vii Relations Between Plants and Animals 127 



which the insects cannot cross, as we see in some of the 

 catchflies, e.g. Lychnis viscaria and others. There may 

 be very slippery stems on which the climbers can find 

 no foothold, " and the flowers are often pendulous, as in 

 snowdrop and cyclamen, creeping creatures being thus kept 

 out of them, just as the pendulous nests of the weaver bird 

 are a protection against snakes and other enemies." Sir 

 John Lubbock quotes from Kerner, whose charming Flowers 

 and their Unbidden Guests is most full of information on 

 this head, the case of Polygonum amphibium, a common 

 pond-weed, which, as its name suggests, lives amphibiously, 

 sometimes in water, sometimes on land. " So long as it 

 grows in water it is protected by the water, and its stem is 

 smooth ; but, on the other hand, those specimens which 

 live on land throw out certain hairs, which terminate in 

 sticky glands, and thus prevent small insects from creeping 

 up to the flowers. In this case, therefore, the plant is not 

 sticky, except just when this condition is useful." In many 

 cases, too, the flowers, like those of the snapdragon, are 

 virtually closed boxes which can be opened by the bees, 

 but not by the small ants, unless indeed these bite a hole 

 through the base of the petals. 



But it may be asked, if the ants are excluded from the 

 flowers in any of these ways, why do they visit the plants 

 at all ? Partly, no doubt, by way of experiment, partly for 

 the sake of the booty of smaller insects which they find about 

 the leaves, but also because the secretion of nectar is not 

 always confined to the flowers, but may take place on other 

 parts of the plant in what are called extra-floral nectaries, 

 of which something will afterwards be said. 



But the web has another mesh. As long ago as 1688 

 John Ray noted the constant occurrence of ants in the 

 hollow stems of the South American Cecropia palmata, and 

 many other naturalists had called attention to the same 



