CHAP, x The Problems of Growth 455 



observation. Pfeffer found in 1873 that the growth of 

 leaves is not thus affected by varying temperature. 



Pfeffer showed further, in 1875, that the opening and 

 closing of flowers at the onset of day and night is due 

 rather to temperature changes, than to the influence of 

 the varying light. He found that the perianth leaves of the 

 crocus opened when the temperature reached 9 C., and 

 continued to expand up to 27 C. ; at 28 C. closing began, 

 but was incomplete till 367 C. was reached; when it was 

 complete. Fall of temperature always caused an open 

 flower to close. Pfeffer held these phenomena to be the 

 accompaniments of growth on alternate sides of a zone of 

 tissue at the base of the perianth leaves. Vines spoke of 

 them as instances of induced epinasty and hyponasty. 



This view was combated many years later, just at the 

 close of the century, by Biirgerstein, who attributed the 

 alteration of the position of the perianth leaves to turgor 

 variations, not necessarily accompanying growth. Farmer 

 confirmed him, having found that in the tulip there exists 

 a layer of active cells on the outer face of the young perianth 

 leaf, running longitudinally down its centre, which shows 

 a variation of turgor as the temperature changes. When 

 this layer is isolated and put into water it curves, the 

 curvature being greatest at the base but still extending 

 up to and including the apex, the cells changing their 

 shape more rapidly than those of the internal layers. 



The influence of temperature on the movement of the 

 lateral leaflets of Hedysarum was examined by Kabsch in 

 1862. He found that the optimum temperature for them is 

 35 C. and that they cease at 22 C. Sachs, the next year, 

 showed that the leaflets of Mimosa gradually lose their 

 motility if kept at 15 C. or if merely exposed to 40 C., the 

 loss taking place more quickly as the temperature of 

 exposure is higher. 



The movements of rotation or circulation were shown 



