390 University of California Puhliaxtions i}i Botany [Vol.5 



b. REACTION TIME IN "SPONTANEOUS" ABSCISSION 



Exact data in regard to the reaction time can be given only in 

 two definite eases. Tlie observations in these cases were made on 

 small shoots of the plant to be considered, which were placed in water 

 and inserted under a bell-jar containing- 1.5 per cent illuminating- 

 gas. After several hours, the material was shaken every fifteen min- 

 utes to determine when the first flower fell. F^ H179 and N. Tahaciim 

 "Maryland" were selected as material for the experiments because 

 these forms were found most sensitive and thus react regularly and 

 quickly to stimuli. Abscission occurs in the pedicel of F^ H179 seven 

 hours after insertion into 1.5 per cent illuminating- gas at a tempera- 

 ture of approximately 19° C. The smaller buds begin to fall first, 

 but are followed in a short time by the open flowers. Abscission 

 occurs in A. Tabacum "Maryland" in eight hours under the above 

 conditions. 



The remainder of the data having- to do with the reaction time in 

 spontaneous abscission is in the form of approximate estimates derived 

 from the results of experiments on the induction of abscission. In 

 the case of abscission induced by illuminating gas most species which 

 shed their flowers in 1.5 per cent illuminating gas do so after ten or 

 fifteen hours at room temperature. 



There remains now to be considered the reaction time in cases of 

 flower-fall due to mechanical injury-. The results along this line are 

 largely derived from tables 2, 3, 4, and 5, which, however, were 

 arranged to show more particularly the comparative effect of different 

 t^'pes of injury, as causing or not causing abscission in flowers of 

 various ages. These tables might as well be presented under the 

 heading "Experimental Induction of Abscission by Mechanical In- 

 jury" (page 405), but since it is necessary to draw certain conclusions 

 from them in regard to the time of abscission they are presented and 

 explained at this time. 



Tables 2, 3, 4, and 5, which follow, serve to record the results of 

 a number and varietj' of experiments all designed to show the relation 

 of mechanical injury to abscission. It was very soon discovered while 

 carrying on the experiments that the effect of injury depends to a 

 large extent upon the age of the flower. Now the age of the flower 

 can be most conveniently measured by determining the increase in 

 size of growing parts such as the corolla and ovary. Thus it was 

 necessary in each case to record the size of the flower — size being a 



