144 BOTANY. [chap. iv. 



There is a lip on which the insect alights as in the dead- 

 nettle, bat the special contrivance of the sage consists 

 in an arrangement by which the insect, in thrusting its 

 head into the tube of the corolla, causes the mature 

 anther to come suddenly down upon its back and shed 

 its pollen, after which it shrivels and disappears, the 

 second' anther then being ready to act in a similar man- 

 ner. Eventually the style curves down as in the dead- 

 nettle. 



The advantage of the latter method over that of the 

 white dead-nettle is as follows. In the last-named 

 plant the stamens, when mature, curve down into posi- 

 tion and shed their pollen whether an insect visits the 

 flower or not, whereas in the sage the stamen remains 

 under the upper lip until mechanically forced down by 

 the presence of an insect ; then the force with which it 

 comes down on the insect's back causes the pollen to 

 escape from the anther. 



Numerous other equally effective and interesting 

 arrangements for preventing self-fertilization, and at the 

 same time favouring cross-fertilization, exist; space 

 alone forbids a detailed description of such. In some 

 exotic plants, fertilization is effected by small birds. 



The various colours of flowers are not, as might be 

 supposed, the result of chance, and without any special 

 object in view ; but, remembering that the only known 

 use of colour in the flower is that of an advertisement 

 indicating their presence to insects or birds in connec- 

 tion with fertilization, so we find that the evolution of 

 colour is, as a rule, contemporaneous with that of struc- 

 ture bearing on fertilization. Yellow is the primitive 

 colour of flowers, and characteristic of those that are 



