TOXIC EXUDATES AND SOIL TOXINS. 155 



The effects of various methods of culture upon the growth and pro- 

 duction of the apple have been investigated by Green and Ballou 

 (1906). An orchard was divided into four plots, each of which was 

 given different treatment. In the cover-crop plot, the ground was 

 plowed or disked early in the spring, cultivated until late in July, 

 and then sown to a cover-crop. The plot with clean culture was 

 treated similarly, except that cultivation continued through the 

 growing-season and no cover-crop was employed. The sod-culture 

 method consisted in planting the trees directly in sod and in culti- 

 vating a circle of 3 or 4 feet around the tree throughout the season. 

 The grass was cut several times each season and allowed to lie. In 

 the sod-mulch plots, the treatment was the same, except that the 

 circular area was mulched with straw and the cut grass used to main- 

 tain the mulch. Continuous clean culture was abandoned after four 

 seasons, owing to the washing of the soil and the removal of the vege- 

 table matter. The trees made the heaviest and most uniform growth 

 in the sod-mulch plot, in comparison with good growth in the cover- 

 crop plot, and much poorer in the sod-culture one. The average 

 diameter of the trees in the three plots was 10.56, 9.71, and 8.55 

 inches, respectively. The greater growth under sod-mulch was 

 ascribed to the greater supply of food-material under the mulch. 



Dandeno (1909 : 24) assumed from field observations that grain 

 grew better when associated with Canada thistle, and tested this 

 experimentally by planting oats, barley, wheat, buckwheat, and flax 

 separately in pots, as well as with a vigorous underground shoot of 

 Canada thistle in a second series of pots. In another series young 

 elm trees were planted singly in 6-inch pots, and oats grown with 

 them. With the exception of buckwheat, all the plants grew as well 

 or better with the Canada thistle as alone, the stimulating effect 

 being most pronounced at 22 days after planting. On the contrary, 

 the elm tree had an injurious effect, as all species grew more poorly 

 with it. It was suggested that the results were due on the one hand 

 to the excretion of substances that stimulate growth or release plant 

 food, and on the other to the excretion of harmful substances. 



Howard (1910, 1915) has found that grass has a marked effect 

 upon most species of fruit trees at Pusa. The leaves are few, very 

 small, and pale yellow; the leaves and flowers appear much later than 

 normally, and the leaves fall early. Very little new wood is formed 

 and the growth in height is much less. The fruit from trees under 

 grass is smaller, less abundant, as well as less juicy and of poorer 

 flavor. The effect of grass is greater with small trees than with large 

 ones, and Cynodon dactylon is more injurious than Imperata arun- 

 dinacea. This difference in the two grasses seems to be explained by 

 the fact that Cynodon requires more air, and correspondingly reduces 

 the supply to the tree roots. Moreover, the Pusa soil packs to such 



