2 THE FERTILISATION OF FLOWERS. [PART I. 
(Myosotis palustris, L.), and speculated on the meaning of the yellow 
ring round the mouth of the corolla, which forms a pleasing con- 
trast to the azure-blue of the limb; and he conceived the idea 
that this might serve to guide insects on their way to the honey. 
On examination of other flowers he found that coloured dots and 
lines and other figures occur especially at the entrance to the 
nectaries, or point towards it, and he was accordingly confirmed in 
this idea of path-finders or honey-guides. The next step was easy, 
and Sprengel could scarcely remain long without perceiving that, as 
the special colour of one part of the corolla serves to guide the 
insect after it has settled upon the flower, the bright colour of the — 
whole flower serves to attract the notice of insects while still at a 
distance. So far, Sprengel had looked upon flowers as contrived 
simply for the use of insects, but the study of some species of 
iris, in the summer of 1789, led him to the further discovery that 
many flowers are absolutely incapable of being fertilised without 
the aid of insects; and so he concluded that the secretion of honey 
in flowers, its protection against rain, and the bright colours of the 
corolla are contrivances of use to the flower itself by bringing 
about its fertilisation by insects. Thus were laid the foundations 
of a theory of honey-containing flowers, which Sprengel enunciates 
in the following propositions : (1) These flowers are fertilised by 
some one species of insect, or by several species ; (2) the insects, 
in approaching the honey, brush pollen from the anthers with 
various hairy parts of their bodies and convey it to the stigma. The 
application of this theory to the various plants that came within 
Sprengel’s reach led to the production of the above-mentioned 
book, which is marked throughout by a wealth of patient observa- 
tion and acute reasoning. In it the following five features are | 
described in several hundred species of flowers, partly native and 
partly cultivated, as proof of the correctness of the theory :— 
(1) A honey-gland or nectary, i.e. a part which elaborates and 
secretes honey; (2) a honey-receptacle, which receives and stores 
the honey secreted by the gland; (3) a contrivance to shelter 
the honey from rain (Saftdecke); (4) contrivances to enable the 
insect to find the honey easily (Sa/tmal); bright colour and ex- 
tension of the corolla, odour, and above all coloured spots near 
the entrance to the honey-receptacles (path-finders); (5) the 
‘impossibility of mechanical fertilisation, 7.e. spontaneous self-fer- 
tilisation, or of fertilisation by the wind, and in many cases the 
direct observation of fertilisation by insects in nature. Sprengel 
discussed these five points in numerous honey-secreting flowers, 
