406 The Biology of Flowers 
The majority of orchids differ from other seed plants (with the 
exception of the Asclepiads) in having no dust-like pollen. The 
pollen, or more correctly, the pollen-tetrads, remain fastened together 
as club-shaped pollinia usually borne on a slender pedicel. At the 
base of the pedicel is a small viscid disc by which the pollinium is 
attached to the head or proboscis of one of the insects which visit 
the flower. Darwin demonstrated that in Orchis and other flowers 
the pedicel of the pollinium, after its removal from the anther, under- 
goes a curving movement. If the pollinium was originally vertical, 
after a time it assumed a horizontal position. In the latter position, 
if the insect visited another flower, the pollinium would exactly hit 
the sticky stigmatic surface and thus effect fertilisation. The relation 
between the behaviour of the viscid disc and the secretion of nectar 
by the flower is especially remarkable. The flowers possess a spur 
which in some species (e.g. Gymnadenia conopsea, Platanthera 
bifolia, etc.) contains honey (nectar), which serves as an attractive 
bait for insects, but in others (e.g. our native species of Orchis) the 
spur is empty. Darwin held the opinion, confirmed by later investi- 
gations, that in the case of flowers without honey the insects must 
penetrate the wall of the nectarless spurs in order to obtain a nectar- 
like substance. The glands behave differently in the nectar-bearing 
and in the nectarless flowers. In the former they are so sticky that 
they at once adhere to the body of the insect; in the nectarless 
flowers firm adherence only occurs after the viscid disc has hardened. 
It is, therefore, adaptively of value that the insects should be detained 
longer in the nectarless flowers (by having to bore into the spur),— 
than in flowers in which the nectar is freely exposed. “If this 
relation, on the one hand, between the viscid matter requiring some 
little time to set hard, and the nectar being so lodged that moths are 
delayed in getting it; and, on the other hand, between the viscid 
matter being at first as viscid as ever it will become, and the nectar 
lying all ready for rapid suction, be accidental, it is a fortunate 
accident for the plant. If not accidental, and I cannot believe it 
to be accidental, what a singular case of adaptation?!” 
Among exotic orchids Catasetum is particularly remarkable. One 
and the same species bears different forms of flowers. The species 
known as Catasetum tridentatum has pollinia with very large viscid 
discs; on touching one of the two filaments (antennae) which occur 
on the gynostemium of the flower the pollinia are shot out to a fairly 
long distance (as far as 1 metre) and in such manner that they alight 
on the back of the insect, where they are held. The antennae have, 
moreover, acquired an importance, from the point of view of the 
physiology of stimulation, as stimulus-perceiving organs. Darwin 
1 Fertilisation of Orchids (1st edit.), p. 53. 
