THE VEGILTATION OF WOODLANDS. 135 



the tree-canopy, but it is thin and poor below a thick canopy; 

 of course, bracken does not occur on all soils, but mainly on fairly 

 deep, moist, open sands or loams. On the other hand, spring- 

 flowering plants, such as celandine, wood hyacinth, primrose 

 and anemone, leaf and flower several weeks in advance of the tree- 

 canopy, and can thus continue to grow in woods : if the ground- 

 vegetation of a wood consists entirely of such plants, it becomes 

 quite bare during the summer, and this indicates that the canopy 

 is thick enough to exclude most of the light. Dog's mercury 

 ( Merciinalis) and wood garlic {Allium) grow under trees through- 

 out early summer, and their main leafing period is later than the 

 spring-flowering species, hence they can only flourish under a 

 thin canopy and are exterminated in close woods. The growth of 

 blaeberry ( Vaccmiuni myrtillus) also takes place mainly during the 

 summer, hence it is only abundant in woods with a poor canopy. 

 Evergreens, like heather {Callufia) and hair grass {Atraflexuosa), 

 although soon suppressed under evergreen coniferous trees, may 

 remain under larch or deciduous trees, where the canopy in 

 summer is moderately thick, because during autumn and spring 

 they are able, by means of their evergreen leaves, to form sufficient 

 food supplies. When the canopy is allowed to become very thm, 

 birch, rowan, thorn, hazel and other shrubs soon make themselves 

 evident. It is hardly necessary to point out that when a strong 

 undergrowth is established, neither seedlings nor transplants of 

 the more valuable timber trees have much chance. 



Loss of canopy also acts indirectly on the soil, and may lead to 

 even worse results than rank undergrowth. The condition of 

 closed woodland leads to accumulation of leaf mould or " mild 

 humus," a soil-covering of the greatest importance in tree-growth ; 

 an open canopy results in destruction of this mild humus. 

 This subject of humus has been carefully investigated by several 

 workers on forestry questions, and was dealt with in these 

 Transactions recently^. The main facts are here summarised. 

 Organic remains of plants and animals lie in the soil and are 

 gradually destroyed by a number of agents. If these do their 

 work well, the organic matter is quickly removed, as in the case 

 of good farm and garden soils ; if the various agents work slowly, 

 then the organic matter accumulates. In woods, it may become 

 a valuable element of the soil if converted into mild humus, but 

 if it is left as raw humus it may become a serious enemy of good 



' Hardy, M., '"Humus as a Geographical Agency," vol. xvii. p. 256, 1904. 



\ 



