INSECTS AND FLOWERS 233 



wards as a tubular spur. The solitary anther consists of 

 two rather widely separated cells, which are open in front ; 

 and each cell contains a pollen-mass, or pollinium, which 

 has a tapering stalk ending below in a round, sticky disc. 

 These discs lie loosely in a cup-shaped envelope called the 

 rostellum, which is easily ruptured. The stamen is con- 

 fluent with the pistil, and they form together the structure 

 called the " column " — the two receptive areas of the 

 stigma lying just within the opening which leads to the 

 spur. Although no nectar is secreted, Darwin showed 

 that the flower is visited by bees, which are able to obtain 

 a sweet juice by probing the inner Avails of the spur. In 

 so doing the insect's head presses against the rostellum, 

 which it ruptures ; and when it flies away it carries the 

 pollinia attached by their viscid discs to its head or pro- 

 boscis. The discs contract unequally in drying, with the 

 result that the pollinia bend forward and diverge slightly 

 from one another. Darwin found that this act of depres- 

 sion is accomplished in the course of thirty seconds on an 

 average. Thus, when the bee visits another flower, the 

 pollinia are in such a position that they are brought into 

 contact with the sticky stigmatic surfaces, and cross- 

 pollination is effected. Other long-spurred orchids, in 

 which the alighting platform is reduced or absent, rely 

 upon the visits of moths, which carry off the pollinia on 

 their sucking-trunks. The species known as Angrcecum 

 sesquipedale, from Madagascar, has a spur more than 

 eleven inches long, from which Darwin inferred the exis- 

 tence of a hawk-moth with a proboscis equally long — 

 a deduction which subsequently proved to be correct. 



Still more remarkable are the contrivances for pollina- 

 tion in certain tropical orchids. We may quote two 

 instances from Darwin's classical work on the subject. 

 The first refers to a species known as Coryanthes speciosa. 



