414 University of California Publications in Botany [Vol. 5 



of 19° C. It occurs in Xicotiana Tabacum "Maryland" in eight hours 

 under the same conditions. The actual time involved in the process of 

 cell separation in the above-mentioned cases lies within thirty to forty 

 minutes in the hybrid and within forty-five to sixty minutes in the 

 Tabacum variety. Normal abscission in these forms is much slower 



4. The length of the reaction time in cases of flower-fall due to 

 mechanical injury shows that this length of time depends more on 

 the age of the flower than on the type of injury. 



5. Temperature is the most important conditioning factor in esti- 

 mates of the time of abscission. 



Experimental Induction of Abscission 



1. Floral abscission is induced, in a large number of the species 

 investigated, by illuminating gas or laboratory air. The increase in 

 resistance to abscission stimulated in this manner takes place suddenly 

 in some species, since abscission will not occur after the opening of 

 the corolla. In other species this condition does not exist. 



2. It is possible to induce the process of abscission with illuminat- 

 ing gas in small isolated pieces of the pedicels or in longitudinal sec- 

 tions of the pedicel cut free-hand from fresh material. 



3. Abscission in Nicotiana and Lycopersicum is induced by certain 

 types of severe injury and not by others. Injury to the ovary seems 

 more effective in causing abscission than injury to other parts of the 

 flower. In the case of these other flower parts, it seems necessary that 

 a certain amount of tissue be actually removed or destroyed before 

 fall occurs. Injury to the pedicel does not cause abscission unless it 

 breaks entirely the connection between floral organs and stem. 

 Flower-fall in Lycopersicum is not readily induced by injury. Floral 

 abscission in this genus is more dependent upon physiological condi- 

 tions brought on by abnormal soil conditions. 



4. Experiments on the induction of abscission in small isolated 

 pieces and in flowers with only a small portion of the stem proximal 

 to the separation layer attached indicate that the stimulus produced 

 by the action of external factors such as illuminating gas and mechan- 

 ical injury can cause abscission by acting directly on the cells in close 

 proximity to the separation zone. The action of external factors is 

 thus largely independent of such physiological processes as transpira- 

 tion which might enter in. This statement is supported by experi- 

 ments which show that abscission is not necessarily induced by 

 checking transpiration from the flower. 



