86 LEAF CANOPY AND PBUNING. [Dec, 



appearance of still more unwelcome classes of weeds. Finally, 

 deterioration manifests itself, when the crop has been removed, by 

 a difficulty in renewing the covert, amounting sometimes to the 

 impossibility of restoring wood of an equally high and exacting 

 class. Hardwood may, for instance, have to be succeeded by 

 coniferous kinds, or Larch may have to be followed by Scotch Pine. 

 As to the sentence ciuoted, it is unfortunately capable of a rather 

 wider interpretation than was intended, owing partly to haste and 

 partly to the desire of making a forcible statement. The idea in the 

 mind of the writer was, that plentiful weeds are at once a sure sign 

 and an active cause of danger to the forest soil. It would have been 

 safer to have qualified the sentence, and written it : ' If the covert be 

 very open, the soil is liable to be deteriorated by exhausting annual 

 crops of grass, brackens, and weeds, and ])y the other incidents of its 

 exposure.' It is the nature of trees maintained in close order to give 

 back to the soil every year many valuable products, and to improve 

 its qualities in the various ways already indicated in return for the 

 very moderate quantity of mineral substances of i which it deprives 

 the ground. Soil producing a hundred tons of saleable timber, sa}^ 

 every seventy years, need not thereby be appreciably impoverished . 

 From a table with which Mr. M'Corquodale himself supplied us, in 

 March, 1882, on p. .787 of the 'Journal of Foresiiiy; we may 

 gather that of these hundred tons only between one ton and half a 

 ton is composed of mineral substances contributed directly by the 

 soil. The remaining ninety-nine tons and more have their bulk 

 composed of watery and gaseous substances, absorbed indeed from the 

 ground, but originally derived from the atmosphere. The trees, it 

 may thus be calculated, take only a few pounds or stones of mineral 

 substances, or solid ingredients, each year from an acre of ground. 

 On the supposition that each year only a small proportion of soil 

 minerals become dissolved by weathering and the other processes, the 

 available yearly supply of soluble mineral nutriment must be very 

 limited. It must then be extremely prejudicial that any great 

 quantity of weeds should enter into competition with the trees for 

 the mineral necessaries of plant life. "Weeds and grasses also promote 

 the evaporation of moisture from the soil, and prevent the dews from 

 reaching it, a point, perhaps, of not very great importance in a moist 

 climate. They also, to some extent, hinder the formation of even and 

 continuous layers of leaf humus. Weeds when allowed to decay on 

 the ground must certainly restore some of the useful substances 

 which they had filched ; but they do not in general form so valuable 

 a humus as tree leaves. Still worse in many respects than the 

 annuals are some of the woody weeds, such as broom and heather. 

 The humus produced by decay of heather is the principal ingredient 



