2] UPON THE DIRECTION OF GROWTH 441 



in the region of the dark heat rays, we find an effect still pro- 

 duced. This effect of dark heat rays will be referred to again 

 in the next chapter. 



The Responding Region. We have already seen that the 

 curvature begins a short way below the tip of the stem of the 

 seedling. Further study shows that this region of first curva- 

 ture is also that of maximum growth. The response does not, 

 however, end here, but passes basalwards even after the seed- 

 ling is transferred to the dark. Also in roots, the region of 

 maximum negative curvature is that of most rapid growth 



(MULLER, '76). 



The Perceptive Region. In most cases the region of re- 

 sponse is also that of perception. But DARWIN ('81) found 

 that in some organs this is not the case. When, for example, 

 the phototropic cotyledons of the seedlings of grasses and 

 grains were deprived of their tips for a distance of 2.5 to 4 

 mm., they exhibited no phototropism ; but when only 1.3 

 mm. of the tip was cut off, the curving occurred, although 

 in diminished degree. Again, when the tips of some of the 

 cot}'ledons were covered with opaque caps made of glass 

 thickly painted with India ink, while others were covered with 

 transparent glass, the first lot remained straight or nearly so, 

 whereas the second curved normally. From such results DAR- 

 WIN concluded that the tip of the cotyledon is the chief per- 

 ceptive region. That it is not the only perceptive part, even 

 in cotyledons, follows from the observations of ROTHERT ('94), 

 who finds that a slight curvature succeeds the illumination of 

 the basal part alone of the cotyledon. In some seedlings of 

 dicotyledons, indeed, the perceptive region exists nearly equally 

 developed along the whole stem. 



In those cases where the tip of the plant is alone percep- 

 tive there must be the transmission of an impulse from the per- 

 ceptive to the bending region. The rate of this transmission 

 is variable ; in favorable cases it is about 2 cm. per hour 

 (ROTHERT, '94, p. 209). If we define "irritation" or "stimu- 

 lation " as the condition of the protoplasm immediately ante- 

 cedent to its response, as the chemical transformation lying 

 behind the visible result, then, since in these plants the 

 response occurs some distance from the perceptive region, we 



