PROTECTIVE ADAPTATIONS IN PLANTS 



123 



species of nettle, e.g. Urtiea stimulata in Java, and others, which have 

 an effect similar to that of snake-poison and produce tetanoid spasms, 

 and so on. In addition to formic acid these hairs contain an 

 undefined ferment, a so-called Enzyme. It need scarcely be said 

 that these stinging-hairs must have much more severe effects on 

 the mucous membrane of the mouth of grazing animals than on the 

 human skin, and that they are therefore an excellent protection for 

 the plants. As a matter of fact we never find our nettle patches 

 eaten away, and even the donkey, which eats thistles freely, turns 



M 



M 



Fig. 19. Barbed bristles 

 of Opuntia raflnesquii ; en- 



larged. 



Fig. 20. Vertical section through 

 a piece of a leaf of the Stinging-nettle 

 {Urtiea dioica), bearing two stinging- 

 hairs ; magnified 85 times ; adapted 

 from Kerner and Haberlandt. 



away from the stinging-nettle. But even these stinging-hairs, like 

 all other protective devices, do not afford an absolute protection. The 

 caterpillars of several of our diurnal butterflies feed exclusively on 

 the stinging-nettle, and they eat up the leaves, stinging-hairs and all. 

 This is the case with five species of the genus Vanessa, namely: 

 Vanessa io, the 'peacock,' Vanessa uHicw, the small tortoiseshell, 

 Vanessa prorsa, Vanessa C. album, the C. butterfly, and Vanessa 

 atalanta, the admiral. 



We are all familiar with our mulleins {Verhascuw), those 



I 3 



