374 MEANS OP CROSS-FERTILISATION. Chap. X. 



that these marks have really been developed in corre- 

 lation with the nectary. The two upper petals of the 

 common Pelargonium are thus marked near their bases; 

 and I have repeatedly observed that when the flowers 

 vary so as to become peloric or regular, they lose their 

 nectaries and at the same time the dark marks. 

 Wnen the nectary is only partially aborted, only one 

 of the upper petals loses its mark. Therefore the nec- 

 tary and these marks clearly stand in some sort of close 

 relation to one another ; and the simplest view is that 

 they were developed together for a special purpose ; 

 the only conceivable one being that the marks serve as 

 a guide to the nectary. It is, however, evident from 

 what has been already said, that insects could discover 

 the nectar without the aid of guiding marks. They 

 are of service to the plant, only by aiding insects to 

 visit and suck a greater number of flowers within a 

 given time than would otherwise be possible ; and thus 

 there will be a better chance of fertilisation by pollen 

 brought from a distinct plant, and this we know is of 

 paramount importance. 



The odours emitted by flowers attract insects, as I 

 have observed in the case of plants covered by a muslin 

 net. Nageli affixed artificial flowers to branches, 

 scenting some with essential oils and leaving others 

 unscented ; and insects were attracted to the former in 

 an unmistakable manner.* It would appear that they 

 must be guided by the simultaneous action of sight 

 and smell, for M. Plateau f found that excellently 

 made, but not scented, artificial flowers never deceived 

 them. It will be shown in the next chapter that the 

 flowers of certain plants remain fully expanded for 



* ' Enstehung, &c, der Natur- Assoc, lor the Advancement of 

 hist. Art.' 1865, p. 23. Science,' 1876. 



t ' Proceedings of the French 



