THE ORIGIN OF FLOWERS 



181 



petals of the wood-cranesbill (Geranium sylvatlcum) protect the 

 nectar of the flower from being diluted with rain, and he drew the 

 conclusion, correct enough, though far removed from our modern 

 ideas as regards the directly efficient cause, that the nectar was there 

 for the insects. 



He was also impressed by the fact that the sky-blue corolla of 

 the forget-me-not (Myosotis patustris) has a beautiful yellow ring 

 round the entrance to the corolla-tube, and he interpreted this as 

 a means by which insects were shown the way to the nectar which 

 lies concealed in the depths of the tube. 



We now know that such ' honey-guides ' are present in most of the 

 flowers visited by insects, in the form of spots, lines, or other marking, 

 usually of conspicuous colour, that is, of a colour contrasting with tip 

 ground colour of the 

 flower. Thus, in species 

 of Iris, regular paths of 

 short hairs lead the way 

 to the place where the 

 nectar lies. In the 

 spring potentilla (Poten- 

 tilla verna) (Fig. 40) the 

 yellow petals (A, Bl) 

 become bright orange- 

 red towards their bases, 

 and this shows the way 

 to the nectaries, which 

 lie at the bases of the 

 stamens (st), and are 

 protected by hairs, the 

 so-called ' nectar-covers ' (Saftdecke) of Sprengel, from being washed 



by rain. 



The recognition of the honey-guides led Sprengel on to the idea 

 that the general colouring of the flower effects on a large scale what 

 the honey-guides do in a more detailed way — it attracts the attention 

 of passing insects to where nectar is to be found ; indeed, he went an 

 important step further by recognizing that there are flowers which 

 cannot fertilize themselves, in which the insect, in its search for 

 honey, covers itself with pollen, which is then rubbed off on the 

 stigma of the next flower visited, fertilization being thus effected. 

 He* demonstrated this not only for the Iris, but for many other 

 flowers, and he drew the conclusion that ' Nature does not seem to 

 have wished that any flower should be fertilized by its own pollen.' 



Fig. 40. Potentilla verna, after Hermann Miiller. A, 

 seen from above. Kbl, sepals. Bl, petals. Nt, nectaries 

 near the base of the stamens. B, section through the 

 flower. Gr, stigma. St, stamen. Xt, nectary. 



