256 EVOLUTION OF PLANTS 



The stamens (here reduced to a single one) and 

 pistil are grown together into a " column " or " g}^no- 

 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, d). 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 being withdrawn will drag away the pollinia.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 (Fig. 57, G, s), 

 lower down than the anther. 



Many other even more remarkable instances might 

 be cited, but space forbids a further discussion of this 



