312 THE BIOLOGY OF FLOWERING PLANTS 



and I believe that the plant largely escapes grazing through 

 this lack of inviting appearance." Stahl found that goats 

 vi^ere in fact prevented from eating the shrub. Goebel 

 points out the danger of drawing conclusions from the 

 behaviour of the plant in the east, v^^here it is not native, 

 or towards an animal like the goat, which does not occur 

 in South America, the plant's original home. Humboldt's 

 observation that horses and cows refused to eat it in South 

 America must also be applied with caution, since these 

 animals are again introduced. Good evidence that the 

 movement is protective in natural conditions is therefore 

 wanting, and, on the other side, we must place Goebel 's 

 discovery that plants subjected to frequent shock are much 

 retarded in their development. There is even less evidence 

 in favour of the theory that the movement enables the plant 

 to shake off marauding insects. 



Finally, we may refer very shortly to the galls formed by 

 so many plants in relation to the attacks of definite insects. 

 These are teratological structures formed on leaf, stem, 

 root, flower, or fruit after deposition in the tissues, or on 

 the growing point, of an insect egg. They are often complex 

 in structure, and are closely related to the requirements of 

 the larva, which may even have at its disposal a special 

 nutritive tissue, and a preformed opening for its escape at 

 the proper time. How the plant comes to form such a 

 thing, apparently without use to itself, and exactly fitted 

 to the needs of the animal, is a question on which even 

 speculation is silent. It is possible that the origin of the 

 gall and its primary significance lies in a limitation of the 

 activities of the larva, and is thus protective. Every one 

 knows the destruction caused by burrowing larvae in the 

 leaves of such garden plants as the chrysanthemum, the 

 nasturtium, and the marguerite. If the grub were restricted 

 in its wanderings the leaf would benefit, and in such restric- 

 tion by means of the formation of a special resistant tissue 

 the gall may have taken its origin. 



Protection against Parasitic Plants. — The means of protec- 

 tion against plant parasites have been little investigated. No 



