306 SCIENCE AND FRUIT GROWING 



pumice stone between the trays and the earth beneath probably 

 produces some reduction in the toxic effect (XVII). 



These results are quite in accordance with those obtained in 

 a chemical examination, now in progress, of the properties of the 

 aqueous extracts from soils in which plants have been growing, 

 for it has been found that such extracts become materially 

 modified in the course of only a few hours. 



The conversion of the product of plant-growth from a harmful 

 to a beneficial substance may result in the proximity of grass to 

 trees proving actually beneficial to the latter. Standard apples 

 were planted in land which had been grassed some years pre- 

 viously, this grass being replaced over the tree-roots in the case 

 :of one plot, and either removed entirely, or replaced up to a 

 certain distance from the stems, in others. The records of these 

 trees during the succeeding years, as given by the annual growth 

 measurements, are of considerable interest. 



Relative growth during 

 1910. 1911. 1912. 1913. 1914. 1915. 1916. 1917. 



1. Tilled ground . . 100 100 100 100 100 100 100 100 



2. Grass up to stems . 76 30 6 5 n 7 20 



3. Grass to ij ft. from 



stems .... 132 145 78 61 90 77 79 



4. Grass to 3 ft. from 



stems .... 104 94 82 61 88 63 103 



5. Gradually grassed . 101 38 12 10 



The effect of complete grassing is to reduce the vigour of the 

 tree in three years to an average of 10 per cent, of that of trees 

 in tilled ground (2 compared with i), whilst in both cases where a 

 space has been kept open round the trees (3 and 4) the reduction, 

 after the first two years, has been only to about 80 percent. : but 

 during these first two years, whilst the trees in the larger open 

 areas (4) were practically unaffected by the grass, those in the 

 smaller areas (3) were actually benefited by it, to the extent of 

 nearly 40 per cent. : the toxin formed by the grass had evidently 

 had time to become changed into food-material before reaching 

 the tree-roots, and it was only after these had extended so as to 

 be actually within the grassed area that the effect of this toxin 

 was felt. 



Another set of trees in this series the last of those entered 

 above affords information as to the comparative effect of 

 grassing trees at once, and of allowing the grass to establish 

 itself gradually : in the latter case there was no effect during 

 the first season, and that produced in subsequent years resulted, 



