March, 1922] NEWCOMBE BEHAVIOR OF SENSITIVE STIGMAS 
ularia, are in contact over the most or the whole of their inner surfaces. 
It requires an appreciable pressure to call forth a response, and pollen may 
be artificially applied to the stigmas without causing closing. Besides 
pressure, the stigmas will respond to an electric current, stigmas of some 
species to various vapors perhaps, and the stigmas of Catalpa hignonioides 
and Mimulus glahratus var. Jamesii to crushing of the style, or to pinching 
of the style. 
As far as good evidence goes, the most of the species so far reported show 
a sensitiveness to pressure on the inner surface only. In my own work, 
Catalpa hignonioides showed itself sensitive on both the outer and the inner 
surfaces of the stigma lobes, though more sensitive on the inner surface. 
The conduction of a stimulus from one stigma lobe to another has been de- 
termined positively for only the three species so reported by Oliver — Mar- 
tynia lutea, M. proboscidea, and Mimulus cardinalis. On the other hand, 
Lloyd found the stigma of Diplacus glutinosus, and my study showed the 
stigmas of Catalpa hignonioides, Mimulus punctatus, and Torenia fournieri 
not transmitting the stimulus from one lobe to the other. 
As to the degree "of relative sensitiveness, or the speed of response, in 
the various species, little that is certain can be said. It is certain that 
Digitalis purpurea is the slowest of all reported; but the stigma of this 
species does not carry out an effective closing, and it should not be con- 
sidered in the same class as the others. Most of the stigmas whose time of 
response has been reported have shown a complete closing within 10 seconds 
of the time of stimulation. Stigmas, reported by Heckel as the slowest in 
response, I have found the quickest, namely, those of Torenia closing com- 
pletely in 2 seconds, and those of Tecoma in 3 seconds after stimulation. 
But the responses of all stigmas vary so much with temperature and other 
conditions that a statement of relative sensitiveness is not now possible. 
The obvious benefit to the plant of the closure of the stigma is its aid in 
securing cross-pollination. As the stigmas are generally in such a position 
as to be touched by the insect, or other visitor, before the pollen of the same 
flower is encountered, and as the stigmatic surface is in a few seconds after 
touching turned so as to be out of reach of the flower visitor as he with- 
draws, self-pollination can be possible only in those rare cases of very 
slowly acting stigmas or unusually placed stigmas. 
The present study has shown that the phenomenon of stigmas opening 
soon after pollination is more general than hitherto supposed. We knew 
from the work of Lloyd that Diplacus glutinosus regularly opened its stigma 
after closing at the time of pollination. The study here reported shows 
that the stigmas of Utricularia vulgaris and Mimulus glahratus also always 
open after pollination, that the stigmas of Torenia fournieri and Catalpa 
hignonioides sometimes open in not very dry air, and half of them, or more, 
will open in moist weather. Other species have shown stigmas opening in 
moist air after being closed at the time of pollination. 
