CHAPTER XXVIII 



THE TOXIC ACTION OF ONE CROP ON ANOTHER 

 (Reports, XIV, 46 and Appendix ; XVII l ) 



IT has been seen how the results obtained at Woburn, first by 

 the exclusion seriatim of other possible explanations, and then 

 by the presentation of more positive evidence, led to the con- 

 clusion that the effect of grass on trees must be due to toxic 

 action. Before, however, entering more fully into this ques- 

 tion, it will be well to give some account of experiments which 

 showed that the effect of grass on trees was but one special 

 instance of the general effect of one crop on another. 



The investigation of the growth of other plants under a surface 

 crop was conducted chiefly by means of pot experiments, and 

 some modification was necessary in the construction of the pots 

 used for trees. They measured internally 17^ inches in diameter, 

 and 12 inches in depth, and the tray, or collar, containing the 

 surface crop was made of earthenware, instead of sheet iron : 

 the central opening in it was 5j inches in diameter, so as to leave 

 sufficient space for the growth of the plants in the pots through it. 

 Fig. 39, B, p. 292, will make the construction clear. The pots held 

 about 70 Ibs. of earth, and the trays 20 Ibs. The plants were 

 raised either from seeds, or by transplanting seedlings, and these 

 were thinned out by removing the more weakly ones, till 

 (generally) two or three plants only were left. The plants in the 

 trays were raised a few weeks ahead of those in the pots, and 

 suitable means were adopted for preventing them from over- 

 shadowing those in the pots. As in the experiments with 

 trees, all the water supplied was added to the trays, and the 

 perforations in these those in the check experiments included 

 were covered by a sheet of fine copper gauze. 



In this way, and by other experiments in the field, it was 

 found that the action of a surface crop is a general one : the 

 plants which have so far been found to be susceptible to the 



1 The majority of the results dealt with in this and the following chapter 

 are described in the XVIIth Report, which at the present time of writing 

 is still unpublished. 



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