1918] Kendall: Abscission of Flowers and Fruits in Solanaceae 399 



esculentum, Iochroma tuberosa, Solanum nigrum, and S. verbasci- 

 folium, upon subjection to 3 per cent illuminating gas for twenty 

 hours, throw off all flowers up to those two or three days past an thesis. 

 No abscission occurred, however, in any concentration of gas, in 

 Nicotiana Bigelovii, N. quadrivalvis, N. multivalvis, Lycium australis. 

 Petunia hybrida, Salpiglossis stinuata, or Salpichrora rhomboidea. 



A peculiar condition exists in Solanum umbelUferum, which throws 

 off buds in the illuminating gas but never under any conditions, in- 

 cluding temperature or the presence of narcotic vapors, throws off 

 flowers in which the corolla has fully opened. A corresponding con- 

 dition seems to exist in Nicotiana Tabacum var. macrophylla, FiHloi, 

 N. Sanderae, N. rustica var. brasilia, and in one other variety of JV. 

 rustica, all of which seldom under any conditions detach fully opened 

 flowers, although flowers up to that stage are freely abscissed. Thus 

 there seems to be, in certain species and at about the time of the open- 

 ing of the corolla, a sudden increase in resistance to the external 

 stimulus which is causing abscission. In other species this sudden 

 increase in resistance does not take place, abscission commonly occur- 

 ring at any stage in the development of the flower or fruit and the 

 increase in resistance taking place very gradually. In addition, there 

 seems to be an intergradation of forms between those in which the 

 increase in resistance takes place suddenly and those in which it takes 

 place gradually. 



The next subject to be taken up is a consideration of experiments 

 5, 6, 7, 8, and 9 on the induction of abscission in small isolated pieces 

 of the pedicel. The main purpose of devising these experiments was to 

 throw some light, if possible, on the direct or indirect action of the 

 external factor in causing "spontaneous" abscission. The pedicel of 

 Fj H179 was again chosen as material for the following experiments, 



Fig. 9 



