410 University of California Fnhlic-ations in Boianij [Vol.5 



4. The Ability of Certain Species to Throw off Pedicels 



PROM which All the Floral Organs Have Been 



Kemoved, as Related to the Induction op 



Abscission by Mechanical Injury 



It was soon noticed in the experiments that all plants of a species 

 in which floral abscission occurs throw off the remains of the pedicel 

 when this organ is severed at any point distal to the separation layer. 

 If after such an operation no abscission occurs, it can lie safely con- 

 cluded that floral abscission never occurs in that species. Petnnia 

 hyhrida, Salpiglossis sinuata, Salpichrora rhonihoidea, and Lycium 

 australis are the only species of the list in table 6 which do not absciss 

 flowerless pedicels in this way. Nicotiana Bigelovii, N. quadrivalvis, 

 and iV. niidtivalvis occasionalh^ do not throw off pedicels under such 

 conditions. The reaction time in cases where the last three species do 

 absciss severed pedicels is very slow (four to fourteen days). 



Turning now to the relation of these observations to the induction 

 of abscission b}^ mechanical injury, it is first necessary to recall the 

 controls used in Experiments 5 and 6 (cf. pages 399 and 400). A fur- 

 ther consideration of the reaction of these controls will suggest that 

 mechanical injury can induce abscission by the action of the stimulus 

 directly on the cells in the vicinity of the separation zone. The con- 

 trol used in Experiment 5, it will be remembered, showed that abscis- 

 sion does not occur under normal conditions in a series of flowers cut 

 as in figure 9. From the control used in Experiment 6 it is evident 

 that merely cutting off the flower at varying distances from the sep- 

 aration laj'er, forming pieces as represented in figure 10, causes ab- 

 scission to occur, evidently as the result of no other stimulus than 

 that of severing the pedicel. Now, if the cut be made through the 

 pedicel at a point approximately 1 mm. distal to the separation layer 

 in flowers, as represented on the extreme right of figure 9, abscission 

 will occur in the remaining piece, which is now scarcely 2 mm. in 

 length. It is evident that the stimulus caused by severing the pedicel 

 must act directly on the cells in close proximity to the separation 

 zone. Practicall}^ the same results are obtained when the transverse 

 cut is made through the base or middle of the calyx. There is no 

 reason to suppose that the stimulus set up by cutting through the 

 flower near the base or middle of the calyx differs in any fashion from 

 that offered by a cut severing only the pedicel. 



