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 con&psea, 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 1 !" 



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. 



