202 



PERIODICITY OF GROWTH 



manner as arc nyctitropic movements. These are partly the result of 

 corresponding accelerations and retardations of growth, and are partly due 

 to variations of turgor, but do not involve an inherited tendency to 

 periodicity. They cease gradually in fact under continuous illumination, 

 whereas autonomic movements having a periodic rhythm of from one to 

 a few hours are exhibited to a marked extent by some plants, but to a less 

 degree by others. If such a plant is now placed in darkness, a nyctitropic 



o p.m. 12 p.m. 6a.m. 12 a.m. 6 p m. 12p.m. Gam. 12a.m. (jp.m. 12p.m. 



23 May 



24 May 



25 May 



Fid. 30. Daily curve ot growth of a stem of Dahlia variabilis (after Sachs). Tin- growth was registered by a 

 lever magnifying twelve times, and each mm. growth in length is represented by a 2mm. ordinate The curve is is 

 constructed from hourly, that marked ^s from tri-hourly observations. The curve /" gives the temperatures, which 

 begin at 139 R. Each square has a side of 4 mm. ; each Reaumur degree co\ers 2 cm. (5 squares). The dark 

 areas represent the night periods. 



movement ensues which is repeated a few times with gradually decreasing 

 intensity. Under normal conditions this after-effect, together with the 

 repeated daily stimulation, gradually brings the periodic daily movements 

 into play with their full amplitude. In darkness, however, the cumulatively 

 induced daily periodicity gradually disappears in from a few days to a fort- 

 night according to the plant examined. 



Similar relationships probably apply to the induced periodicity of 



