110 SCIENCE AND FRUIT GROWING 



uniform, and the average results, as shown by the relative vigour 

 of the trees, has been 



Ground tilled. Green-manuring. Permanent crop. 



ioo 84 45 



Thus, the green-manuring has, as compared with the tilled 

 ground, done nothing but harm, though this harm amounts to 

 only one-third or one-quarter as much as that done by the 

 permanent surface crop. 1 



The reduction in the deleterious effect of the surface crop in 

 this case depends, as indicated above, on the growth of the 

 surface crop not being coincident with the period of greatest 

 activity of the trees, and to obtain more definite information 

 on this point, the rate of growth of trees throughout the season 

 was determined in two instances 1906 and 1918 by making 

 weekly measurements of the length of new wood formed by 

 apple trees. In the first series of experiments, 48 young trees 

 were taken, representing three different varieties, half of them 

 having been established in the ground for one year, and half 

 having been planted in the previous winter. They all exhibited 

 substantially the same rate of growth, which is portrayed by the 

 upper curve in Fig. 16. Growth had ceased by about the 1st of 

 August, and 90 per cent, of the total growth had occurred by 

 the second week in July (XIII, 58). Below the growth-curve is 

 given the rainfall for the seven -days preceding each measure- 

 ment, and apparently, in this case, there is no direct connection 

 between the two : the rainfall was small and fairly uniform 

 throughout, except for the week preceding July 2, and the 

 heavy fall then evoked no response in the growth of the trees. 



In the second series, 1918, measurements were made on trees 

 of Gascoyne's Scarlet Seedling, eleven years of age, and the 

 values are also depicted in Fig. 16, with the weekly rainfall 

 below. Here the results are very different, but are easily 

 explicable by differences in the character of the two seasons. 

 In 1918, a spell of dry weather throughout the greater part of 

 June had brought active growth prematurely to a standstill by 

 June 17, and then, when a heavy fall took place after the first 

 week in July, the growing energy of the trees not having been 

 exhausted, active growth was resumed. There seems also to 

 have been some response during this season to the increased 

 rainfall of May 20-27. 



1 Green-manuring (" cover crops ") is a practice often successfully 

 employed in America, but it has been found inferior to clean cultivation 

 in Illinois. T. J. Burrill and J. C. Blair, Illinois Agric. Expt. Sta. Bull., 52, 



