777 



normal value for some minutes, and then later becomes quite normal 

 again. 



This is a sliort résumé of the reaction, which these growing cells 

 execute after illumination by the above-mentioned definite quantity 

 of light and there was among the dozens of cells, which I investigated 

 in this manner not a single one which did not clearly show this 

 i-emarkably strong reaction. Moreover the phenomenon equally occurred 

 both in slow-growing and in rapidly growing cells. 



Of this series of experiments, only a few examples will for the 

 sake of brevity, be given in tables T, U, and III, of which the third 

 is also represented graphically in fig. 1. If observations are not made 

 every two minutes but at long intervals, the reaction then does not 

 appear to be so striking, whilst with observations made at still 

 shorter intervals than two minutes, for a very short time perhaps a 

 still higher figure for the maximum growth might be found, than 

 has here been noted in observations taken every two minutes. 



If the cell is illuminated with the same intensity of light for a 

 4 times shortei- period, there likewise always occurs a distinct 

 acceleiation of growth, but the latter reaches a somewhat lower 

 value, about IV2 to 2 times the normal ; if the illumination is 4 times 

 as strong and 4 times as long, that is to say 16 times as great, 

 then growth increases not nearly so much as with the lesser illumination 

 and reaches a value of IV3 to V/^ times the normal. 



Whilst these experiments are being continued in greater detail and 

 more accurately in order to determine on the one hand, with how 

 small a quantity of light a measurable reaction still occurs and to 

 trace, on the other hand, what happens further after giving a much 

 greater C[nantity of light, my immediate purpose is to report the fact, 

 that the growth of the cells responds with a sharply accentuated 

 reaction to illumination with a certain quantity of light, a reaction 

 which shows the typical character of what hitherto has been called 

 in botanical literature a stimulus- reaction. This reaction of growth 

 to light I should like to name photo-gro^oth-reaclion, but considering, 

 that many as yet unanalysed phenomena in which light has an 

 influence on growth or form, may also be included under this general 

 name, I will in order to prevent confusion, distinguish this reaction 

 as primary photo-growth-reaction. In the case of an accelaration 

 of growth we can then speak of a positive, in the case of retardation 

 of a negative photo-growth-reaction. 



With regard to the existence of a sharply-defined reaction of this 

 kind, practically nothing can be deduced from the literature-references, 

 at least the general opinion about the infiuence of light on growth 



50* 



