January 20, 192 1] 



NATURE 



667 



dioxide water." They were of a purely negative 

 character, and in no way indicated that the toxic 

 action of the surface crop was modified by the 

 differential treatment. .Also bearing on this point 

 are observations, direct and indirect, in several 

 of the numerous series of experiments made which 

 tend to show that the toxin after "oxidation" 

 actually serves as a nutrient to plants exposed to 

 its action. As to the nature of this substance 

 and the manner in which it is communicated by 

 the roots to the soil, no positive views are put 

 forward, but Mr. Pickering held that there was 

 no reason to assume that it is excreted by the 

 roots, and he was inclined to attribute its origin to 

 tht debris which roots furnish to the medium in 

 which they grow. 



The later phases of Mr. Pickering's in- 

 vestigations were mainly directed towards 

 proving that the grass injury to fruit trees 

 is only a particular , case of the action 

 of one growing crop on another. By means 

 of experiments on lines similar to those 

 already reviewed, he showed for a wide variety 

 of plants a corresf>onding toxic effect, and, 

 moreover, demonstrated that the action is recip- 

 rocal. It is not confined to plants of a different 

 kind ; it is at least equally marked when the asso- 

 ciated plants are similar. Further, it follows that 

 the individual plant tends to restrict its own 

 ujrowth through the toxin which it produces so 

 long as that remains "unoxidiscd" in the imme- 

 diate range of the root system. 



A new light is thus thrown on the question of 

 lil drainage, it being evident that soil conditions 

 l;icilitating rapid removal of the toxin or its oxida- 

 tion must tend to promote healthier growth, pro- 

 \ idcd that food supplies do not escape. DifTer- 

 I noes in efficiency of drainage may accordingly 

 l>e held to account for the divergences of grass 

 effect on trees which have been recorded in 

 various localities. 



With every plant exerting a direct toxic effect 

 on all others within its range, the phenomena of 

 •oot-interferencc do not merely represent the out- 

 ome of competition for food supplies. Mr. 

 i'ickering gave particular attention to the 

 ispect of the case where similar plants are grown 

 nassed together, and his observations are intcrest- 

 rig not only in them.sclvcs, but also because of 

 considerations raised by them of economic signifi- 

 cance as applied to agricultural and horticultural 

 crops. He found that where the mass of soil 

 available is below a certain limit, the total amount 

 of plant growth produced is independent of the 

 number of plants present. This holds whether the 

 individual plants are grown with their roots in 

 separate compartments of the soil-containing re- 

 ceptacle, so that root-interference is eliminated, 

 or in a similar-sized vessel without divisions which 

 permits unrestricted root-interference ; but it applies 

 only to cases where the plants arc of the same 

 age. When some are younger than others, the 

 latter grow more vigorously at the expense of the 

 former in the undivided containers, but the total 



NO. 2673, VOL. 106] 



combined growth falls considerably short of the 

 amount which the mass of soil is capable of pro- 

 ducing with plants of equal age. The latter point 

 is, however, reached when plants of unequal age 

 are grown in the divided vessels with no root- 

 interference. In such cases the toxic action of the 

 older plant on the younger was thus definitely 

 illustrated where root-interference was possible 

 and the available growth standard was not nearly 

 reached, indicating that the plants were prevented 

 from utilising all the nutrient present. 



The question of range of root-interference has 

 become latterly of considerably increased import- 

 ance in fruit culture. The present high cost of 

 labour has driven fruit-growers to consider how 

 to reduce the expenses of cultivation of their 

 orchards and plantations. In some cases this has 

 been attempted by grassing them down, but the 

 trees generally suffer so severely that this method 

 can be practised successfully only where local con- 

 ditions minimise grass influence. Cover cropping, 

 followed by the ploughing in of the cover crop 

 to serve as green manure, is also receiving atten- 

 tion, particularly on account of the increasing 

 difficulty of obtaining adequate supplies of farm- 

 yard and stable manure. Where the trees are 

 grown on strong and relatively deep-rooting types 

 of root-stocks, such as the stronger free stocks 

 for apples, the toxic effect of the cover crop or 

 grass may be relatively negligible, provided that 

 the soil is deep and well-drained. With super- 

 ficial rooting types of root-stock, such as the 

 Paradise or dwarfing stocks for apples, however, 

 the toxic influence of the surface crop will cer- 

 tainly be more strongly marked, and may be suffi- 

 ciently pronounced to render grassing or cover 

 cropping other than for periods of short dura- 

 tion not only dangerous in many cases, but 

 entirely impracticable in some. The use of the 

 latter forms of root-stock is accordingly open to 

 objection on this score, and, in so far as the 

 grower may be debarred from these alternatives 

 in the treatment of his plantations, and confined 

 to clean cultivation, the modern English policy 

 of advocacy of dwarfing stocks may perhaps prove 

 to be misguided. 



The recent announcement of the closing down of 

 the Woburn Experimental Fruit Farm, followed 

 so closely by that of the regretted death of its 

 distinguished director, marks the end of the most 

 important systematic attempt to grapple with the 

 problems of fruit culture since the days of Thomas 

 Andrew Knight. Hotly disputed as some of the 

 conclusions reached there have been, the general 

 value of its contributions to pomological science 

 stands, nevertheless, beyond question. Although 

 further development of the subject in this country 

 must now be left to other stations, the influence of 

 Woburn will persist, and future investigators will 

 find their work materially simplified not only by 

 the constructive results achieved there, but also by 

 the illustrations afforded of the pitfalls to which 

 field experiments in pomology arc liable. 



B. T. P. B. 



