INITIATION OF FLOWERS 



349 



[Chap. XXXI 



and the formation of tubers, are also more variable under these photo- 

 periods, because the influence of other environmental factors on them is 

 relatively more pronounced. 



Plants may also be classified as long-day and short-day types with 

 respect to tuber and bulb formation, root growth, stem growth (Figs. 

 148 and 149 ) , deciduous and evergreen habit, kinds of flowers, and yield 

 of crop plants. 



Tubers ordinarily develop at the ends of rhizomes of Jerusalem arti- 

 choke during the short days of autumn. They will also develop during 



Fig. 150. Photographs of the roots, underground stems, and bases of the aerial 

 stems of Jerusalem artichokes (Helianthiis tuberosus) . The plants on the left were 

 exposed to the long days of summer, those in the center and on the right were 

 covered with a dark cloth from 4:30 to 9 a.m. each day, but only the tips of 

 those on the right were covered. The elongated rootstocks of the plants on the 

 left, and the thickened tubers of the others are evident. Photo by P. W. Zimmer- 

 man and A. E. Hitchcock, Boyce Thompson Institute. 



midsummer if the photoperiod is artificially shortened each day by plac- 

 ing the plants in a dark room or by covering the leaves and stems above- 

 ground with a black cloth. Darkening of the stem tips alone is sufficient 

 to initiate tuber formation in artichokes ( Fig. 150 ) . 



Girdling. In certain habitats young apple trees and other orchard trees 

 may remain vegetative for several years before they bear flowers and 



