1238 



THE AGRICULTURAL NEWS. 



July 25, 1908. 



SCIENCE NOTES. 



A Toxic Substance Excreted by the Roots of Plants. 



A short note giving dojcriptioiis of investigations 

 into ;i to.xic snbstance excreted by the roots of a variety 

 -of plants has lecently been published by Mr. F. 

 Fietchrr. M.A., B.Sc., Deputy Director of Agricnlture, 

 Bombay, as No. 3 of Vol II, Botanical Series, of the 

 Mcmiiirs of the Department of Agriculture in India. 



It has frequently been suggested that plants 

 •excrete (from their roots) some material that is no longer 

 ■of use to them or that are by-products of the 

 process of metabolism. Further it has been held that 

 it is possible that such substances may be injurious 

 to the kinds of vegetation by which they are e.xcreted. 



Ob.servations iiiack^ by Mr. Fletcher some years ago 

 (while in Egypt) lead to the conclusion that certain phe- 

 nomena shown by crojis growing in the field could only be 

 explained by the theory of excretion. This was especially 

 the case with cutton crops in which a grass was allowed to 

 grow as a weed. Poor crops were often observed, and experi- 

 ments showed that neither lack of water or manures, nor 

 faidty aeration was the cause of the poor growth. 



Ob.servations in the field were subsequently resumed in 

 India, and the.se tended very materially t(j strengthen the 

 ■view that .substances injurious to other plants were excreted 

 by the roots of certain conunon crops. This was especially 

 the case with surghum. 



Pints of various crops were grown side by side at the 

 Surat Kx))criment Station, each plot l.ieing sown by means of 

 a diill in such a way that the rows wei'e '2 feet apart, and were 

 pai'allel in all the jilots. Further, several [ilots were left fal- 

 low, as it -was found that plants growing on the border af 

 a fallow gave a return ten times as great as that yielded liy 

 others in the centre of the plot. Also, cotton and snigliuni 

 were sown in alternate rnm-s in the same plot. 



Observations were then made on the following points : — 



(1) The yield of the row of each crop bordering 

 on fallow. 



(2) The yield f'f the row of each crop bordering on 

 a plot bearing another cro]). 



(."5) The yield of a row of each crop in the centre 

 of a ])lot bearing only that crop. 



(4) TIk' yield of a row of cotton when grown with 

 a row of sorghum on each side (at a distance 

 of 2 feet). 



(5) The vield of the row of sorghum wdien grown 

 with a row of cotton on each side (at 

 a distance of 2 feet). 



Under the particnlai- conditions of soil and climate under 

 which the experiments were conducted, the following con- 

 clusions were arrived at, a.s the result of these observations : — 



(1) All plants excrete substances wliicli are toxic both 

 to themselves and to other species. 



(2) The iiuantity of material excreted by the ditierent 

 crops varies when reckoned per unit area of a field sown in 

 the ordinary way. 



(3) The sensitiveness of croi)s to the same (|uantity of 

 the excreted substance varies with the variety of the crop. 



(4) The sub.stance excreted by all cr()[is is probably 

 identical. 



In tlie experiment where cotton and sorghum were grown 

 in alternate rows, very interesting results were obtained. 

 The experiments were made on | acre plots, on land, the crop 



and manurial record '■f wliicli had been known fur the 

 previous nine years. The snrghum yielded nuich better after 

 cotton than after sorghum and the converse also appeared 

 to be the case ; that is, cotton yielded less after cotton 

 than after sorghum, s<i that the whole results obtained would 

 appear to be an example of the benefits of lotation. 



A large number of water cultures with different plants 

 were carried out at the Dharwar Experiment .Station, and 

 solutions of the excreta from their roots were obtained. 

 Seedlings placed with their roots in these solutions sooa 

 withered, and the data given show that all the jilants under 

 experiment withered in the .same order in the ditf'erent solu- 

 tions. All did worst in the excretory solution from Cicer 

 arittinuiii, followed in order by sesamum {Si'xninmn indicum), 

 wheat, cotton, cajanus, and sorghum. The fact of this 

 regularity appears to favoiu- the view put forward that the 

 substa'ices excreted by vai'ious plants are identical, and that 

 the solutions used differ only in concentration and not in kind. 

 Preliminary chemical tests of the toxic substance 

 excreted by the jilants have been made and tend to prove that 

 it is an alkaloid, and that the substances excreted by different 

 crops aie ap])arently identical. The ab.sence of sufficient 

 literature for reference has rendered it, as yet, impossible to 

 compare its reactions with those obtained with any of the 

 known alkaloids. The amount of .substance given out by the 

 roots is not incniisiili'rable. 



The toxic substance is [u-ecipitated by most of the 

 mineral manures in connnon use. This suggests the manner 

 in which many manures may possibly act towards increasing 

 crop yields, and indicates that the .supply of so many units 

 of .some fertilizing substance may not be the last word in 

 correctly interpreting the effects of manures. Further, the 

 precipitation of the .substance by tannic acid raises many 

 interesting points in connexion with the manurial value of 

 leaves of ditferent varieties of trees both as manure and a.s 

 mulch. In this connexion, the author of this note states that 

 in spire gardens and rice lields in C'anara, leaves (containing 

 tannic acid) are systematically used as manure, and that the 

 cultivator's opinion as to the manurial value of the leaves of 

 any variety of trce'appeared to correspond with the amount 

 of tannic acid which analysis showed to be contained in these 

 leaves. That it is not the ash constituents of these leaves 

 which produce the manurial etfect is obvious from the fact 

 that if the leaves be burnt and the ashes applied to peppers — 

 one of the spices to wliich the leaf manure is api]lied — the 

 pepiier plants are killed. Similarly, neither irrigation nor 

 farmyard manure serves the purpose of the leaves. The 

 latter, therefore, serve neither for the storage and regulation 

 of water nor as a supply of nitrogen. 



The isolation of this toxic substance in solution, 

 anil the experimental ])roof of its presence and etfect.s 

 in the case of a large number of plants, are of consider- 

 able interest, and constitute another advance in scientific 

 agriculture. Further experiments with other plants, 

 and in ditfereul parts of the world will be looked for 

 with interest. The etfect of the result of this inquiry 

 on the (piestion of rotation of crops is obvious. Each 

 crop, through the excretion of this toxic substance, 

 fouls the soil for a crop of the same variety, whose roots 

 will occupy the same layer of soil as the previous crop 

 more than for a crop who.se roots spread in another layer.* 



