PERIODICAL LITERATURE 503 



attention to extensive researches in England made some years ago in 

 an effort to show the effect of grass on fruit trees. In this investiga- 

 tion it was shown that the soil and climate are determining factors 

 in the amount of injury from grass. Fruit trees make a much better 

 fight against grass on deep, loamy soil in the west of England than 

 on the gravel and chalk of Kent or Surrey. It is pointed out that 

 fruit trees are often killed by grass; that recovery of health only 

 occurs where the roots extend beyond the grassed area and that 

 ground that gradually grasses over while the trees are growing is less 

 harmful than when the grass is sown and comes on at once. 



Forest trees were effected in the same way as fruit trees when 

 grass was sown immediately after planting, but on light soil conifers 

 were effected much less than fruit trees and some recovery occurred 

 in time, whereas in the case of fruit trees the effect was intensified 

 by time. The writer states that the injurious effect of grass was proved 

 not to be due to : 



(a) Interference with the circulation of atmospheric air in the soil. 



(b) The amount of carbonic acid in the soil. 



(c) Soil temperature. 



(d) Food supply. 



(e) Physical condition of the soil, or 



(f) The micro-flora of the soil. 



He states that it was found to be due to a substance produced by 

 the grass roots, which acts as a poison to the roots of most trees. The 

 evidence, however, from which the above conclusion is drawn is not 

 presented in the article under review. The toxic substance is said to 

 be easily destroyed by contact with air, which accounts for the fact 

 that trees will make but poor growth on an old pasture while the turf 

 from the pasture when torn up and decomposed greatly stimulates 

 growth. 



Ash and larch planted in 1912 on ground covered with grass 

 showed in 1913 and 1914 that: 



(1) The effect of the grass was evident at the close of the first 

 season. 



(2) The terminal bud often failed to mature and the foliage was 

 stunted and short. 



(3) The annual shoots were much shorter than those produced by 

 trees on plots where the ground was kept bare. 



