PROTECTION AGAINST ANIMALS 309 



These, and chemical protection in general, may be much 

 more effective than mechanical protection. Stahl states 

 that usually in arid regions with sparse vegetation grazing 

 animals avoid juicy, apparently unprotected, plants, and 

 may be seen wandering round bushes bristling with thorns, 

 nibbling here and there, picking with care and trouble each 

 available leaflet. In Algeria the sharp-needled Juniperus 

 oxycedrus is trimmed round by goats and sheep, while the 

 softer leaved J. phoenicea is left severely alone ; the latter 

 is poisonous. In Britain the sheep-nibbled whin bush is 

 familiar, while the broom, which contains bitter substances 

 and a poisonous alkaloid, is avoided. 



Acid sap is a protection against snails, as in Oxalis 

 Acetosella and Rum ex Acetosa. In many plants alkaloid 

 poisons protect against grazing animals — coniin in the 

 hemlock ; in the foxglove there is a poisonous glucoside, 

 digitalin. The latex of many plants is poisonous for at least 

 some of their enemies. We have already mentioned the 

 case of the cypress spurge, which is protected by bitter 

 latex against all animals except the caterpillar of one moth. 

 The latex of the para rubber tree does not protect against 

 boring beetles. Ethereal oils are said to be protective, as 

 in the Labiatas. 



Inflammation may be set up, not only by the contents of 

 stinging hairs, but by substances exuded by glandular hairs, 

 as in Primula sinensis, or liberated w'hen the leaf is crushed 

 as in Rhus toxicodendron, the poison ivy of America. The 

 nature of these ver}' active poisons is obscure. Systematic 

 accounts of poisonous plants are given by Long (1917) and 

 Pammel (191 1). 



Special Cases.— Three very doubtful cases may be 

 referred to. In tropical South America great damage is 

 done to foliage by leaf-cutting or parasol ants, which use 

 the material thus obtained in the preparation of their fungus 

 culture beds. It has been long supposed that some trees are 

 protected against such enemies by maintaining an army of 

 fighting ants which drive oflF the invaders. These fighting 

 ants live in hollow stems, e.g. in Cecropia adenopus, the 



