DAILY PERIODICITY OF GROWTH. 



559 



of growth occurs; or, in other words, the daily period indicated, the increase 

 of growth during the night, and the progressive diminution during the day, cannot 

 be regarded as a result of the daily variation in temperature. This fact, however, 

 comes out sharp and clear only when the hourly variations of temperature are by 

 tenths of degrees, and only amount to a few whole degrees in the course of the day : 

 where the variations in temperature are large, the movement of the curve of growth 

 follows in the main the ups and downs of the curve of temperature, and this is the 

 chief reason why the daily period, which is independent of temperature, was either not 



"y 



*r 



Fig. 349.— The curve i z represents the hourly gfrowth, and the curve 3« the growth every three hours of a shoot-: 

 of Dahlia. The curve t indicates the temperature. On the abscissa, 6a denotes 6p.m. ; 12 « ■= midnight ; 6/- 6 a.i 

 and 12?« = noon. The dark areas correspond to the night, the light ones to the day. 



recognised at all or only indistinctly by the earlier observer.s, because their plants 

 were exposed in the open to variations of temperature which were for the most 

 part very large. 



On the other hand, however, the assumption suggests itself that the daily 

 period of growth, so far as it is independent of variations of temperature, may 

 be chiefly induced by the daily alternation of illumination ; for that the light causes 

 a retardation of the growth of stems and green leaves may be proved in various ways, 

 and since the plant remains exposed to light from sunrise till evening, its retarding 

 effects would assert their influence more and more in the course of several days, 

 i.e. the hourly growths would become smaller anil smaller. When, however, the 



