442 University of California Publications in Botany [Vol. 5 



The effects of the pure constituents of illuminating gas were investi- 

 gated only in the case of Nicotiana. Table 2, which follows, summarizes 

 the results of these experiments. 



Table 2 



Effects of the Pure Constituents of Illuminating Gas in Causing 



Flower-fall in N. Tahacum var. macropliylla pxirpurea 



The figures given represent the times in hours between the start of 



the experiment and the fall of the first flower 



Experiment 

 number 



6 



7 



In two experiments (table 1, experiment 4, and table 2, experi- 

 ment 7) the material was subjected to the gases in a dry atmosphere. 

 The bell-jar already described as fitted with a ground-glass plate sealed 

 with vaseline was used in these experiments with a large amount of 

 fused calcium chloride scattered over this ground-glass bottom. In 

 all the other experiments with 25/06 water-sealed bell-jars were used. 



In an additional experiment a lateral branch of 25/06 was exposed 

 in an atmosphere containing 5% by volume of ethylene, but no 

 abscission of the leaves could be induced, even after a considerable 

 period had elapsed. In Nicotiana Tahacum there seems to be no direct 

 leaf-fall in the sense that leaf bases are cleanly detached from the stem 

 by the death or separation of cells. The dead, dry leaves remain 

 attached to the plant often for months until the blades are broken 

 away by the wind or other agencies. Thereafter the torn, dry leaf 

 base may or may not come away cleanly from the stem. It seems 

 apparent, however, that there is no abscission mechanism present in 

 the leaf base, at least there is none which may be stimulated to 

 activity by ethylene. Further investigation in this connection would 

 appear to be profitable and necessary. 



