[8] _Das neu-entdeckte Geheimniss der Natur im Bau u. der Befruchtung der Blumen_, Berlin, 1793. Sprengel was completely dominated by the idea of an all-wise Creator, who 'has not created even a single hair without intention,' and, guided by this idea, he endeavoured to penetrate into the significance of many little details in the structure of flowers. Thus he recognized that the hairs which cover the lower surface of the petals of the wood-cranesbill (_Geranium sylvaticum_) 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 palustris_) 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 is concealed in the depths of the tube. [Illustration: FIG. 40. _Potentilla verna_, after Hermann Müller. _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. _Nt_, nectary.] 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 the 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 (_Potentilla 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.' How near Sprengel was to reaching a complete solution of the problem is now plain to us, for he even discovered that many flowers, such as _Hemerocallis fulva_, remained infertile if they were dusted with their own pollen.