92 PERIOD IV. 



found in spiders' nests, gave him the hint as to the way 

 in which the fertilisation of orchids is effected. Definite 

 questions must be put if observation is to be profitable. 

 What is the use of honey to the plant of this coloured 

 spot of these hairs? He notes the peculiarities of 

 wind-fertilised and insect-fertilised flowers, the relative 

 abundance of the pollen, the form of the stigma, the 

 presence or absence of honey, the size, colour, and 

 scent of the corolla. Here is a pretty illustration from 

 his pages. Pluck a branch of hazel, aspen, or alder, 

 with unexpanded catkins, and also one from the male 

 sallow ; place them in water, and keep them in a sunny 

 window until the anthers are ripe. A vigorous puff 

 will then discharge a cloud of pollen from the 'wind- 

 fertilised catkins, but none from the insect-fertilised 

 catkin of the sallow. What Linnaeus said about the 

 flowers of trees appearing before the leaves, in order 

 that the pollen may more easily reach the stigmas, 

 holds good, Sprengel remarks, only of wind-fertilised 

 trees. The lime, which is insect-fertilised, flowers in 

 the height of summer, when all the branches are 

 crowded with leaves. 



Sprengel left it to later biologists to complete his 

 discovery. " That wonderfully accurate observer, 

 Sprengel," says Darwin, 1 "who first showed how im- 

 portant a part insects play in the fertilisation of flowers, 

 called his book The Secret of Nature Displayed ; yet he 

 only occasionally saw that the object for which so many 

 curious and beautiful adaptations have been acquired, 

 was the cross-fertilisation of distinct plants ; and he 

 knew nothing of the benefits which the offspring thus 

 receive in growth, vigour, and fertility." Not even 



1 Cross and Self-Fertilisation of Plants, chap. xi. 



