360 University of California Publications in Botany [Vol. 5 



b. INDUCTION BY MECHANICAL INJURY 



Beequerel (1907), in a brief paper on the effect of wounding 

 flowers of Nicotmna, notes that even after fifteen days flowers without 

 sepals, anthers, or stigmas do not fall. After the same length of time, 

 flowers without corollas or flowers in which the corolla or stamens are 

 only half removed, have fallen. He points out that this result is more 

 conspicuous in young flowers but did not investigate this point suffi- 

 ciently to arrive at any definite conclusions. According to Hannig, 

 removal of various organs of flowers frequently causes abscission but 

 wounding of the pedicel does not. He concludes, therefore, that in- 

 jury itself does not cause abscission but only acts indirectly by inter- 

 fering with important physiological processes in the treated tissues. 



According to Lloyd (1914a), shedding of very young cotton-bolls 

 can be induced by removal of the styles before pollination, but fall in 

 this case can be assigned, as Fitting has shown, to lack of fertilization. 

 It appears that in the cotton flower (Lloyd, 1916&) there is an inhibi- 

 tion period which starts with the opening of the corolla and during 

 which premature abscission as the result of sudden stimuli very sel- 

 dom occurs. Also, cotton-bolls larger than 30 mm. in diameter are 

 very seldom shed under any conditions. Other results obtained by 

 Lloyd on the effect of injury on the abscission of cotton flowers are 

 discussed above under "Time of Abscission" (page 357). Lloyd 

 (1914&) also notes the effect of injury on abscission of internodes in 

 Impatiens Sultani. Plants of this species, when a cut is made across 

 the stem, cast off the remainder of the severed internode. He gives 

 results of experiments on the effect of different types of injury, noting 

 that some severe injuries do not cause abscission. Gortner and Harris 

 (1914) have obtained similar results with the same species. They 

 find that when the cut is made across the internode, very close to 

 the separation layer, abscission usually occurs, but occasionally it does 

 not. They state, as does Lloyd, that the shape and location of the 

 separation layer may vary slightly according to the type of injury. 



c. THE DIRECT OR INDIRECT ACTION OP THE EXTERNAL 

 STIMULUS 



In all the above investigations the question naturally arises, 

 whether the narcotic vapors and injuries or any stimulus conducive 

 to abscission act indirectly through their influence on the physiolog- 

 ical condition of the plant or directly, through their action on the 

 cells of the separation zone. Most investigators, except Wiesner, ex- 



