TOXIC ACTION OF ONE CROP ON ANOTHER 293 



baleful influence of a surface crop were : mustard, tobacco, 

 tomatoes, barley, clover, two varieties of grasses, Brussels sprouts, 

 pears, plums, cherries, many varieties of apples, beech, oak, ash, 

 larch, spruce and firs ; whilst those found capable of affecting 

 other plants were : mustard, tobacco, tomatoes, clover, apple 

 trees, and sixteen varieties of grasses. In no case have negative 

 results been obtained, either as to the susceptibility of a plant 

 to the influence of surface growth, or to the deleterious effect of a 

 plant when grown as a surface crop/ It would be premature, 

 however, to conclude that no such exceptions exist, and, as will 

 be seen more clearly from what follows, the existence of such 

 exceptions, extending even to cases where the action of a 

 surface crop may be beneficial, are quite conceivable. 



Some illustrations of the results obtained are shown in Figs. 40 

 and 41, and they include one wherein the action has been very 

 great, for the clover, grown there as a surface crop, has so 

 stunted the tobacco in the pot, that the latter has not developed 

 sufficiently to make its appearance above the rim of the latter. 



Very great variations were observed as to the extent of the 

 effect in different cases : taking cases where the same plant was 

 grown in the pots, but different plants in the trays, the vigour 

 of the former, as determined by the weight of the plants when 

 dried after growth had finished, was found to have been reduced 

 from 100 to the following values 



Plant in Plants in Relative vigour of 



the pots. the trays. plant in the pots. 



9 series Mustard Various (i to 77 =) 33 



6 ,, Tobacco (3 to 78 =) 47 



3 ,, Tomatoes ,, (33 to 54 =) 44 



Equally great variations were observed when comparison was 

 made of cases where the plant in the trays was the same, but 

 those in the pots different, a result in accordance with the differ- 

 ence observed in the susceptibility of different plants to the 

 toxin in heated soil (p. 254). 



Plants in Plant in Relative vigour of the 



the pots. the trays. plants in the pots. 



10 series Various Grass (24 to 85 =) 53 



6 ,, ,, Mustard (i to 40 =) 21 



3 Clover (3 to 77 =) 32 



and, even in cases where there was no alteration in the nature 



