YoL. VII. No. 1(5!S. 



THE AGRICULTUKAL NEWS. 



315 



SCIENCE NOTES. 



Excretion of Toxic Substances by the 

 Roots of Plants. 



The following review on the subject of the 

 excretion of toxic substances by the roots of plants 

 appeared in Nature of August 27 last, and should be of 

 interest, since it deals with a (_|uestion that has of late 

 received a considerable amount of attention in the 

 United States, and by scientific experimenters in other 

 countries. A summai-ized account of the work in 

 connexion with this subject of Mr. F. Fletcher, J\I.A., 

 B.Sc, recently Deputy Directorof Agriculture, Bombay, 

 was given in the A</ricultural Nevs (Vol. VII, p. 

 238) : — 



The idea formulated a century ago by de (.'andolle that 

 plant roots excrete toxic substances has recently been very 

 much pushed forward by the American l>ureau of Soils to 

 explain the effects of fertilizers and the advantage of a rotation 

 of crops. The American method (rf 'experiment is to grow 

 .seedlings in water cultnie for a few days, and measure the 

 amount of transpiration, which is considered to be an index 

 of the amount of growth. The seedlings are then removed 

 and replaced by a second batch, without changing the water ; 

 the rate of transpiration is found to be diminished, showing 

 (it is stated) that a toxic body excreted by the roots of the 

 first batch is adversely affecting the .second. Kurthei, seedlings 

 grown in an acjueous extract of certain poor soils are found to 

 transpire less water tlian others grown in distilled water, and 

 it is concluded that these soils contain .sonic toxic material, 

 presumably excreted by plants. The toxic body is, however, 

 jirecipitated on addition of charcoal; ferric hydrate, and 

 .solutions of various manures ; and the Bureau of Soils argues 

 that the function of fertilizers, in some cases at any rate, is 

 not to feed the plant, but to precipitate the toxin excreted by 

 previous plants. Rotations of crops are of advantage, because 

 the toxin (poisonous material) excreta by one plant is not 

 necessarily harmful to plants of a different order. 



It cannot be .said that any very convincing evidence 

 is offered in support of this view by the woricers in the 

 United iStates, and the assumption that transpiration is 

 a measure of plant gmwth is not liorne out by any of the 

 figures tpujted. 



Another weak point is that the experiments are made 

 with seedlings, and last only a feW'flays instead of being 

 carried on to the end of the j)lant's life. The nutrition of 

 the seedling is not the same as that of the plant, and even if 

 it were demonstrated that excretion from seedling roots took 

 place, it would not follow that there was a siniilitr excretifm 

 from the roots of fully grown plants. 



In the last Jiiilli'tln from the Soil Bureau (\o. 48) an 

 account is given of more than l.S.OOO pot trials with soils 

 frolin different parts of the United States. The results show, 

 as might be expected, that addition of manures increases the 

 crop, and that each ULanurial substance exerts a specific effect 

 which is not shown by an} other ; with this statement every- 

 one would agree. The further cimclusion is drawn that the 

 character of fertilizer required depends more upon local 

 conditions and practices (such as the crops grown, etc.) than 

 on the type of soil or the geological formation to which it 

 belongs, so that the fertilizers required for the same ty])e 

 •of soil occurring in different localities usually vary more than 

 those required for very difTerent types when in the same 

 locality and subjected to similar environment. If this 



generalization turned out to be true, it would be more easy 

 to reconcile it with the view that the chief use of manures is 

 to precipitate the poisonous matter excreted by the roots 

 of plants than with the ordinarily accepted idea that 

 manures are applied in order to pro\ide the neccessary plant 

 food. An examination of the tables given, howe\'cr, dfics not 

 show that there is any proof. 



Mr. Fletcher's work has been partly on the above lines. 

 He obtained a 'solution of excreta ' by growing plants in 

 water culture, and then used this solution as a medium for 

 plant growth. It [ji-oved to be toxic, and the conclusion is 

 drawn that the plant ficsj, used excreted some poisonous 

 body. The experiment, however, is not a very good one. 

 It is well known by those who have worked with water 

 cultures that bacterial decompositions are liable to take place 

 in the solution, producing substan('es injuri<ius to plants ; 

 pre<'autions always have to be taken to prevent development 

 of bacteria. It does not appear that any such ]irecautions 

 were taken by Mr. Fletcher ; indeed, the conditions under 

 which he worked seem to have been favouiable to bacterial 

 development ; well water was used and the ' solution of 

 excreta ' was allowed to eva])orate at ordinary temperature 

 until sufficiently concentrated for the second part of the 

 experiment. There is no eviilence that the toxic substance 

 Wiis excreted by the plant : it might erpially well have been 

 a bacterial product. 



In another set of exj)eriments, crops were grown in rows 

 side b}' side, and three lots of measurements were taken : — 

 (1) the yield on the outside row, bordering on the bare 

 ground ; (2) the yield in the middle row ; ^3) the yield in 

 a row bordering on another crop. The first is the highest, 

 th'- second shows the effect of the plant on others of the 

 same kind, and the third shows the effect on others of a differ- 

 ent kind. The falling off in yield in the second and third 

 cases is regarded liy Mr. Fletcher as proof of a toxic excre- 

 tion. Such a falling off is generally exjilained as due to lack of 

 water ov food, and no satisfactory evidence is adduced against 

 this view ; indeed, Mr. Fletcher states that the reductions in 

 crop are less marked under a more evenly distributed rainfall. 



SISAL HEMP IN GERMAN EAST 

 AFRICA. 



Attention has been given to si.sal hemp cultivation in 

 (icrman East Africa .since''1893, and the increasing value of 

 the industry is evident from tlie fact that the exports of fibre 

 during 1906 were valuer! at £66,900, as compared with 

 £43,900 in 190.5 and £28,300 in 1904. 



The industry was started by the importation of a small 

 number of plants from Florida fifteen years ago. Machinery 

 for the extraction of the fibre was imported in 1899, and the 

 first exports were made in 1900. In 1904, the number of plants 

 dealt with was 1,300,000, which yielded 624 tons of fibre, 

 this being equivalent to an average yield of 17 oz. of fibre 

 pel' plant. In the following year the average return of fibre 

 I'ose to 2.5 oz. per plant, but in 1906 it dropped to 22 oz. 

 It is calculated that if 80() plants per acre are grown, an 

 annual crop of 900 to 1,200 lb. per acre .should be obtained. 



The machine einplo3ed for the extraction of the fibre i.s 

 the one used in Yucatan, and is known as the " MoUa.' It 

 costs £650, is capable of irealing with about 100,000 leaves 

 in ten hours, and requires 48 horse power to drive it. In 

 order to keep this machine sufficiently employed, a plantation 

 of at least 600,000 plants ■ is necessary. This (allowing 

 distances of 3-J x 8^ feet between the plants) represents an 

 area of about 310 acres. 



