PROTECTIVE ADAPTATIONS IN PLANTS 129 



prevent the snails from creeping up the stalks. We never find the 

 comfrey (Symphytum officinale) of our meadows eaten by snails Poi 

 it is thickly covered over with stiff bristles, which are most dis- 

 agreeable to the snail, and the stinging-nettle (Urtica dicrica) is 

 similarly protected by bristle hairs, while, as we have already 

 seen, its stinging-hairs secure immunity from the attacks of larger 

 animals. 



And although it is true that the majority of plants do not 

 prevent the snails from creeping up their stalks, yet they do not 

 serve them in any great degree as food, since the green parts often 

 offer resistance to mastication and digestion. Thus the lime encrus- 

 tations which cover the stoneworts (Char a) prevent snails from 

 eating them. If the lime be dissolved by means of acids, and the 

 plants then offered to the snails, they will eat them greedily. The 

 same is true of the silicifying of the cell-walls, so widely distributed 

 among mosses and grasses, and when this occurs in a high degree it 

 forms an effective protection even against the large herbivores. Our 

 slightly siliceous grasses are secure from snails, and that it is really 

 the presence of the silicic acid which deters them from an otherwise 

 welcome kind of food is proved by Stahl's experiment of growing 

 maize in pure water, and so obtaining plants poor in silica. These 

 were devoured without ceremony by the snails. 



Of the many other protective peculiarities which make it difficult 

 for snails to eat plants I shall only recall the so-called ' Raphides,' 

 those microscopic crystal-like needles of oxalate of lime, pointed at 

 both ends, which lie close together in the tissues of many plants. 

 Cuckoo pint (Arum maculatum), the narcissi, the snowdrops (Leu- 

 cojum), the squill (ticilla), and the asparagus contain them, and all 

 these plants are spared by snails obviously because during masti- 

 cation they are unpleasantly affected by the raphides. Even the 

 voracious field-slug rejects these. 



Of course it cannot be said that these raphides protect againsl 

 all other enemies. They are effective against rodents and ruminants, 

 and also against locusts, but a number of caterpillars seek out by 

 preference just those plants which contain raphides. Thus certain 

 caterpillars of the Sphingidse feed on species of Galium and Epi- 

 lobium, the leaves of the vine, and the wild balsam (Impatient). 

 The caterpillar of Chcerocampa, elpenor, which especially prefers 

 Vitis and Epilobium, has transferred its affections to the fuchsias 

 in our gardens, which came from South America; the butterfly not 

 infrequently lays its eggs on these plants, and the caterpillars devour 

 them readily ; but the fuchsias ma} T also contain raphides. 



