INDIRECT BENEFITS DERIVED FROM INSECTS. 295 



by plants as to their nutriment, there can be no doubt 

 that many species perform an important function with 

 regard to their impregnation, which indeed without 

 their aid would in some cases never take place at all. 

 Thus, for the due fertilization of the common Barberry 

 (Berberis vulgaris) it is necessary that the irritable sta- 

 mens should be brought into contact with the pistil by 

 the application of some stimulus to the base of the fila- 

 ment ; but this would never take place were not insects 

 attracted by the melliferous glands of the flower to in- 

 sinuate themselves amongst the filaments, and thus, 

 while seeking their own food, unknowingly fulfil the 

 intentions of nature in another department^. 



The agency of these little operators is equally in- 

 dispensable in the beautiful tribe of Iris. In these, as 

 appears from the observations of Kcilreuter, the true 

 stigma is situated on the upper side of a transverse 

 membrane {arcus eminens of Haller) which is stretched 

 across the middle of the under surface of the petal-like 

 expansion or style-flag, the whole of which has been 

 often improperly regarded as fulfilling the office of a 

 stigma. Now as the a,nther is situated at the base of the 

 style-flag which covers it, at a considerable distance 

 from the stigma, and at the same time 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 (Saft- 

 maal) or spots of a different colour from the rest of the 



' Smith's Tracts, 165. KiJlreuter Ann. of Bot. ii. 9. 



