24S Oliver . — On the Sensitive Labellum 
cally active substances (Pfeffer), is a question which I think 
would be unsuitable for discussion here, involving as it does 
physiological principles of fundamental importance. The ex- 
tended condition is arrived at by the gradual re-absorption 
by these cells of water, the former equilibrium between the 
two halves being re-inaugurated. In M asdevallia this occurs 
some twenty minutes after contraction ; the act of descent 
of the blade not occupying more than five minutes when once 
it has commenced. 
I made an extremely delicate cut across the upper 
surface of the neck, at a point nearer the blade than the 
emergence, severing the epidermis and subjacent layers, but 
leaving the bundles intact, and the labellum in the single case 
thus operated on remained in the contracted position for 
many hours and finally moved down only through about 
io°, and in this position remained until the fading of the 
flower. This experiment is the counterpart of cutting through 
the contractile half of a pulvinus of Mimosa , and has a result 
quite similar. 
It remains for me to describe certain experiments under- 
taken to ascertain the course followed by the stimulus. 
(1) A clear transverse cut was made across the crest of the 
blade (Fig. 18, A) separating it into a proximal and distal 
portion. Care was taken that this cut should not reach the 
plane of the vascular bundles. When the labellum had as- 
sumed the extended position the crest was stimulated with 
a hair, in the ordinary way, on the distal side of the cut 
(at X, Fig. 18). A normal contraction immediately occurred, 
the cut not impeding the course of the stimulus. 
(2) The cut was then continued further down, so as to 
sever the three vascular bundles. On stimulating again at 
X no contraction took place. If, however, the crest was 
stimulated on the proximal side of the cut an ordinary con- 
traction resulted. 
These two experiments point to the fact that the stimulus 
— initiated at the surface of the crest — passes, not along the 
surface or subjacent tissue to the contractile cells of the neck, 
