FERTILIZATION OF FLOWERS. 361 



powdery pollen. Each mass of pollen grains, or pollinium 

 (7^, C, p), is a pear-shaped body provided with a short stalk or 

 caudicle. The stalk is continued downwards into the rostellum 

 as shown in C, where it ends in a membranous disc attached 

 to the sticky substance. When the pollen is ripe the pollinia are 

 exposed by rupture of the sacs in which they lie, so that in the 

 mature flower they are only attached by means of their stalks or 

 caudicles, loosely fixed by the sticky substance of the rostellum. 



If we take any slender object, such as a pencil point, and poke 

 it gently into the mouth of the nectary, we shall find on with- 

 drawing it again that it will bring with it either one or both of 

 the pollinia, attached to it by sticky cement. The pencil has, of 

 course, touched the rostellum and pushed down the little cap 

 which covers it. The sticky substance of the rostellum, thus 

 exposed, has cemented on to the pencil the disc at the lower end 

 of the caudicle, and thus the pollinium itself, or both of them, is 

 pulled bodily out with the pencil. The cement rapidly hardens 

 on exposure to the air. 



When first attached to the pencil the pollinium is in a nearly 

 upright position (D), so that if the pencil were inserted into 

 another flower it would j- imply go back into its old place, without 

 touching the stigma and of course, therefore, without effecting 

 fertilization. After being exposed to the air for a short time, 

 however, the disc by which the pollinium is attached to the 

 pencil begins to contract, and in such a manner that the polli- 

 nium is pulled down until it comes to project straight forwards 

 (E). If now the pencil be inserted into a flower it will be found 

 that the pollinium exactly strikes against one of the stigmas and 

 dusts it with pollen. 



In the economy of nature the proboscis of some insect takes the 

 place of our experimental pencil. Bees and moths have fre- 

 quently been observed with the pollinia of various species of 

 Orchis attached to them ; indeed Darwin figures one instance in 

 which no less than fourteen pollinia are attached to the proboscis 

 of a moth. Every time such an insect visits a flower in search 

 of honey it will effect cross-fertilization by pushing the pollinia 

 which it brings with it against the stigmas, and may perhaps 

 carry off another pollinium into the bargain. 



In this case one can hardly fail to be astonished at the number 

 and complexity of the adaptations which have arisen in the 

 flower for the purpose of ensuring cross-fertilization. There is 



