134 



GROWTH HORMONES IN PLANTS 



where k can be any desired constant, the sensitivity to light may 

 be computed. A comparison of the experiments of Lange and of 

 Sierp and Seybold is given in Table 4, where the values obtained 



Table 4. — Relative Sensitivity to Light in Different Zones of the 



AVENA CoLEOPTILE 



by the latter authors have been recomputed on the basis of Lange's 

 formula. Even though the two series of numbers deviate con- 

 considerably from each other in the second and third half- 

 millimeter zones, the results are essentially in agreement. 



Since Lange worked with zones as narrow as 50 microns, he has 

 given us very exact information concerning the distribution of 

 sensitivity to light in the extreme tip region. It is clear that the 

 uppermost zone, which is only about the width of a single cell, 

 is the point most sensitive to light (see Table 5). Phototropic 



Tabi,e 5. — Relative Sensitivity to Light in the Upper Half Millimeter 



OF the Tip of the Avena Coleoptile 



Zones, Microns Sensitivity 



0- 50 7,600 



50-100 6,422 



100-200 5,665 



200-300 4,106 



300-400 3,046 



perception diminishes rapidly when the tip is removed (Koch, 

 1934). The reduction in sensitivity becomes more marked with 

 the removal of tip pieces up to 1 mm. in length, just as has been 

 observed by darkening cUfferent zones. With still further 

 decapitation, the increase is much more than that found by the 

 darkening method. After some time has elapsed following 

 decapitation, a "physiological tip" is regenerated; at the same 

 time, phototropic sensitivity is increased, and the ability to react 



