256 EVOLUTION OF PLANTS 
The stamens (here reduced to a single one) and 
pistil are grown together into a “column” or “gyno- 
stemium.” Like the milkweed, this orchid has the 
pollen-spores in two pollinia, club-shaped masses con- 
verging toward the base, where each terminates in 
a sticky disk covered over with a delicate membrane 
just above the opening of the spur (G, @). Each pol- 
linium lies in a little pocket from which it can be dis- 
lodged only through some external agency. An insect 
alighting upon the lip and probing the spur for nectar, 
must hit against the membrane which covers the base 
of the pollinia, and this is ruptured, and the adhesive 
disks are thus brought into contact with the head or 
tongue of the insect, to which they become firmly at- 
tached by the “setting” of the cement-like substance 
composing the disk. As the insect backs out of the 
flower, the two pollinia are dragged out of their recep- 
tacles and carried away. The action of the insect is 
easily imitated by inserting into the flower a slender 
stalk of grass, or the fine point of a pencil, which on 
being withdrawn will drag away the pollinia. The 
latter at first stand nearly vertical and diverge widely 
(H); but very quickly they change position, bending 
downward and forward until they lie nearly parallel 
and point almost directly forward (I, J). Thrusting 
the pencil-point with the pollinia in this position, into 
another flower, it will be found that the pollinia come 
into immediate contact with the two stigmatic surfaces 
on either side of the opening of the spur Cig. 57, G, st), 
lower down than the anther. 
Many other even more remarkable instances might 
be cited, but space forbids a further discussion of this 
