626 The Outline of Science 



8 



How Plants Protect themselves 



Even this short survey shows that the power of limited move- 

 ment in plants is widespread. The tropisms are specially im- 

 portant, for with their aid the plant arranges its organs so as to 

 make the best of its environment. The ultimate position of leaf, 

 stem, and root is the result of balanced reactions to all the 

 influences that play on the plant throughout its growth. But 

 the power of movement is nearly always restricted and gentle, 

 confined to arranging the members of a stationary organism. 

 Active locomotion belongs to the animal kingdom. Perhaps the 

 nearest approach is in the telegraph plant (Desmodium gyrans) 

 from the plains of the Ganges, in which there are continuous 

 movements of the leaflets in little orbits; but no one knows why 

 this plant should be so busy. 



We have said that plants live their lives very much as ani- 

 mals live theirs; like animals they have to protect themselves 

 against natural enemies. 



Many animals live exclusively on a vegetable diet; as there 

 are plants that strongly object to being eaten, they take effective 

 measures to protect themselves; if they did not, the herbivorous 

 animal itself would suffer, for its means of subsistence would in 

 time disappear from the face of the earth. One means of defence 

 employed by the plant is poisons and corrosive fluids, which it 

 uses to good effect. But sometimes what is one animal's food 

 is another's poison. For instance, the leaves of the deadly night- 

 shade form the most important food of a small beetle, but the 

 foliage is poison to the larger grazing animals. 



It is not clear to us how grazing animals discriminate be- 

 tween what is harmful to them and what is not harmful. Many 

 plants have characteristic odours offensive to us. Others are 

 odourless to the olfactory nerves of man, but they may make 

 themselves known to the animal's sense of smell. Wild animals 

 probably recognise the dangerous plant by colour, smell, or taste. 



