TIME OF SOWING 261 



spring and earlier in autumn if they be followed in northward, east- 

 ward or vertical directions. The law states that the variation in 

 time is at the rate of four days for each degree of latitude, for each 

 five degrees of longitude and for each 400 feet in altitude. The 

 hypothesis has a purely empirical basis, and was originally founded 

 upon data afforded by certain beetles destructive to forests. 

 Subsequent studies by Hopkins (1919) and other observers have 

 led to its application to other biological phenomena, the most 

 important being in relation to the time of incidence of the autumn 

 generation of the Hessian fly (Mayetiola destructor). In the case 

 of this insect time of sowing of wheat has a very definite bearing 

 in relation to subsequent attack. The method is to sow wheat 

 sufficiently late, so that it will appear above ground in the 

 susceptible stage, at a time when the second generation of the 

 Hessian fly has died out. At the same time the crop must be 

 sown early enough to have attained sufficient growth to enable it 

 to withstand the winter. Based on the bioclimatic law, the 

 critical dates for sowing range from about September 10th in the 

 north-east to October 25th in the south-west. It has been found 

 that the actual emergence dates of the insect, taken over a very 

 wide area, all appear to fall within one week of the calculated 

 time under the bioclimatic law. Flint and Larrimer (1928) have 

 examined the problem in some detail with respect to the State of 

 Illinois. They conclude that under normal weather conditions 

 the relatively " fly-free " date is September 15th in the northern- 

 most counties in the state, and October 9th for the southernmost. 

 They present experimental data which show, without exception, 

 that wheat heavily infested with Hessian fly gives appreciably 

 lower yields than wheat grown during the same year, and in the 

 same field, but sown later according to the law enunciated. 



A " law " of so comprehensive a character has its limitations 

 and is based upon climate, and not upon weather. Due allowance 

 has to be made for local fluctuations of the latter, and exceptional 

 seasons. West of the Rocky Mountains it breaks down, and it 

 does not appear to hold good for the conditions prevailing over 

 continental Europe. 



Crop Nutrition and Soil Conditions. The problem of regulating 



