Fertilisation of Scrophularia aquatica omd S. nodosa. 38' 



Sci'ophularia is the commonest of the very few entomo- 

 philoiis plants which are habitually cross-fertilised by the 

 agency of wasps, but the exact mode by which it is accom- 

 plished has been misunderstood. Professor Stephen "Wil- 

 son, in a paper read before the British Association in 1878, 

 " On the Association of an Inconspicuous Corolla with Pro- 

 terogynous Dichogamy," states that indefinite inflorescences 

 which have proterogynously dichogamic flowers have the 

 younger pistillate flowers above the older flowers in the 

 male stage, and selecting 8, nodosa as an example, he 

 describes how wasps alight on the top flower, pass in a 

 somewhat irregular manner to the lower ones, and leave 

 the plant " from the lowest flowers." He says that this 

 is the mode by which cross-fertilisation of diS'erent in- 

 dividuals is effected ; and Mliller adopts his opinion. But 

 in Scroplmlaria there is no limitation of the flowers in 

 either of the two sexual stages to a particular portion of 

 the raceme ; they are irregularly intermixed, and both 

 may be found on the same cyme. There are, of course, 

 successive crops of flowers centripetally developed, but 

 the different generations are not distinctly separated, but 

 overlap, as it were; and thus there is no parity with a 

 proterandrous indefinite inflorescence, such as Digitalis, 

 where the development of the dichogamy has a simple 

 positional relation to the whole inflorescence. The wasps, 

 then, if they kept to one raceme and pursued a descend- 

 ing mode of visitation, would fertilise the great majority 

 of its flowers with pollen from flowers higher up, and 

 this would be very detrimental to the continuance of the 

 species. 



I have watched wasps visiting the flowers of S. nodosa 

 and of 8. aquatica on numerous occasions, but they never 

 adopted a regular descending method of working, nor indeed 



* The figures within brackets give the percentage proportions. 



