250 Oliver . — On the Sensitive Labellnm 
parenchyma in the stigma of Martynia is diffused, in Masdevallia 
muscosa concentrated ; in relation to this it must be remem- 
bered that in the former the movement is a slow one, but in 
the latter very rapid ; indeed we have to deal with a highly 
specialised mechanism in this case. A parallel case is found 
in the concomitancy of nerves and blood-vessels in a highly 
developed animal. 
The presence of so much tannin in the cells of the supposed 
conducting sheath would tend to point it out, other things 
being equal, as a stimulus-conducting tissue. Gardiner 1 has 
shown that the amount of tannin present in a motile organ is 
often proportional to its irritability. 
The. conduction is probably facilitated by the c continuity of 
protoplasm ’ which may be demonstrated in these parts. I 
have shown it for the parenchyma of the crest (Fig. 14), and 
also for the conducting sheath. 
Biological Significance. 
This manifestation of movement in the labellum seems to be 
simply one of the numerous ways chanced on by orchids in 
promoting cross-fertilization by the agency of insects. A 
small fly alighting on the extended labellum is immediately 
enclosed as it were in a box, and the only mode of exit is at 
the top, the aperture being bounded laterally by the two 
lateral petals, and posteriorly by the rostellum (Fig. 10). 
The insect will have to climb up the face of the column 
(Fig. 8). and in passing through the aperture the pollinia will 
be removed, the mucilage from the end of the rostellum stick- 
ing them firmly to the insect. It would be almost impossible 
for the insect to escape without removing them. I have often 
imitated this action with a blunt needle, passing its end 
through the aperture. The pollinia were always brought 
away w 7 ith it. 
For the completion of the act of cross-fertilization, it must 
be supposed that the insect next visits another flower, and 
1 Gardiner, On Tannins in Vegetable Cells, in Proc. Camb. Phil. Soc. vol. 
iv. p. 391. 
