304 SCIENCE AND FRUIT GROWING 



affected by it, and, so long as the tree has not become hide- 

 bound and stunted beyond recall, it is only necessary to remove 

 the surface growth in order to restore it to vigour. The effect 

 of weeds as a surface crop is certainly less than that of grass, 

 and Fig. 43, A and B, illustrate the effect which weeds have on a 

 tree when they are allowed to occupy the ground for only one 

 season after the planting. (In the particular instance shown 

 the only one now available the effect of the weeds was increased 

 by an absence of cutting back.) After the first season, the 

 weeds were removed, and the photograph reproduced in Fig. 43, C, 

 shows how complete the recovery was after two years ; indeed, 

 the recovery commenced immediately; in the season following 

 the removal of the weeds, the deficit in vigour of the trees 

 amounted to only 10 per cent., whereas in the previous season, 

 before the removal of the weeds, the deficiency had been 56 per 

 cent. (I, 162; II, 90). 



Another instance in point has been afforded by the increase in 

 the vigour of trees when grown in soil from previously grassed, 

 instead of from tilled, ground (p. 288), and in that case only 

 two or three months had elapsed between the removal of the 

 soil and the growth of the trees, so that the disappearance of 

 the toxic property must have occurred in that interval. Similarly, 

 when plants are grown in soil which has been moderately heated, 

 although the amount of toxin present is probably much greater 

 than that produced by merely growing plants in it, the presence 

 of the toxin becomes unrecognisable after a few weeks (p. 250). 

 But the destruction of the toxin, at any rate when in the dissolved 

 condition, appears to be a matter of hours, rather than of weeks 

 or months, for when, in the pot experiments, the leachings 

 from the surface crop in the trays, instead of being allowed to 

 run directly on to the plant growing beneath, are collected and 

 not applied to it till some hours afterwards, they are found to 

 have lost their toxic property, and, indeed, to have become 

 slightly beneficial. In two series of experiments where the 

 surface crop was grass, and the plants in the pots were apple 

 trees, the average increase in vigour of the trees watered with 

 aerated leachings was 18 per cent., whereas there was a decrease 

 in their vigour amounting to 40 per cent, when the leachings 

 reached the trees without aeration (XIII, 86). In other 

 experiments with grass as the surface crop and tobacco as the 

 plant in the pots, the aerated leachings gave an increase of 

 7 per cent., against a decrease of 22 per cent, with the unae' rated 

 leachings. Even the interposition of a layer of two inches of 



