144 LIGHT, VEGETATION AND CHLOROPHYLL 



variety; other varieties may react differently. When potato 

 seeds are selected, the daylength should be considered with 

 the planting date and the latitude of the district in which the 

 crop will be cultivated. 



The form of dahlia roots, tuberous or fibrous, also depends 

 on the length of the day; the tubers containing stored food 

 appear only when the day becomes short. The Silverskin 

 species of onion, if it is kept with a rather short daylength of 

 ten hours, remains more than twelve months without forming 

 either bulb or flower. On the other hand, if the day lengthens, 

 as it does naturally in the summer in our latitudes, the bulb 

 forms and grows and flowers are produced, the plant then 

 becoming dormant and resuming later, during the short days 

 of autumn, a new activity of growth. 



As a general rule, the formation of tubers is checked 

 when the days are too long and the formation of bulbs when 

 the days are too short. But here again, diff*erent species and 

 varieties behave in a different way. 



The distinction between annuals, biennials and perennials 

 is generally a consequence of the seasonal variation of the 

 daylength. This would be shown by the diff'erence in vegetation 

 in diff'erent latitudes if the temperature and humidity con- 

 ditions were the same, and is shown by experiments in the 

 daylength modified artificially. 



The light conditions which promote flowering accelerate 

 the life-cycle of annual plants and cause them to die earher. 

 But if they are brought into conditions of daylength which 

 prevent flowering, they resume the course of their growth. 

 Thus, with the soya bean, two cycles of development and 

 growth can be made to alternate in four months. 



Ligneous plants, hke trees, are greatly influenced by the 

 length of the day. Salix lanata, for example, which thrives at 

 high latitudes, with very long summer days, does not grow well 

 at Leningrad, where the days are shorter, until the beginning 

 of the winter. 



On the other hand, Robinia pseudoacacia, originating from 

 lower latitudes, with shorter summer days, is overtaken by 



