250 mDIRECT BENEFITS DERIVED FROM IISTSECTS. 



cut off from all access to it by the intervening barrier formed 

 by the arcus eminens, it is clear that but for some extraneous 

 agency the pollen could never possibly arrive at the place of 

 its destination. In this case the humble-bee is the operator. 

 Led by instinct, or, as the ingenious Sprengel supposes, by 

 one of those honey marks (^SaftmaaV) or spots of a different 

 colour from the rest of the corolla, which, according to him, 

 are placed in many flowers expressly to guide insects to the 

 nectaries, she pushes herself between the stiff style-flag and 

 elastic petal, which last, while she is in the interior, presses 

 her close to the anther, and thus causes her to brush ofl" the 

 pollen with her hairy back, which ultimately, though not at 

 once, conveys it to the stigma. Having exhausted the nectar, 

 she retreats backwards ; and in doing this is indeed pressed by 

 the petal to the arcus eminens ; but it is only to its lower or 

 negative surface, which cannot influence impregnation. She 

 now takes her way to the second petal, and insinuating her- 

 self under its style-flag, her back comes into close contact 

 with the true stigma, which is thus impregnated with the 

 pollen of the first visited anther ; and in this manner migrating 

 from one part of the corolla to another, and from flower to 

 flower, she fructifies one with pollen gathered in her search 

 after honey in another. Sprengel found that not only 

 are insects indispensable in fructifying the different species 

 of Iris, but that some of them, as /. xiphium, require 

 the agency of the larger humble-bees, which alone are strong 

 enough to force their way beneath the style-flag ; and hence, 

 as these insects are not so common as many others, this Iris 

 is often barren, or bears imperfect seeds. ^ Sprengel also 

 contends, that insects are essentially necessary in the impreg- 

 nation of AsclepiadecB ; in which opinion he is confirmed by 

 the conclusive testimony of the celebrated botanist Robert 

 Brown, Esq., who states ^ that there can be no doubt that the 

 agency of insects is very frequently, though not always, em- 

 ployed in the fecundation of OrchidecB, "but that in those 



1 Chr. Conr. Sprengel, Entdecktes Geheimmss, Sec. Berlin, 1793, 4to. ; quoted 

 in Ann. of Bot. i. 414. 



2 On the Organs and Mode of Fecmidation in Orchidece and AsclepiadecE. Linn. 

 Trans, xvi. 731. 



