PROTECTIVE ACTIVITIES 



375 



instincts that have to do with the getting of food. Thus, while 

 the ferocity of the tiger or the dog finds expression through 

 organs that are related to food-getting, the strictly vegetarian 

 rhinoceros or mountain sheep will fight fiercely and coura- 

 geously with horns or hoofs. The branching horns of the 

 deer or elk seem never 

 to be used aggressively 

 except against mem- 

 bers of their own spe- 

 cies, as when two males 

 are in combat. 



413. Shedding of 

 leaves. The dropping 

 of leaves in the autumn, 

 while it does not in- 

 volve movements like 

 those of muscles, may 

 properly be considered 

 a protective act. 



The shedding of leaves 

 seems to be related to 

 the water factor as well 

 as to the temperature 

 factor, which we usually 

 associate with the change 

 of seasons. As the au- 

 tumn advances and the 

 water in the soil becomes 



Fig. 196. Insect galls 



It is probable that by the formation of such galls 

 many plants are really protected against serious in- 

 jury, although many of the galls may simply represent 

 the behavior of protoplasm when injured in a certain 

 way, rather than a useful way of behaving. It is in- 

 teresting to note that the galls are always specific. 

 Thus, both of these galls are on the same species — 

 the white oak {Qitcrciis alba) — but are produced by 

 different species of insects: i, hy Bior/iiza fortiLornis ; 

 2, by Holcaspis globulus 



scarcer, transpiration is 



interfered with. Evaporation from the leaves, however, continues so 

 long as there is water in the cells. If the loss of water cannot be 

 compensated by the absorption of the roots, the live cells of the plant 

 must suffer injury. The leaf cells are the first to be affected. The 

 loss of the leaves prevents the complete drying up of the plant, and 

 it also prevents the freezing of live cells (see Fig. 195). The relation 

 of water to the fall of the leaf has been determined experimentally. 



