SIGNIFICANCE OF THE BEHAVIOR OF SENSITIVE STIGMAS ^ 
F. C. Newcombe 
(Received for publication June ii, 1921) 
It has been known for a long time that in the famiUes of the Bignoniaceae, 
Scrophulariaceae, Martyniaceae, and Lentibulariaceae there are certain 
genera whose stigmas respond to pressure. In nature, the stimulating 
object is the body of a visiting insect or of a humming bird. The external 
structure of these sensitive stigmas is similar, though not closely similar, 
in all species. The stigma consists, in most of the species, of two tongue- 
shaped lobes which ordinarily diverge 180° or more. The pollen is de- 
posited on the inner surface of these gaping lobes by the visiting insect, 
and the push of the insect's body stimulates the lobes to close together 
completely, except in the Lentibulariaceae, in which there is but partial 
closing. 
I. The Primary Closing of Stigmas 
I. Stimuli Causing Primary Closing 
As stated above, the primary closing of stigmas includes that striking 
behavior in which the two divergent stigma lobes close together as a result 
of the pressure of an insect or other body, the response being therefore 
thigmotropic. 
As long ago as 1841, sensitive stigmas were described by Henderson (i), 
and in 1861 by Kabsch (2). More recently some dozen authors have pub- 
lished their observations and experiments, till, at the present time, we know 
some two dozen species and varieties with sensitive stigmas. A list of 
these species, with the names of authors publishing them, will be found 
under the "Summary," at the end of this paper. 
The notion that the stigmas are anywhere sensitive (or responsive) to 
the slight pressure called touch is erroneous, as shown by Lloyd (14) for 
Diplacus glutinosus, and as I have found for six species I have tested. A 
human hair may be glued to a style, leaving two centimeters free, and with 
the tip of the hair one may stroke the stigmas anywhere without causing 
closing. I have commonly used this instrument in the pollination tests, 
the hair carrying the pollen very well. 
To locate the sensitive region of the stigma, various authors have 
explored the stigma lobes with various instruments. Heckel (8) found 
both stigma lobes equally sensitive, but the inner surfaces of the lobes more 
sensitive than the outer surfaces. Oliver (10), working in part with the 
^ Contribution 184 from the Botanical Department of the University of Michigan. 
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