872 



SCIENCE 



[N. S. Vol. XXV. No. 648 



not cover the entire lawn area, this cause 

 could not be the only one. The converse 

 effect, i. e., a deleterious effect of grass on 

 trees, was found by the Duke of Bedford and 

 his co-vporkers.' In 1897 they began to notice 

 the peculiar effect produced by grass upon 

 their fruit trees, esjmcially apple and pear 

 trees. The soil on this farm is shallow, 

 eighteen to twenty-four inches of soil over- 

 lying an impervious calcareous subsoil. 



Their first supposition was the removal of 

 plant food, and so they inaugurated experi- 

 ments to determine if this assumption was 

 correct, but all their experiments answered tho 

 question in the negative. They then tried if 

 the removal of water by the grass was the 

 cause, but here again they received a negative 

 answer. They tried the effect of carbon 

 dioxide on the tree roots, thinking this might 

 be given off in such large quantities by the 

 grass as to be harmful. This not proving to 

 be the cause, they tried the effect of the ex- 

 clusion of oxygen, and also of the effect of 

 packing imitating the impervious sod, but in 

 all cases they were baffled, finding no evident 

 effect of any of these factors on the trees. 



Having ruled out all the above-mentioned 

 factors, they found by other experiments that 

 only the most actively growing portions of the 

 tree root system was affected by the grass. 

 A circular sod of a few feet in diameter 

 around the tree had no effect, but as the 

 circle was increased, the tree began showing 

 the detrimental effect, viz., premature falling 

 of the leaves, and entire change of the normal 

 ripe color of the fruit, from green to red, and 

 a dwarfing of the tree. In very many in- 

 stances the trees were killed outright. They 

 also found by excavating the ground around 

 the trees and by removing the root system, 

 that the pernicious effect of the grass was 

 strongly marked even when only one thou- 

 sandth to two thousandths of the root system 

 of the tree was exposed to the action of the 



They finally, after about seven years' work, 

 concluded the pernicious effect of the grass 

 could be due only to some poisonous substance 



• Wobum Experimental Fruit Farm, 3d Kept., 

 1903, and 4th Kept., 1904. 



formed in the soil around the tree roots, leav- 

 ing the question open as to whether these 

 substances were due to direct excretions from 

 the grass or to a changed bacterial action in 

 the soil induced by the presence of the grass. 



Jones and Morse' have described a similar 

 relation existing between the shrubby cinque- 

 foil {Potentilla fructicosa) and the butternut 

 tree (Juglans cinerea), the latter killing the 

 former for an area equal to and often much 

 greater than that of the tree top. Excava- 

 tions showed in every case of the dead or 

 dying cinquefoil that the butternut tree roots 

 were in close physical relation with those of 

 the shrub. Young birch, beech, maple, cherry, 

 apple and pine trees growing among the 

 cinquefoil in the same field had no such influ- 

 ence on the latter. More recently an antag- 

 onism between peach trees and several herba- 

 ceous plants, commonly used as cover crops in 

 orchards, has been reported by Hedrick.' 



In work done in these laboratories. Reed 

 found unquestionable evidence that plants do 

 produce toxic conditions in the medium in 

 which they grow. Agar in which wheat had 

 grown was decidedly toxic to a second crop of 

 wheat. Agar in which corn or cowpeas had 

 grown was scarcely, if at aU, toxic to wheat. 

 Agar in which oats had grown was quite 

 toxic to wheat, but not as toxic as that in 

 which wheat itself had previously grown. Ap- 

 parently excretions from the roots of a given 

 plant, or its near relatives, are more toxic to 

 that species than the excretions from plants 

 belonging to more distantly related species. 



It was decided to try the effect of tree seed- 

 lings on the growth of wheat under control 

 of external factors, and accordingly a number 

 of tree seedlings were dug up in the forest 

 in the early part of June, 1906. The species 

 gathered were pine, tulip, maple, dogwood, 

 and cherry, and varied in height from about 

 15 to 40 cm., care being taken to get the 

 entire root system. These were planted in 

 paraffined wire pots,' using soil already made 



•Rept. Vt. Expt. Sta., 16 (1903), 173-190. 



'Proc. Soc. Eort. 8oi., 1905, 72. 



• This method of pot culture is fully described 

 in Cir. No. 18, Bureau of Soils, U. S. Dept. of 

 Agriculture. 



