of Masdevallia muscosa , Rchb. f. 251 
being similarly entrapped, leaves the pollinia sticking to the 
viscid stigmatic surface, up which it has to crawl in making its 
escape. I have often caused small house-flies and ants to 
become thus entrapped ; but unfortunately they did not make 
their escape, but after struggling awhile became quiescent. 
This of course is one of the disadvantages attendant on bio- 
logical experiment made on plants removed from their natural 
surroundings. 
In Pterostylis the part played by the mechanism is similar. 
Fitzgerald 1 has shown that in this genus small insects alight 
on the labellum, and, from its irritability, become enclosed 
within the flower. He has been fortunate enough to observe 
the removal of the pollinia by their agency. In Pterostylis , as 
in Masdevallia , the one way of escape is by crawling up the 
column and past the rostellum. The upper part of the column 
is provided with wings which bound the passage, just as the 
lateral petals do in Masdevallia . Fertilization is due, he 
says, chiefly to small dipterous insects. 
As in Pterostylis the labellum is sensitive equally on upper 
and under surfaces, the insects very often fail to be en- 
trapped, the labellum closing from any casual contact they 
may make with its under surface. This will be especially the 
case in those species in which the labellum is partly enclosed 
in the flower, and from its position very liable to be touched 
on the wrong side. 
In Masdevallia muscosa the irritability is limited to the crest 
on the upper surface of the blade, and, so far as can be judged, 
it appears to possess altogether a more highly specialized 
mechanism. In either case it is not obvious that the pos- 
session of irritability is a very advantageous mechanism for 
securing cross-fertilization. Many other orchid-mechanisms 
will, I think, compare favourably with it for efficiency, since the 
disturbance caused by even a gentle breeze is quite sufficient 
to bring about the closing of the flowers, which will not open 
again for from twenty minutes to an hour. 
As to the very numerous emergences present on the 
1 Fitzgerald, Australian Orchids, vol. i. Sydney, 1882. 
