LIGHT AND VEGETATION 145 



the winter and destroyed by frost before it enters into its 

 dormant period. Dormancy can even be retarded, with the 

 same results, by supplementary Hghting at the top of the tree. 



The Economic Importance of Photoperiodism 



The influence of the daylength, natural or artificially 

 modified, therefore has considerable practical importance. 

 Since the daylength and its seasonal variations depend on 

 latitude, the selection of crops ought to take into account 

 the effects of photoperiodism scientifically studied and com- 

 bined with other effects which vary with the geographical 

 situation, such as temperature, rainfall, humidity of the 

 atmosphere and nebulosity. 



One example is wheat. Australian varieties are early and 

 do not do well in England; Enghsh varieties are late and give 

 poor results in AustraUa. These different varieites have been 

 tried with diff'erent daylengths, modified artificially, and it has 

 been shown that the length of the summer day, which is 

 shorter in Australia (latitude 38° at Melbourne) than in 

 England (latitide 52°), is at least one of the causes of these 

 failures. 



