2 9 o SCIENCE PROGRESS 



carbonic acid thereby produced playing an important part in 

 rendering the mineral constituents of the soil soluble and 

 assimilable by the plant ; aeration is also necessary for the 

 existence of the bacteria on which the plant is dependent for 

 its supply of soluble nitrogen. The importance of an air-supply 

 to the roots is rendered evident by the failure to grow plants in 

 water unless this be well aerated ; also, no surer way of damaging 

 or killing trees exists than that of allowing the soil to become 

 water-logged while they are in active growth. Thousands of 

 trees were killed in this way during the wet summer of 1903. 

 On the other hand, it is surprising how limited the supply 

 of air to the roots may be without interfering materially with 

 the growth of a tree. In some experiments at Woburn a 

 number of apple-trees were each surrounded by an iron drum, 

 3 feet in diameter, 18 inches deep; when this had been driven 

 down into the soil, the soil within the area enclosed by the 

 drum was covered with a 2-inch layer of cement. Each tree 

 was thus enclosed in a sort of tub and its roots could only 

 obtain such moisture and air as permeated through the stiff 

 clay subsoil 18 inches below the surface. Yet these trees 

 flourished during four years just as well as and even slightly 

 better than similar trees which were not enclosed ; and though 

 afterwards they began to fall behind-hand, owing to the ex- 

 haustion of the limited amount of soil available for their 

 growth, they are still — after thirteen years — fairly healthy trees. 

 Trees planted in towns, often with their roots covered by 

 paving-stones, afford familiar instances of the extent to which 

 they will thrive with a very limited access of air to their roots. 

 One very striking illustration may be noticed just outside 

 St. Pancras Station. The Midland Railway line passed over 

 a burial-ground in which there were some trees with stems 

 up to about a foot in diameter. This burial-ground was done 

 away with about twelve years ago and the ground made up 

 to the level of the railway line by dumping on to it some 

 13 feet of earth and rubbish: the trees were, consequently, 

 buried to this depth, leaving only their heads above ground : 

 yet they have continued to live and are still in a fairly flourishing 



condition. 



Trenching 



The question of trenching or double-digging the soil pre- 

 paratory to planting fruit-trees is one of considerable importance 



