, resulting in the burning-out of the humus from the bare surface-soil during the long, hot summers, and an injurious compacting impossible to remedy by the most careful tillage. It thus happens that green-manuring, the natural remedy for this evil, cannot safely be done there with summer crops, but must be accomplished with winter crops, such as can be turned under before the dry season begins. The same objection holds against the growing of summer crops between the orchard-rows. DISTRIBUTION OF MOISTURE IN THE SOIL AS AFFECTED BY VEGETATION. The investigations of Wollny and others have long shown quantitatively what common experience has taught the farmer, viz., that a field in crops or grass is always drier within the soil-mass penetrated by the roots than is a cultivated field bare of crops, unless perhaps when heavily crusted on the surface. The depletion of moisture caused by grass sward is the most easily observed because of the shallowness of the root-system; and this is one cause at least why grass sward does not occur naturally in the arid region, and when planted cannot be maintained without irrigation repeated at short intervals. Deeper-rooted plants of course deplete the soil at different and varying levels; and where surface roots are few or absent it may readily happen that the surface soil is moister than the subsoil.